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    SubjectRe: [PATCH] mm: Fix kswapd livelock on single core, no preempt kernel
    On Wed, 2011-12-14 at 12:45 +0800, Mike Waychison wrote:
    > On Tue, Dec 13, 2011 at 8:36 PM, Mike Waychison <> wrote:
    > > On Tue, Dec 13, 2011 at 6:24 PM, Shaohua Li <> 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.


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