lkml.org 
[lkml]   [2019]   [Jul]   [12]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [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

\
 
 \ /
  Last update: 2019-07-12 11:49    [W:0.036 / U:3.960 seconds]
©2003-2018 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site