From 0eb927c0ab789d3d7d69f68acb850f69d4e7c36f Mon Sep 17 00:00:00 2001 From: Mel Gorman Date: Tue, 21 Jan 2014 15:51:05 -0800 Subject: mm: compaction: trace compaction begin and end The broad goal of the series is to improve allocation success rates for huge pages through memory compaction, while trying not to increase the compaction overhead. The original objective was to reintroduce capturing of high-order pages freed by the compaction, before they are split by concurrent activity. However, several bugs and opportunities for simple improvements were found in the current implementation, mostly through extra tracepoints (which are however too ugly for now to be considered for sending). The patches mostly deal with two mechanisms that reduce compaction overhead, which is caching the progress of migrate and free scanners, and marking pageblocks where isolation failed to be skipped during further scans. Patch 1 (from mgorman) adds tracepoints that allow calculate time spent in compaction and potentially debug scanner pfn values. Patch 2 encapsulates the some functionality for handling deferred compactions for better maintainability, without a functional change type is not determined without being actually needed. Patch 3 fixes a bug where cached scanner pfn's are sometimes reset only after they have been read to initialize a compaction run. Patch 4 fixes a bug where scanners meeting is sometimes not properly detected and can lead to multiple compaction attempts quitting early without doing any work. Patch 5 improves the chances of sync compaction to process pageblocks that async compaction has skipped due to being !MIGRATE_MOVABLE. Patch 6 improves the chances of sync direct compaction to actually do anything when called after async compaction fails during allocation slowpath. The impact of patches were validated using mmtests's stress-highalloc benchmark with mmtests's stress-highalloc benchmark on a x86_64 machine with 4GB memory. Due to instability of the results (mostly related to the bugs fixed by patches 2 and 3), 10 iterations were performed, taking min,mean,max values for success rates and mean values for time and vmstat-based metrics. First, the default GFP_HIGHUSER_MOVABLE allocations were tested with the patches stacked on top of v3.13-rc2. Patch 2 is OK to serve as baseline due to no functional changes in 1 and 2. Comments below. stress-highalloc 3.13-rc2 3.13-rc2 3.13-rc2 3.13-rc2 3.13-rc2 2-nothp 3-nothp 4-nothp 5-nothp 6-nothp Success 1 Min 9.00 ( 0.00%) 10.00 (-11.11%) 43.00 (-377.78%) 43.00 (-377.78%) 33.00 (-266.67%) Success 1 Mean 27.50 ( 0.00%) 25.30 ( 8.00%) 45.50 (-65.45%) 45.90 (-66.91%) 46.30 (-68.36%) Success 1 Max 36.00 ( 0.00%) 36.00 ( 0.00%) 47.00 (-30.56%) 48.00 (-33.33%) 52.00 (-44.44%) Success 2 Min 10.00 ( 0.00%) 8.00 ( 20.00%) 46.00 (-360.00%) 45.00 (-350.00%) 35.00 (-250.00%) Success 2 Mean 26.40 ( 0.00%) 23.50 ( 10.98%) 47.30 (-79.17%) 47.60 (-80.30%) 48.10 (-82.20%) Success 2 Max 34.00 ( 0.00%) 33.00 ( 2.94%) 48.00 (-41.18%) 50.00 (-47.06%) 54.00 (-58.82%) Success 3 Min 65.00 ( 0.00%) 63.00 ( 3.08%) 85.00 (-30.77%) 84.00 (-29.23%) 85.00 (-30.77%) Success 3 Mean 76.70 ( 0.00%) 70.50 ( 8.08%) 86.20 (-12.39%) 85.50 (-11.47%) 86.00 (-12.13%) Success 3 Max 87.00 ( 0.00%) 86.00 ( 1.15%) 88.00 ( -1.15%) 87.00 ( 0.00%) 87.00 ( 0.00%) 3.13-rc2 3.13-rc2 3.13-rc2 3.13-rc2 3.13-rc2 2-nothp 3-nothp 4-nothp 5-nothp 6-nothp User 6437.72 6459.76 5960.32 5974.55 6019.67 System 1049.65 1049.09 1029.32 1031.47 1032.31 Elapsed 1856.77 1874.48 1949.97 1994.22 1983.15 3.13-rc2 3.13-rc2 3.13-rc2 3.13-rc2 3.13-rc2 2-nothp 3-nothp 4-nothp 5-nothp 6-nothp Minor Faults 253952267 254581900 250030122 250507333 250157829 Major Faults 420 407 506 530 530 Swap Ins 4 9 9 6 6 Swap Outs 398 375 345 346 333 Direct pages scanned 197538 189017 298574 287019 299063 Kswapd pages scanned 1809843 1801308 1846674 1873184 