On Fri, 16 May 2014 11:47:53 +0200 Vlastimil Babka <vbabka@xxxxxxx> wrote: > Compaction uses compact_checklock_irqsave() function to periodically check for > lock contention and need_resched() to either abort async compaction, or to > free the lock, schedule and retake the lock. When aborting, cc->contended is > set to signal the contended state to the caller. Two problems have been > identified in this mechanism. > > First, compaction also calls directly cond_resched() in both scanners when no > lock is yet taken. This call either does not abort async compaction, or set > cc->contended appropriately. This patch introduces a new compact_should_abort() > function to achieve both. In isolate_freepages(), the check frequency is > reduced to once by SWAP_CLUSTER_MAX pageblocks to match what the migration > scanner does in the preliminary page checks. In case a pageblock is found > suitable for calling isolate_freepages_block(), the checks within there are > done on higher frequency. > > Second, isolate_freepages() does not check if isolate_freepages_block() > aborted due to contention, and advances to the next pageblock. This violates > the principle of aborting on contention, and might result in pageblocks not > being scanned completely, since the scanning cursor is advanced. This patch > makes isolate_freepages_block() check the cc->contended flag and abort. > > In case isolate_freepages() has already isolated some pages before aborting > due to contention, page migration will proceed, which is OK since we do not > want to waste the work that has been done, and page migration has own checks > for contention. However, we do not want another isolation attempt by either > of the scanners, so cc->contended flag check is added also to > compaction_alloc() and compact_finished() to make sure compaction is aborted > right after the migration. What are the runtime effect of this change? > Reported-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx> What did Joonsoo report? Perhaps this is the same thing.. > > ... > > @@ -718,9 +739,11 @@ static void isolate_freepages(struct zone *zone, > /* > * This can iterate a massively long zone without finding any > * suitable migration targets, so periodically check if we need > - * to schedule. > + * to schedule, or even abort async compaction. > */ > - cond_resched(); > + if (!(block_start_pfn % (SWAP_CLUSTER_MAX * pageblock_nr_pages)) > + && compact_should_abort(cc)) This seems rather gratuitously inefficient and isn't terribly clear. What's wrong with if ((++foo % SWAP_CLUSTER_MAX) == 0 && compact_should_abort(cc)) ? (Assumes that SWAP_CLUSTER_MAX is power-of-2 and that the compiler will use &) -- 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>