Re: [PATCH 10/11] mm, oom: protect !costly allocations some more

[Vlastimil Babka]

On 04/05/2016 01:25 PM, 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 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@xxxxxxxx>

Looks good.

Acked-by: Vlastimil Babka <vbabka@xxxxxxx>