On Tue, 5 Apr 2016 13:25:32 +0200 Michal Hocko <mhocko@xxxxxxxxxx> 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. > > @@ -3369,14 +3425,6 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order, > if (is_thp_gfp_mask(gfp_mask) && compaction_withdrawn(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; > - > /* Try direct reclaim and then allocating */ > page = __alloc_pages_direct_reclaim(gfp_mask, order, alloc_flags, ac, > &did_some_progress); Hugh's patches moved this elsewhere. I'll drop this hunk altogether - please carefully review the result. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>