Re: [PATCH] mm: Fix kswapd livelock on single core, no preemptkernel

[Shaohua Li]

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

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