1861089 Kswapd pages reclaimed 1806972 1798684 1844219 1870509 1858622 Direct pages reclaimed 197227 188829 298380 286822 298835 Kswapd efficiency 99% 99% 99% 99% 99% Kswapd velocity 953.382 970.449 952.243 934.569 922.286 Direct efficiency 99% 99% 99% 99% 99% Direct velocity 104.058 101.832 153.961 143.200 148.205 Percentage direct scans 9% 9% 13% 13% 13% Zone normal velocity 347.289 359.676 348.063 339.933 332.983 Zone dma32 velocity 710.151 712.605 758.140 737.835 737.507 Zone dma velocity 0.000 0.000 0.000 0.000 0.000 Page writes by reclaim 557.600 429.000 353.600 426.400 381.800 Page writes file 159 53 7 79 48 Page writes anon 398 375 345 346 333 Page reclaim immediate 825 644 411 575 420 Sector Reads 2781750 2769780 2878547 2939128 2910483 Sector Writes 12080843 12083351 12012892 12002132 12010745 Page rescued immediate 0 0 0 0 0 Slabs scanned 1575654 1545344 1778406 1786700 1794073 Direct inode steals 9657 10037 15795 14104 14645 Kswapd inode steals 46857 46335 50543 50716 51796 Kswapd skipped wait 0 0 0 0 0 THP fault alloc 97 91 81 71 77 THP collapse alloc 456 506 546 544 565 THP splits 6 5 5 4 4 THP fault fallback 0 1 0 0 0 THP collapse fail 14 14 12 13 12 Compaction stalls 1006 980 1537 1536 1548 Compaction success 303 284 562 559 578 Compaction failures 702 696 974 976 969 Page migrate success 1177325 1070077 3927538 3781870 3877057 Page migrate failure 0 0 0 0 0 Compaction pages isolated 2547248 2306457 8301218 8008500 8200674 Compaction migrate scanned 42290478 38832618 153961130 154143900 159141197 Compaction free scanned 89199429 79189151 356529027 351943166 356326727 Compaction cost 1566 1426 5312 5156 5294 NUMA PTE updates 0 0 0 0 0 NUMA hint faults 0 0 0 0 0 NUMA hint local faults 0 0 0 0 0 NUMA hint local percent 100 100 100 100 100 NUMA pages migrated 0 0 0 0 0 AutoNUMA cost 0 0 0 0 0 Observations: - The "Success 3" line is allocation success rate with system idle (phases 1 and 2 are with background interference). I used to get stable values around 85% with vanilla 3.11. The lower min and mean values came with 3.12. This was bisected to commit 81c0a2bb ("mm: page_alloc: fair zone allocator policy") As explained in comment for patch 3, I don't think the commit is wrong, but that it makes the effect of compaction bugs worse. From patch 3 onwards, the results are OK and match the 3.11 results. - Patch 4 also clearly helps phases 1 and 2, and exceeds any results I've seen with 3.11 (I didn't measure it that thoroughly then, but it was never above 40%). - Compaction cost and number of scanned pages is higher, especially due to patch 4. However, keep in mind that patches 3 and 4 fix existing bugs in the current design of compaction overhead mitigation, they do not change it. If overhead is found unacceptable, then it should be decreased differently (and consistently, not due to random conditions) than the current implementation does. In contrast, patches 5 and 6 (which are not strictly bug fixes) do not increase the overhead (but also not success rates). This might be a limitation of the stress-highalloc benchmark as it's quite uniform. Another set of results is when configuring stress-highalloc t allocate with similar flags as THP uses: (GFP_HIGHUSER_MOVABLE|__GFP_NOMEMALLOC|__GFP_NORETRY|__GFP_NO_KSWAPD) stress-highalloc 3.13-rc2 3.13-rc2 3.13-rc2 3.13-rc2 3.13-rc2 2-thp 3-thp 4-thp 5-thp 6-thp Success 1 Min 2.00 ( 0.00%) 7.00 (-250.00%) 18.00 (-800.00%) 19.00 (-850.00%) 26.00 (-1200.00%) Success 1 Mean 19.20 ( 0.00%) 17.80 ( 7.29%) 29.20 (-52.08%) 29.90 (-55.73%) 32.80 (-70.83%) Success 1 Max 27.00 ( 0.00%) 29.00 ( -7.41%) 35.00 (-29.63%) 36.00 (-33.33%) 37.00 (-37.04%) Success 2 Min 3.00 ( 0.00%) 8.00 (-166.67%) 21.00 (-600.00%) 21.00 (-600.00%) 32.00 (-966.67%) Success 2 Mean 19.30 ( 0.00%) 17.90 ( 7.25%) 32.20 (-66.84%) 32.60 (-68.91%) 35.70 (-84.97%) Success 2 Max 27.00 ( 0.00%) 30.00 (-11.11%) 36.00 (-33.33%) 37.00 (-37.04%) 39.00 (-44.44%) Success 3 Min 62.00 ( 0.00%) 62.00 ( 0.00%) 85.00 (-37.10%) 75.00 (-20.97%) 64.00 ( -3.23%) Success 3 Mean 66.30 ( 0.00%) 65.50 ( 1.21%) 85.60 (-29.11%) 83.40 (-25.79%) 83.50 (-25.94%) Success 3 Max 70.00 ( 0.00%) 69.00 ( 1.43%) 87.00 (-24.29%) 86.00 (-22.86%) 87.00 (-24.29%) 3.13-rc2 3.13-rc2 3.13-rc2 3.13-rc2 3.13-rc2 2-thp 3-thp 4-thp 5-thp 6-thp User 6547.93 6475.85 6265.54 6289.46 6189.96 System 1053.42 1047.28 1043.23 1042.73 1038.73 Elapsed 1835.43 1821.96 1908.67 1912.74 1956.38 3.13-rc2 3.13-rc2 3.13-rc2 3.13-rc2 3.13-rc2 2-thp 3-thp 4-thp 5-thp 6-thp Minor Faults 256805673 253106328 253222299 249830289 251184418 Major Faults 395 375 423 434 448 Swap Ins 12 10 10 12 9 Swap Outs 530 537 487 455 415 Direct pages scanned 71859 86046 153244 152764 190713 Kswapd pages scanned 1900994 1870240 1898012 1892864 1880520 Kswapd pages reclaimed 1897814 1867428 1894939 1890125 1877924 Direct pages reclaimed 71766 85908 153167 152643 190600 Kswapd efficiency 99% 99% 99% 99% 99% Kswapd velocity 1029.000 1067.782 1000.091 991.049 951.218 Direct efficiency 99% 99% 99% 99% 99% Direct velocity 38.897 49.127 80.747 79.983 96.468 Percentage direct scans 3% 4% 7% 7% 9% Zone normal velocity 351.377 372.494 348.910 341.689 335.310 Zone dma32 velocity 716.520 744.414 731.928 729.343 712.377 Zone dma velocity 0.000 0.000 0.000 0.000 0.000 Page writes by reclaim 669.300 604.000 545.700 538.900 429.900 Page writes file 138 66 58 83 14 Page writes anon 530 537 487 455 415 Page reclaim immediate 806 655 772 548 517 Sector Reads 2711956 2703239 2811602 2818248 2839459 Sector Writes 12163238 12018662 12038248 11954736 11994892 Page rescued immediate 0 0 0 0 0 Slabs scanned 1385088 1388364 1507968 1513292 1558656 Direct inode steals 1739 2564 4622 5496 6007 Kswapd inode steals 47461 46406 47804 48013 48466 Kswapd skipped wait 0 0 0 0 0 THP fault alloc 110 82 84 69 70 THP collapse alloc 445 482 467 462 539 THP splits 6 5 4 5 3 THP fault fallback 3 0 0 0 0 THP collapse fail 15 14 14 14 13 Compaction stalls 659 685 1033 1073 1111 Compaction success 222 225 410 427 456 Compaction failures 436 460 622 646 655 Page migrate success 446594 439978 1085640 1095062 1131716 Page migrate failure 0 0 0 0 0 Compaction pages isolated 1029475 1013490 2453074 2482698 2565400 Compaction migrate scanned 9955461 11344259 24375202 27978356 30494204 Compaction free scanned 27715272 28544654 80150615 82898631 85756132 Compaction cost 552 555 1344 1379 1436 NUMA PTE updates 0 0 0 0 0 NUMA hint faults 0 0 0 0 0 NUMA hint local faults 0 0 0 0 0 NUMA hint local percent 100 100 100 100 100 NUMA pages migrated 0 0 0 0 0 AutoNUMA cost 0 0 0 0 0 There are some differences from the previous results for THP-like allocations: - Here, the bad result for unpatched kernel in phase 3 is much more consistent to be between 65-70% and not related to the "regression" in 3.12. Still there is the improvement from patch 4 onwards, which brings it on par with simple GFP_HIGHUSER_MOVABLE allocations. - Compaction costs have increased, but nowhere near as much as the non-THP case. Again, the patches should be worth the gained determininsm. - Patches 5 and 6 somewhat increase the number of migrate-scanned pages. This is most likely due to __GFP_NO_KSWAPD flag, which means the cached pfn's and pageblock skip bits are not reset by kswapd that often (at least in phase 3 where no concurrent activity would wake up kswapd) and the patches thus help the sync-after-async compaction. It doesn't however show that the sync compaction would help so much with success rates, which can be again seen as a limitation of the benchmark scenario. This patch (of 6): Add two tracepoints for compaction begin and end of a zone. Using this it is possible to calculate how much time a workload is spending within compaction and potentially debug problems