linux-kernel - Re: [Question] Should direct reclaim time be bounded?

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Date:   Fri, 12 Jul 2019 10:49:19 +0100
From:   Mel Gorman <mgorman@...e.de>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Mike Kravetz <mike.kravetz@...cle.com>,
        Hillf Danton <hdanton@...a.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Johannes Weiner <hannes@...xchg.org>
Subject: Re: [Question] Should direct reclaim time be bounded?

On Thu, Jul 11, 2019 at 09:12:45AM +0200, Michal Hocko wrote:
> On Wed 10-07-19 16:36:58, Mike Kravetz wrote:
> > On 7/10/19 12:44 PM, Michal Hocko wrote:
> > > On Wed 10-07-19 11:42:40, Mike Kravetz wrote:
> > > [...]
> > >> As Michal suggested, I'm going to do some testing to see what impact
> > >> dropping the __GFP_RETRY_MAYFAIL flag for these huge page allocations
> > >> will have on the number of pages allocated.
> > > 
> > > Just to clarify. I didn't mean to drop __GFP_RETRY_MAYFAIL from the
> > > allocation request. I meant to drop the special casing of the flag in
> > > should_continue_reclaim. I really have hard time to argue for this
> > > special casing TBH. The flag is meant to retry harder but that shouldn't
> > > be reduced to a single reclaim attempt because that alone doesn't really
> > > help much with the high order allocation. It is more about compaction to
> > > be retried harder.
> > 
> > Thanks Michal.  That is indeed what you suggested earlier.  I remembered
> > incorrectly.  Sorry.
> > 
> > Removing the special casing for __GFP_RETRY_MAYFAIL in should_continue_reclaim
> > implies that it will return false if nothing was reclaimed (nr_reclaimed == 0)
> > in the previous pass.
> > 
> > When I make such a modification and test, I see long stalls as a result
> > of should_compact_retry returning true too often.  On a system I am currently
> > testing, should_compact_retry has returned true 36000000 times.  My guess
> > is that this may stall forever.  Vlastmil previously asked about this behavior,
> > so I am capturing the reason.  Like before [1], should_compact_retry is
> > returning true mostly because compaction_withdrawn() returns COMPACT_DEFERRED.
> 
> This smells like a problem to me. But somebody more familiar with
> compaction should comment.
> 

Examine in should_compact_retry if it's retrying because
compaction_zonelist_suitable is true. Looking at it now, it would not
necessarily do the right thing because any non-skipped zone would make
it eligible which is too strong a condition as COMPACT_SKIPPED is not
reliably set. If that function is the case, it would be reasonable
remove "ret = compaction_zonelist_suitable(ac, order, alloc_flags);" and
the implementation of compaction_zonelist_suitable entirely as part of
your fix.

-- 
Mel Gorman
SUSE Labs