On Wed, 21 Jan 2015, Vlastimil Babka wrote:

On 01/21/2015 10:34 AM, Vinayak Menon wrote:

quoted

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.

quoted

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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