related to cached pfns for scanning. In combination with the direct reclaim and slab trace points it should be possible to estimate most allocation-related overhead for a workload. Signed-off-by: Mel Gorman Signed-off-by: Vlastimil Babka Cc: Rik van Riel Cc: Joonsoo Kim Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/compaction.c | 4 ++++ 1 file changed, 4 insertions(+) (limited to 'mm/compaction.c') diff --git a/mm/compaction.c b/mm/compaction.c index f58bcd016f43..a03995eddedb 100644 --- a/mm/compaction.c +++ b/mm/compaction.c @@ -970,6 +970,8 @@ static int compact_zone(struct zone *zone, struct compact_control *cc) if (compaction_restarting(zone, cc->order) && !current_is_kswapd()) __reset_isolation_suitable(zone); + trace_mm_compaction_begin(start_pfn, cc->migrate_pfn, cc->free_pfn, end_pfn); + migrate_prep_local(); while ((ret = compact_finished(zone, cc)) == COMPACT_CONTINUE) { @@ -1015,6 +1017,8 @@ out: cc->nr_freepages -= release_freepages(&cc->freepages); VM_BUG_ON(cc->nr_freepages != 0); + trace_mm_compaction_end(ret); + return ret; } -- cgit v1.2.3 From de6c60a6c115acaa721cfd499e028a413d1fcbf3 Mon Sep 17 00:00:00 2001 From: Vlastimil Babka Date: Tue, 21 Jan 2014 15:51:07 -0800 Subject: mm: compaction: encapsulate defer reset logic Currently there are several functions to manipulate the deferred compaction state variables. The remaining case where the variables are touched directly is when a successful allocation occurs in direct compaction, or is expected to be successful in the future by kswapd. Here, the lowest order that is expected to fail is updated, and in the case of successful allocation, the deferred status and counter is reset completely. Create a new function compaction_defer_reset() to encapsulate this functionality and make it easier to understand the code. No functional change. Signed-off-by: Vlastimil Babka Acked-by: Mel Gorman Reviewed-by: Rik van Riel Cc: Joonsoo Kim Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/compaction.c | 9 ++++----- 1 file changed, 4 insertions(+), 5 deletions(-) (limited to 'mm/compaction.c') diff --git a/mm/compaction.c b/mm/compaction.c index a03995eddedb..927de97cab8d 100644 --- a/mm/compaction.c +++ b/mm/compaction.c @@ -1124,12 +1124,11 @@ static void __compact_pgdat(pg_data_t *pgdat, struct compact_control *cc) compact_zone(zone, cc); if (cc->order > 0) { - int ok = zone_watermark_ok(zone, cc->order, - low_wmark_pages(zone), 0, 0); - if (ok && cc->order >= zone->compact_order_failed) - zone->compact_order_failed = cc->order + 1; + if (zone_watermark_ok(zone, cc->order, + low_wmark_pages(zone), 0, 0)) + compaction_defer_reset(zone, cc->order, false); /* Currently async compaction is never deferred. */ - else if (!ok && cc->sync) + else if (cc->sync) defer_compaction(zone, cc->order); } -- cgit v1.2.3 From d3132e4b83e6bd383c74d716f7281d7c3136089c Mon Sep 17 00:00:00 2001 From: Vlastimil Babka Date: Tue, 21 Jan 2014 15:51:08 -0800 Subject: mm: compaction: reset cached scanner pfn's before reading them Compaction caches pfn's for its migrate and free scanners to avoid scanning the whole zone each time. In compact_zone(), the cached values are read to set up initial values for the scanners. There are several situations when these cached pfn's are reset to the first and last pfn of the zone, respectively. One of these situations is when a compaction has been deferred for a zone and is now being restarted during a direct compaction, which is also done in compact_zone(). However, compact_zone() currently reads the cached pfn's *before* resetting them. This means the reset doesn't affect the compaction that performs it, and with good chance also subsequent compactions, as update_pageblock_skip() is likely to be called and update the cached pfn's to those being processed. Another chance for a successful reset is when a direct compaction detects that migration