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SubjectRe: [PATCH 4/4] mm: vmscan: If kswapd has been running too long, allow it to sleep
On Wed, May 18, 2011 at 09:26:09AM +0900, KOSAKI Motohiro wrote:
> >Lets see;
> >
> >shrink_page_list() only applies if inactive pages were isolated
> > which in turn may not happen if all_unreclaimable is set in
> > shrink_zones(). If for whatver reason, all_unreclaimable is
> > set on all zones, we can miss calling cond_resched().
> >
> >shrink_slab only applies if we are reclaiming slab pages. If the first
> > shrinker returns -1, we do not call cond_resched(). If that
> > first shrinker is dcache and __GFP_FS is not set, direct
> > reclaimers will not shrink at all. However, if there are
> > enough of them running or if one of the other shrinkers
> > is running for a very long time, kswapd could be starved
> > acquiring the shrinker_rwsem and never reaching the
> > cond_resched().
>
> OK.
>
>
> >
> >balance_pgdat() only calls cond_resched if the zones are not
> > balanced. For a high-order allocation that is balanced, it
> > checks order-0 again. During that window, order-0 might have
> > become unbalanced so it loops again for order-0 and returns
> > that was reclaiming for order-0 to kswapd(). It can then find
> > that a caller has rewoken kswapd for a high-order and re-enters
> > balance_pgdat() without ever have called cond_resched().
>
> Then, Shouldn't balance_pgdat() call cond_resched() unconditionally?
> The problem is NOT 100% cpu consumption. if kswapd will sleep, other
> processes need to reclaim old pages. The problem is, kswapd doesn't
> invoke context switch and other tasks hang-up.
>

Which the shrink_slab patch does (either version). What's the gain from
sprinkling more cond_resched() around? If you think there is, submit
another pair of patches (include patch 1 from this series) but I'm not
seeing the advantage myself.

>
> >While it appears unlikely, there are bad conditions which can result
> >in cond_resched() being avoided.
> >
>
>

--
Mel Gorman
SUSE Labs


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