Re: [RFC 2/3] mm: throttle on IO only when there are too many dirty and writeback pages

[Johannes Weiner <hannes@cmpxchg.org>]

On Tue, Dec 01, 2015 at 01:56:46PM +0100, Michal Hocko wrote:
> From: Michal Hocko <mhocko@suse.com>
> 
> wait_iff_congested has been used to throttle allocator before it retried
> another round of direct reclaim to allow the writeback to make some
> progress and prevent reclaim from looping over dirty/writeback pages
> without making any progress. We used to do congestion_wait before
> 0e093d99763e ("writeback: do not sleep on the congestion queue if
> there are no congested BDIs or if significant congestion is not being
> encountered in the current zone") but that led to undesirable stalls
> and sleeping for the full timeout even when the BDI wasn't congested.
> Hence wait_iff_congested was used instead. But it seems that even
> wait_iff_congested doesn't work as expected. We might have a small file
> LRU list with all pages dirty/writeback and yet the bdi is not congested
> so this is just a cond_resched in the end and can end up triggering pre
> mature OOM.
> 
> This patch replaces the unconditional wait_iff_congested by
> congestion_wait which is executed only if we _know_ that the last round
> of direct reclaim didn't make any progress and dirty+writeback pages are
> more than a half of the reclaimable pages on the zone which might be
> usable for our target allocation. This shouldn't reintroduce stalls
> fixed by 0e093d99763e because congestion_wait is called only when we
> are getting hopeless when sleeping is a better choice than OOM with many
> pages under IO.
> 
> We have to preserve logic introduced by "mm, vmstat: allow WQ concurrency
> to discover memory reclaim doesn't make any progress" into the
> __alloc_pages_slowpath now that wait_iff_congested is not used anymore.
> As the only remaining user of wait_iff_congested is shrink_inactive_list
> we can remove the WQ specific short sleep from wait_iff_congested
> because the sleep is needed to be done only once in the allocation retry
> cycle.
> 
> Signed-off-by: Michal Hocko <mhocko@suse.com>

Yep, this looks like the right thing to do. However, the code it adds
to __alloc_pages_slowpath() is putting even more weight behind the
argument that the reclaim retry logic should be in its own function.

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