and free scanners meet (which has its own problems addressed by another patch) and sets update_pageblock_skip flag which kswapd uses to do the reset because it goes to sleep. This is clearly a bug that results in non-deterministic behavior, so this patch moves the cached pfn reset to be performed *before* the values are read. Signed-off-by: Vlastimil Babka Acked-by: Mel Gorman Acked-by: Rik van Riel Cc: Joonsoo Kim Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/compaction.c | 16 ++++++++-------- 1 file changed, 8 insertions(+), 8 deletions(-) (limited to 'mm/compaction.c') diff --git a/mm/compaction.c b/mm/compaction.c index 927de97cab8d..f4e2c166880b 100644 --- a/mm/compaction.c +++ b/mm/compaction.c @@ -946,6 +946,14 @@ static int compact_zone(struct zone *zone, struct compact_control *cc) ; } + /* + * Clear pageblock skip if there were failures recently and compaction + * is about to be retried after being deferred. kswapd does not do + * this reset as it'll reset the cached information when going to sleep. + */ + if (compaction_restarting(zone, cc->order) && !current_is_kswapd()) + __reset_isolation_suitable(zone); + /* * Setup to move all movable pages to the end of the zone. Used cached * information on where the scanners should start but check that it @@ -962,14 +970,6 @@ static int compact_zone(struct zone *zone, struct compact_control *cc) zone->compact_cached_migrate_pfn = cc->migrate_pfn; } - /* - * Clear pageblock skip if there were failures recently and compaction - * is about to be retried after being deferred. kswapd does not do - * this reset as it'll reset the cached information when going to sleep. - */ - if (compaction_restarting(zone, cc->order) && !current_is_kswapd()) - __reset_isolation_suitable(zone); - trace_mm_compaction_begin(start_pfn, cc->migrate_pfn, cc->free_pfn, end_pfn); migrate_prep_local(); -- cgit v1.2.3 From 7ed695e069c3cbea5e1fd08f84a04536da91f584 Mon Sep 17 00:00:00 2001 From: Vlastimil Babka Date: Tue, 21 Jan 2014 15:51:09 -0800 Subject: mm: compaction: detect when scanners meet in isolate_freepages Compaction of a zone is finished when the migrate scanner (which begins at the zone's lowest pfn) meets the free page scanner (which begins at the zone's highest pfn). This is detected in compact_zone() and in the case of direct compaction, the compact_blockskip_flush flag is set so that kswapd later resets the cached scanner pfn's, and a new compaction may again start at the zone's borders. The meeting of the scanners can happen during either scanner's activity. However, it may currently fail to be detected when it occurs in the free page scanner, due to two problems. First, isolate_freepages() keeps free_pfn at the highest block where it isolated pages from, for the purposes of not missing the pages that are returned back to allocator when migration fails. Second, failing to isolate enough free pages due to scanners meeting results in -ENOMEM being returned by migrate_pages(), which makes compact_zone() bail out immediately without calling compact_finished() that would detect scanners meeting. This failure to detect scanners meeting might result in repeated attempts at compaction of a zone that keep starting from the cached pfn's close to the meeting point, and quickly failing through the -ENOMEM path, without the cached pfns being reset, over and over. This has been observed (through additional tracepoints) in the third phase of the mmtests stress-highalloc benchmark, where the allocator runs on an otherwise idle system. The problem was observed in the DMA32 zone, which was used as a fallback to the preferred Normal zone, but on the 4GB system it was actually the largest zone. The problem is even amplified for such fallback zone - the deferred compaction logic, which could (after being fixed by a previous patch) reset the cached scanner pfn's, is only applied to