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

[KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>]

> 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.
>