On Wed, 2011-12-14 at 12:45 +0800, Mike Waychison wrote: > On Tue, Dec 13, 2011 at 8:36 PM, Mike Waychison <mikew@xxxxxxxxxx> wrote: > > On Tue, Dec 13, 2011 at 6:24 PM, Shaohua Li <shaohua.li@xxxxxxxxx> wrote: > >> On Wed, 2011-12-14 at 01:44 +0800, Mike Waychison wrote: > >>> On a single core system with kernel preemption disabled, it is possible > >>> for the memory system to be so taxed that kswapd cannot make any forward > >>> progress. This can happen when most of system memory is tied up as > >>> anonymous memory without swap enabled, causing kswapd to consistently > >>> fail to achieve its watermark goals. In turn, sleeping_prematurely() > >>> will consistently return true and kswapd_try_to_sleep() to never invoke > >>> schedule(). This causes the kswapd thread to stay on the CPU in > >>> perpetuity and keeps other threads from processing oom-kills to reclaim > >>> memory. > >>> > >>> The cond_resched() instance in balance_pgdat() is never called as the > >>> loop that iterates from DEF_PRIORITY down to 0 will always set > >>> all_zones_ok to true, and not set it to false once we've passed > >>> DEF_PRIORITY as zones that are marked ->all_unreclaimable are not > >>> considered in the "all_zones_ok" evaluation. > >>> > >>> This change modifies kswapd_try_to_sleep to ensure that we enter > >>> scheduler at least once per invocation if needed. This allows kswapd to > >>> get off the CPU and allows other threads to die off from the OOM killer > >>> (freeing memory that is otherwise unavailable in the process). > >> your description suggests zones with all_unreclaimable set. but in this > >> case sleeping_prematurely() will return false instead of true, kswapd > >> will do sleep then. is there anything I missed? > > Actually, I don't see where sleeping_prematurely() would return false > if any zone has ->all_unreclaimable set. In this case, the order was > 0, so we return !all_zones_ok, which is false because > !zone_watermark_ok_safe(ZONE_DMA32). so the ZONE_DMA32 hasn't all_unreclaimable set, right? if all zones have all_unreclaimable set, all_zones_ok clearly is true. this means kswapd can reclaim some pages in the zone, which looks sane. Thanks, Shaohua -- 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/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>