Re: [PATCH] mm: compaction: fix the page state calculation in too_many_isolated

[David Rientjes]

On Wed, 21 Jan 2015, Vlastimil Babka wrote:

> On 01/21/2015 10:34 AM, Vinayak Menon wrote:
> > Commit "3611badc1baa" (mm: vmscan: fix the page state calculation in
> 
> That appears to be a -next commit ID, which won't be the same in Linus' tree, so
> it shouldn't be in commit message, AFAIK.
> 
> > too_many_isolated) fixed an issue where a number of tasks were
> > blocked in reclaim path for seconds, because of vmstat_diff not being
> > synced in time. A similar problem can happen in isolate_migratepages_block,
> > similar calculation is performed. This patch fixes that.
> 
> I guess it's not possible to fix the stats instantly and once in the safe
> versions, so that future readings will be correct without safe, right?
> So until it gets fixed, each reading will have to be safe and thus expensive?
> 

Yeah, this patch will actually hurt performance for the migration scanner 
but not as much as stalling unnecessarily when the snapshot is the same.

> I think in case of async compaction, we could skip the safe stuff and just
> terminate it - it's already done when too_many_isolated returns true, and
> there's no congestion waiting in that case.
> 
> So you could extend the too_many_isolated() with "safe" parameter (as you did
> for vmscan) and pass it "cc->mode != MIGRATE_ASYNC" value from
> isolate_migrate_block().
> 

Or just pass it struct compact_control *cc and use both cc->zone and 
cc->mode inside this compaction-only function.
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