On Wed, Apr 20, 2016 at 03:47:25PM -0400, Michal Hocko wrote:
> From: Michal Hocko <mhocko@xxxxxxxx>
>
> should_reclaim_retry will give up retries for higher order allocations
> if none of the eligible zones has any requested or higher order pages
> available even if we pass the watermak check for order-0. This is done
> because there is no guarantee that the reclaimable and currently free
> pages will form the required order.
>
> This can, however, lead to situations were the high-order request (e.g.
> order-2 required for the stack allocation during fork) will trigger
> OOM too early - e.g. after the first reclaim/compaction round. Such a
> system would have to be highly fragmented and there is no guarantee
> further reclaim/compaction attempts would help but at least make sure
> that the compaction was active before we go OOM and keep retrying even
> if should_reclaim_retry tells us to oom if
> - the last compaction round backed off or
> - we haven't completed at least MAX_COMPACT_RETRIES active
> compaction rounds.
>
> The first rule ensures that the very last attempt for compaction
> was not ignored while the second guarantees that the compaction has done
> some work. Multiple retries might be needed to prevent occasional
> pigggy backing of other contexts to steal the compacted pages before
> the current context manages to retry to allocate them.
>
> compaction_failed() is taken as a final word from the compaction that
> the retry doesn't make much sense. We have to be careful though because
> the first compaction round is MIGRATE_ASYNC which is rather weak as it
> ignores pages under writeback and gives up too easily in other
> situations. We therefore have to make sure that MIGRATE_SYNC_LIGHT mode
> has been used before we give up. With this logic in place we do not have
> to increase the migration mode unconditionally and rather do it only if
> the compaction failed for the weaker mode. A nice side effect is that
> the stronger migration mode is used only when really needed so this has
> a potential of smaller latencies in some cases.
>
> Please note that the compaction doesn't tell us much about how
> successful it was when returning compaction_made_progress so we just
> have to blindly trust that another retry is worthwhile and cap the
> number to something reasonable to guarantee a convergence.
>
> If the given number of successful retries is not sufficient for a
> reasonable workloads we should focus on the collected compaction
> tracepoints data and try to address the issue in the compaction code.
> If this is not feasible we can increase the retries limit.
>
> Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
> Signed-off-by: Michal Hocko <mhocko@xxxxxxxx>
> ---
> mm/page_alloc.c | 87 ++++++++++++++++++++++++++++++++++++++++++++++++++-------
> 1 file changed, 77 insertions(+), 10 deletions(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 3b78936eca70..bb4df1be0d43 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -2939,6 +2939,13 @@ __alloc_pages_may_oom(gfp_t gfp_mask, unsigned int order,
> return page;
> }
>
> +
> +/*
> + * Maximum number of compaction retries wit a progress before OOM
> + * killer is consider as the only way to move forward.
> + */
> +#define MAX_COMPACT_RETRIES 16
> +
> #ifdef CONFIG_COMPACTION
> /* Try memory compaction for high-order allocations before reclaim */
> static struct page *
> @@ -3006,6 +3013,43 @@ __alloc_pages_direct_compact(gfp_t gfp_mask, unsigned int order,
>
> return NULL;
> }
> +
> +static inline bool
> +should_compact_retry(unsigned int order, enum compact_result compact_result,
> + enum migrate_mode *migrate_mode,
> + int compaction_retries)
> +{
> + if (!order)
> + return false;
> +
> + /*
> + * compaction considers all the zone as desperately out of memory
> + * so it doesn't really make much sense to retry except when the
> + * failure could be caused by weak migration mode.
> + */
> + if (compaction_failed(compact_result)) {
IIUC, this compaction_failed() means that at least one zone is
compacted and failed. This is not same with your assumption in the
comment. If compaction is done and failed on ZONE_DMA, it would be
premature decision.
> + if (*migrate_mode == MIGRATE_ASYNC) {
> + *migrate_mode = MIGRATE_SYNC_LIGHT;
> + return true;
> + }
> + return false;
> + }
> +
> + /*
> + * !costly allocations are really important and we have to make sure
> + * the compaction wasn't deferred or didn't bail out early due to locks
> + * contention before we go OOM. Still cap the reclaim retry loops with
> + * progress to prevent from looping forever and potential trashing.
