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SubjectRe: [PATCH 4/4] mm: vmscan: If kswapd has been running too long, allow it to sleep
> 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.




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




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