the preferred zone and not for the fallbacks. The problem in the third phase of the benchmark was further amplified by commit 81c0a2bb515f ("mm: page_alloc: fair zone allocator policy") which resulted in a non-deterministic regression of the allocation success rate from ~85% to ~65%. This occurs in about half of benchmark runs, making bisection problematic. It is unlikely that the commit itself is buggy, but it should put more pressure on the DMA32 zone during phases 1 and 2, which may leave it more fragmented in phase 3 and expose the bugs that this patch fixes. The fix is to make scanners meeting in isolate_freepage() stay that way, and to check in compact_zone() for scanners meeting when migrate_pages() returns -ENOMEM. The result is that compact_finished() also detects scanners meeting and sets the compact_blockskip_flush flag to make kswapd reset the scanner pfn's. The results in stress-highalloc benchmark show that the "regression" by commit 81c0a2bb515f in phase 3 no longer occurs, and phase 1 and 2 allocation success rates are also significantly improved. Signed-off-by: Vlastimil Babka Cc: Mel Gorman Cc: Rik van Riel Cc: Joonsoo Kim Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/compaction.c | 19 +++++++++++++++---- 1 file changed, 15 insertions(+), 4 deletions(-) (limited to 'mm/compaction.c') diff --git a/mm/compaction.c b/mm/compaction.c index f4e2c166880b..cc46db36e708 100644 --- a/mm/compaction.c +++ b/mm/compaction.c @@ -660,7 +660,7 @@ static void isolate_freepages(struct zone *zone, * is the end of the pageblock the migration scanner is using. */ pfn = cc->free_pfn; - low_pfn = cc->migrate_pfn + pageblock_nr_pages; + low_pfn = ALIGN(cc->migrate_pfn + 1, pageblock_nr_pages); /* * Take care that if the migration scanner is at the end of the zone @@ -676,7 +676,7 @@ static void isolate_freepages(struct zone *zone, * pages on cc->migratepages. We stop searching if the migrate * and free page scanners meet or enough free pages are isolated. */ - for (; pfn > low_pfn && cc->nr_migratepages > nr_freepages; + for (; pfn >= low_pfn && cc->nr_migratepages > nr_freepages; pfn -= pageblock_nr_pages) { unsigned long isolated; @@ -738,7 +738,14 @@ static void isolate_freepages(struct zone *zone, /* split_free_page does not map the pages */ map_pages(freelist); - cc->free_pfn = high_pfn; + /* + * If we crossed the migrate scanner, we want to keep it that way + * so that compact_finished() may detect this + */ + if (pfn < low_pfn) + cc->free_pfn = max(pfn, zone->zone_start_pfn); + else + cc->free_pfn = high_pfn; cc->nr_freepages = nr_freepages; } @@ -1005,7 +1012,11 @@ static int compact_zone(struct zone *zone, struct compact_control *cc) if (err) { putback_movable_pages(&cc->migratepages); cc->nr_migratepages = 0; - if (err == -ENOMEM) { + /* + * migrate_pages() may return -ENOMEM when scanners meet + * and we want compact_finished() to detect it + */ + if (err == -ENOMEM && cc->free_pfn > cc->migrate_pfn) { ret = COMPACT_PARTIAL; goto out; } -- cgit v1.2.3 From 50b5b094e683f8e51e82c6dfe97b1608cf97e6c0 Mon Sep 17 00:00:00 2001 From: Vlastimil Babka Date: Tue, 21 Jan 2014 15:51:10 -0800 Subject: mm: compaction: do not mark unmovable pageblocks as skipped in async compaction Compaction temporarily marks pageblocks where it fails to isolate pages as to-be-skipped in further compactions, in order to improve efficiency. One of the reasons to fail isolating pages is that isolation is not attempted in pageblocks that are not of MIGRATE_MOVABLE (or CMA) type. The problem is that blocks skipped due to not being MIGRATE_MOVABLE in async compaction become skipped due to the temporary mark also in future sync compaction. Moreover, this may follow quite soon during __alloc_page_slowpath, without much time for kswapd to clear the pageblock skip marks. This goes against the idea that sync compaction should try to scan