Re: [PATCH] mm,vmscan: Use accurate values for zone_reclaimable() checks

[Tejun Heo <htejun@gmail.com>]

Hello, Michal.

On Tue, Oct 27, 2015 at 10:16:03AM +0100, Michal Hocko wrote:
> > Seriously, nobody goes full-on RUNNING.
> 
> Looping with cond_resched seems like general pattern in the kernel when
> there is no clear source to wait for. We have io_schedule when we know
> we should wait for IO (in case of congestion) but this is not necessarily
> the case - as you can see here. What should we wait for? A short nap
> without actually waiting on anything sounds like a dirty workaround to
> me.

It's one thing to do cond_resched() in long loops to avoid long
priority inversions and another to indefinitely loop without making
any difference.

> > > guarantee that then I would argue that it should be implicit for
> > > WQ_MEM_RECLAIM otherwise we always risk a similar situation. What would
> > > be a counter argument for doing that?
> > 
> > Not serving any actual purpose and degrading execution behavior.
> 
> I dunno, I am not familiar with WQ internals to see the risks but to me
> it sounds like WQ_MEM_RECLAIM gives an incorrect impression of safety
> wrt. memory pressure and as demonstrated it doesn't do that. Even if you

It generally does.  This is an extremely rare corner case where
infinite loop w/o forward progress is introduce w/o the user being
outright buggy.

> consider cond_resched behavior of the page allocator as bug we should be
> able to handle this gracefully.

We can argue this back and forth forever but we'll either need to
special case it (be it short sleep or a special flag) or implement a
rather complex detection logic which will likely involve some level of
complexity and is dubious in its practical usefulness.  It's a
trade-off and given the circumstances adding short sleep looks like a
reasonable one to me.  If this is more common, we definitely wanna go
for automatic detection.

Thanks.

-- 
tejun

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