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

From: KOSAKI Motohiro
Date: Tue May 17 2011 - 20:26:48 EST

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


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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