> + */
> + if (order <= PAGE_ALLOC_COSTLY_ORDER) {
> + if (compaction_withdrawn(compact_result))
> + return true;
> + if (compaction_retries <= MAX_COMPACT_RETRIES)
> + return true;
> + }
> +
> + return false;
> +}
> #else
> static inline struct page *
> __alloc_pages_direct_compact(gfp_t gfp_mask, unsigned int order,
> @@ -3014,6 +3058,14 @@ __alloc_pages_direct_compact(gfp_t gfp_mask, unsigned int order,
> {
> return NULL;
> }
> +
> +static inline bool
> +should_compact_retry(unsigned int order, enum compact_result compact_result,
> + enum migrate_mode *migrate_mode,
> + int compaction_retries)
> +{
> + return false;
> +}
> #endif /* CONFIG_COMPACTION */
>
> /* Perform direct synchronous page reclaim */
> @@ -3260,6 +3312,7 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
> unsigned long did_some_progress;
> enum migrate_mode migration_mode = MIGRATE_ASYNC;
> enum compact_result compact_result;
> + int compaction_retries = 0;
> int no_progress_loops = 0;
>
> /*
> @@ -3371,13 +3424,8 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
> compaction_failed(compact_result)))
> goto nopage;
>
> - /*
> - * It can become very expensive to allocate transparent hugepages at
> - * fault, so use asynchronous memory compaction for THP unless it is
> - * khugepaged trying to collapse.
> - */
> - if (!is_thp_gfp_mask(gfp_mask) || (current->flags & PF_KTHREAD))
> - migration_mode = MIGRATE_SYNC_LIGHT;
> + if (order && compaction_made_progress(compact_result))
> + compaction_retries++;
>
> /* Try direct reclaim and then allocating */
> page = __alloc_pages_direct_reclaim(gfp_mask, order, alloc_flags, ac,
> @@ -3408,6 +3456,17 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
> no_progress_loops))
> goto retry;
>
> + /*
> + * It doesn't make any sense to retry for the compaction if the order-0
> + * reclaim is not able to make any progress because the current
> + * implementation of the compaction depends on the sufficient amount
> + * of free memory (see __compaction_suitable)
> + */
> + if (did_some_progress > 0 &&
> + should_compact_retry(order, compact_result,
> + &migration_mode, compaction_retries))
Checking did_some_progress on each round have subtle corner case. Think
about following situation.
round, compaction, did_some_progress, compaction
0, defer, 1
0, defer, 1
0, defer, 1
0, defer, 1
0, defer, 0
In this case, compaction has enough chance to succeed since freepages
increase, but, compaction will not be triggered.
Thanks.
> + goto retry;
> +
> /* Reclaim has failed us, start killing things */
> page = __alloc_pages_may_oom(gfp_mask, order, ac, &did_some_progress);
> if (page)
> @@ -3421,10 +3480,18 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
>
> noretry:
> /*
> - * High-order allocations do not necessarily loop after
> - * direct reclaim and reclaim/compaction depends on compaction
> - * being called after reclaim so call directly if necessary
> + * High-order allocations do not necessarily loop after direct reclaim
> + * and reclaim/compaction depends on compaction being called after
> + * reclaim so call directly if necessary.
> + * It can become very expensive to allocate transparent hugepages at
> + * fault, so use asynchronous memory compaction for THP unless it is
> + * khugepaged trying to collapse. All other requests should tolerate
> + * at least light sync migration.
> */
> + if (is_thp_gfp_mask(gfp_mask) && !(current->flags & PF_KTHREAD))
> + migration_mode = MIGRATE_ASYNC;
> + else
> + migration_mode = MIGRATE_SYNC_LIGHT;
> page = __alloc_pages_direct_compact(gfp_mask, order, alloc_flags,
> ac, migration_mode,
> &compact_result);
> --
> 2.8.0.rc3
>
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