these blocks more thoroughly than the async compaction. The fix is to ensure in async compaction that these !MIGRATE_MOVABLE blocks are not marked to be skipped. Note this should not affect performance or locking impact of further async compactions, as skipping a block due to being !MIGRATE_MOVABLE is done soon after skipping a block marked to be skipped, both without locking. Signed-off-by: Vlastimil Babka Cc: Rik van Riel Acked-by: Mel Gorman Cc: Joonsoo Kim Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/compaction.c | 11 +++++++++-- 1 file changed, 9 insertions(+), 2 deletions(-) (limited to 'mm/compaction.c') diff --git a/mm/compaction.c b/mm/compaction.c index cc46db36e708..32a033cb5c65 100644 --- a/mm/compaction.c +++ b/mm/compaction.c @@ -459,6 +459,7 @@ isolate_migratepages_range(struct zone *zone, struct compact_control *cc, unsigned long flags; bool locked = false; struct page *page = NULL, *valid_page = NULL; + bool skipped_async_unsuitable = false; /* * Ensure that there are not too many pages isolated from the LRU @@ -534,6 +535,7 @@ isolate_migratepages_range(struct zone *zone, struct compact_control *cc, if (!cc->sync && last_pageblock_nr != pageblock_nr && !migrate_async_suitable(get_pageblock_migratetype(page))) { cc->finished_update_migrate = true; + skipped_async_unsuitable = true; goto next_pageblock; } @@ -627,8 +629,13 @@ next_pageblock: if (locked) spin_unlock_irqrestore(&zone->lru_lock, flags); - /* Update the pageblock-skip if the whole pageblock was scanned */ - if (low_pfn == end_pfn) + /* + * Update the pageblock-skip information and cached scanner pfn, + * if the whole pageblock was scanned without isolating any page. + * This is not done when pageblock was skipped due to being unsuitable + * for async compaction, so that eventual sync compaction can try. + */ + if (low_pfn == end_pfn && !skipped_async_unsuitable) update_pageblock_skip(cc, valid_page, nr_isolated, true); trace_mm_compaction_isolate_migratepages(nr_scanned, nr_isolated); -- cgit v1.2.3 From 55b7c4c99f6a448f72179297fe6432544f220063 Mon Sep 17 00:00:00 2001 From: Vlastimil Babka Date: Tue, 21 Jan 2014 15:51:11 -0800 Subject: mm: compaction: reset scanner positions immediately when they meet Compaction used to start its migrate and free page scaners at the zone's lowest and highest pfn, respectively. Later, caching was introduced to remember the scanners' progress across compaction attempts so that pageblocks are not re-scanned uselessly. Additionally, pageblocks where isolation failed are marked to be quickly skipped when encountered again in future compactions. Currently, both the reset of cached pfn's and clearing of the pageblock skip information for a zone is done in __reset_isolation_suitable(). This function gets called when: - compaction is restarting after being deferred - compact_blockskip_flush flag is set in compact_finished() when the scanners meet (and not again cleared when direct compaction succeeds in allocation) and kswapd acts upon this flag before going to sleep This behavior is suboptimal for several reasons: - when direct sync compaction is called after async compaction fails (in the allocation slowpath), it will effectively do nothing, unless kswapd happens to process the compact_blockskip_flush flag meanwhile. This is racy and goes against the purpose of sync compaction to more thoroughly retry the compaction of a zone where async compaction has failed. The restart-after-deferring path cannot help here as deferring happens only after the sync compaction fails. It is also done only for the preferred zone, while the compaction might be done for a fallback zone. - the mechanism of marking pageblock to be skipped has little value since the cached pfn's are reset only together with the pageblock skip flags. This effectively limits pageblock skip usage to parallel compactions. This patch changes