Re: [Bugme-new] [Bug 31142] New: Large write to USB stick freezes unrelated tasks for a long time

[Mel Gorman <mel@csn.ul.ie>]

On Fri, Mar 18, 2011 at 01:05:15PM -0500, Alex Villac??s Lasso wrote:
> El 18/03/11 06:13, Mel Gorman escribio:
> >
> >\o/ ... no wait, it's the other one - :(
> >
> >If you look at the stack traces though, all of them had called
> >do_huge_pmd_anonymous_page() so while it looks similar to 12309, the trigger
> >is new because it's THP triggering compaction that is causing the stalls
> >rather than page reclaim doing direct writeback which was the culprit in
> >the past.
> >
> >To confirm if this is the case, I'd be very interested in hearing if this
> >problem persists in the following cases
> >
> >1. 2.6.38-rc8 with defrag disabled by
> >    echo never>/sys/kernel/mm/transparent_hugepage/defrag
> >    (this will stop THP allocations calling into compaction)
> >2. 2.6.38-rc8 with THP disabled by
> >    echo never>
> >/sys/kernel/mm/transparent_hugepage/enabled
> >    (if the problem still persists, then page reclaim is still a problem
> >     but we should still stop THP doing sync writes)
> >3. 2.6.37 vanilla
> >    (in case this is a new regression introduced since then)
> >
> >Migration can do sync writes on dirty pages which is why it looks so similar
> >to page reclaim but this can be controlled by the value of sync_migration
> >passed into try_to_compact_pages(). If we find that option 1 above makes
> >the regression go away or at least helps a lot, then a reasonable fix may
> >be to never set sync_migration if __GFP_NO_KSWAPD which is always set for
> >THP allocations. I've added Andrea to the cc to see what he thinks.
> >
> >Thanks for the report.
> >
>
> I have just done tests 1 and 2 on 2.6.38 (final, not -rc8), and I
> have verified that echoing "never" on either
> /sys/kernel/mm/transparent_hugepage/defrag or
> /sys/kernel/mm/transparent_hugepage/enabled does allow the file copy
> to USB to proceed smoothly (copying 4GB of data). Just to verify, I
> later wrote "always" to both files, and sure enough, some
> applications stalled when I repeated the same file copy. So I have
> at least a workaround for the issue. Given this evidence, will the
> patch at comment #14 fix the issue for good?
> 

Thanks for testing and reporting, it's very helpful. Based on that that
report the patch should help. Can you test it to be absolutly sure please?

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