From fd901c95388b3bd5a6f749ed1d677a672b992298 Mon Sep 17 00:00:00 2001 From: Vlastimil Babka Date: Thu, 28 Apr 2016 16:18:49 -0700 Subject: [PATCH] mm: wake kcompactd before kswapd's short sleep When kswapd goes to sleep it checks if the node is balanced and at first it sleeps only for HZ/10 time, then rechecks if the node is still balanced and nobody has woken it during the initial sleep. Only then it goes fully sleep until an allocation slowpath wakes it up again. For higher-order allocations, waking up kcompactd is done only before the full sleep. This turns out to be an issue in case another high-order allocation fails during the initial sleep. It will wake kswapd up, however kswapd considers the zone balanced from the order-0 perspective, and will just quickly try to sleep again. So if there's a longer stream of high-order allocations hitting the slowpath and waking up kswapd, it might never actually wake up kcompactd, which may be considered a regression from kswapd-based compaction. In the worst case, it might be that a single allocation that cannot direct reclaim/compact itself is waking kswapd in the retry loop and preventing kcompactd from being woken up and unblocking it. This patch makes sure kcompactd is woken up in such situations by simply moving the wakeup before the short initial sleep. More efficient solution would be to wake kcompactd immediately instead of kswapd if the node is already order-0 balanced, but in that case we should also move reset_isolation_suitable() call to kcompactd so it's not adding to the allocator's latency. Since it's late in the 4.6 cycle, let's go with the simpler change for now. Fixes: accf62422b3a ("mm, kswapd: replace kswapd compaction with waking up kcompactd") Signed-off-by: Vlastimil Babka Cc: Andrea Arcangeli Cc: "Kirill A. Shutemov" Cc: Rik van Riel Cc: Joonsoo Kim Cc: Mel Gorman Cc: David Rientjes Cc: Michal Hocko Cc: Johannes Weiner Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/vmscan.c | 28 ++++++++++++++-------------- 1 file changed, 14 insertions(+), 14 deletions(-) diff --git a/mm/vmscan.c b/mm/vmscan.c index c638b28310fc..142cb61f4822 100644 --- a/mm/vmscan.c +++ b/mm/vmscan.c @@ -3318,6 +3318,20 @@ static void kswapd_try_to_sleep(pg_data_t *pgdat, int order, /* Try to sleep for a short interval */ if (prepare_kswapd_sleep(pgdat, order, remaining, balanced_classzone_idx)) { + /* + * Compaction records what page blocks it recently failed to + * isolate pages from and skips them in the future scanning. + * When kswapd is going to sleep, it is reasonable to assume + * that pages and compaction may succeed so reset the cache. + */ + reset_isolation_suitable(pgdat); + + /* + * We have freed the memory, now we should compact it to make + * allocation of the requested order possible. + */ + wakeup_kcompactd(pgdat, order, classzone_idx); + remaining = schedule_timeout(HZ/10); finish_wait(&pgdat->kswapd_wait, &wait); prepare_to_wait(&pgdat->kswapd_wait, &wait, TASK_INTERRUPTIBLE); @@ -3341,20 +3355,6 @@ static void kswapd_try_to_sleep(pg_data_t *pgdat, int order, */ set_pgdat_percpu_threshold(pgdat, calculate_normal_threshold); - /* - * Compaction records what page blocks it recently failed to - * isolate pages from and skips them in the future scanning. - * When kswapd is going to sleep, it is reasonable to assume - * that pages and compaction may succeed so reset the cache. - */ - reset_isolation_suitable(pgdat); - - /* - * We have freed the memory, now we should compact it to make - * allocation of the requested order possible. - */ - wakeup_kcompactd(pgdat, order, classzone_idx); - if (!kthread_should_stop()) schedule(); -- 2.20.1