Messages in this thread | | | Subject | Re: [PATCH 10/11] mm, oom: protect !costly allocations some more | From | Vlastimil Babka <> | Date | Mon, 11 Apr 2016 16:48:40 +0200 |
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On 04/05/2016 01:25 PM, Michal Hocko wrote: > From: Michal Hocko <mhocko@suse.com> > > 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 packing 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. > > Signed-off-by: Michal Hocko <mhocko@suse.com>
Looks good.
Acked-by: Vlastimil Babka <vbabka@suse.cz>
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