compact_finished() so that cached pfn's are reset immediately when the scanners meet. Clearing pageblock skip flags is unchanged, as well as the other situations where cached pfn's are reset. This allows the sync-after-async compaction to retry pageblocks not marked as skipped, such as blocks !MIGRATE_MOVABLE blocks that async compactions now skips without marking them. Signed-off-by: Vlastimil Babka Cc: Rik van Riel Acked-by: Mel Gorman Cc: Joonsoo Kim Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/compaction.c | 4 ++++ 1 file changed, 4 insertions(+) (limited to 'mm/compaction.c') diff --git a/mm/compaction.c b/mm/compaction.c index 32a033cb5c65..3a91a2ea3d34 100644 --- a/mm/compaction.c +++ b/mm/compaction.c @@ -851,6 +851,10 @@ static int compact_finished(struct zone *zone, /* Compaction run completes if the migrate and free scanner meet */ if (cc->free_pfn <= cc->migrate_pfn) { + /* Let the next compaction start anew. */ + zone->compact_cached_migrate_pfn = zone->zone_start_pfn; + zone->compact_cached_free_pfn = zone_end_pfn(zone); + /* * Mark that the PG_migrate_skip information should be cleared * by kswapd when it goes to sleep. kswapd does not set the -- cgit v1.2.3 From 309381feaee564281c3d9e90fbca8963bb7428ad Mon Sep 17 00:00:00 2001 From: Sasha Levin Date: Thu, 23 Jan 2014 15:52:54 -0800 Subject: mm: dump page when hitting a VM_BUG_ON using VM_BUG_ON_PAGE Most of the VM_BUG_ON assertions are performed on a page. Usually, when one of these assertions fails we'll get a BUG_ON with a call stack and the registers. I've recently noticed based on the requests to add a small piece of code that dumps the page to various VM_BUG_ON sites that the page dump is quite useful to people debugging issues in mm. This patch adds a VM_BUG_ON_PAGE(cond, page) which beyond doing what VM_BUG_ON() does, also dumps the page before executing the actual BUG_ON. [akpm@linux-foundation.org: fix up includes] Signed-off-by: Sasha Levin Cc: "Kirill A. Shutemov" Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/compaction.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) (limited to 'mm/compaction.c') diff --git a/mm/compaction.c b/mm/compaction.c index 3a91a2ea3d34..e0ab02d70f13 100644 --- a/mm/compaction.c +++ b/mm/compaction.c @@ -601,7 +601,7 @@ isolate_migratepages_range(struct zone *zone, struct compact_control *cc, if (__isolate_lru_page(page, mode) != 0) continue; - VM_BUG_ON(PageTransCompound(page)); + VM_BUG_ON_PAGE(PageTransCompound(page), page); /* Successfully isolated */ cc->finished_update_migrate = true; -- cgit v1.2.3 From 6c14466cc00ff13121ae782d33d9df0fde20b124 Mon Sep 17 00:00:00 2001 From: Mel Gorman Date: Thu, 23 Jan 2014 15:53:38 -0800 Subject: mm: improve documentation of page_order Developers occasionally try and optimise PFN scanners by using page_order but miss that in general it requires zone->lock. This has happened twice for compaction.c and rejected both times. This patch clarifies the documentation of page_order and adds a note to compaction.c why page_order is not used. [akpm@linux-foundation.org: tweaks] [lauraa@codeaurora.org: Corrected a page_zone(page)->lock reference] Signed-off-by: Mel Gorman Acked-by: Rafael Aquini Acked-by: Minchan Kim Cc: Laura Abbott Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/compaction.c | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-) (limited to 'mm/compaction.c') diff --git a/mm/compaction.c b/mm/compaction.c index e0ab02d70f13..b48c5259ea33 100644 --- a/mm/compaction.c +++ b/mm/compaction.c @@ -523,7 +523,10 @@ isolate_migratepages_range(struct zone *zone, struct compact_control *cc, if (!isolation_suitable(cc, page)) goto next_pageblock; - /* Skip if free */ + /* + * Skip if free. page_order cannot be used without zone->lock + * as nothing prevents parallel allocations or buddy merging. + */ if (PageBuddy(page)) continue; -- cgit v1.2.3