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SubjectRe: [PATCH v5 07/14] mm, compaction: khugepaged should not give up due to need_resched()
On Tue, Jul 29, 2014 at 12:31:13AM -0700, David Rientjes wrote:
> On Tue, 29 Jul 2014, Joonsoo Kim wrote:
>
> > I have a silly question here.
> > Why need_resched() is criteria to stop async compaction?
> > need_resched() is flagged up when time slice runs out or other reasons.
> > It means that we should stop async compaction at arbitrary timing
> > because process can be on compaction code at arbitrary moment. I think
> > that it isn't reasonable and it doesn't ensure anything. Instead of
> > this approach, how about doing compaction on certain amounts of pageblock
> > for async compaction?
> >
>
> Not a silly question at all, I had the same feeling in
> https://lkml.org/lkml/2014/5/21/730 and proposed it to be a tunable that
> indicates how much work we are willing to do for thp in the pagefault
> path. It suffers from the fact that past failure to isolate and/or

Oh... you already suggested the same idea.

> migrate memory to free an entire pageblock doesn't indicate that the next
> pageblock will fail as well, but there has to be cutoff at some point or
> async compaction becomes unnecessarily expensive. We can always rely on
> khugepaged later to do the collapse, assuming we're not faulting memory
> and then immediately pinning it.
>
> I think there's two ways to go about it:
>
> - allow a single thp fault to be expensive and then rely on deferred
> compaction to avoid subsequent calls in the near future, or
>
> - try to make all thp faults be as least expensive as possible so that
> the cumulative effect of faulting large amounts of memory doesn't end
> up with lengthy stalls.

Hmm, if thp faults want to pay cost as least as possible, how about
making thp faults skip async/sync compaction at all?

Thanks.


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