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SubjectRe: [PATCH 0/14] Avoid overflowing of stack during page reclaim V3
On Fri, 2 Jul 2010 12:33:15 -0700
Andrew Morton <akpm@linux-foundation.org> wrote:

> On Tue, 29 Jun 2010 12:34:34 +0100
> Mel Gorman <mel@csn.ul.ie> wrote:
>
> > Here is V3 that depends again on flusher threads to do writeback in
> > direct reclaim rather than stack switching which is not something I'm
> > likely to get done before xfs/btrfs are ignoring writeback in mainline
> > (phd sucking up time).
>
> IMO, implemetning stack switching for this is not a good idea. We
> _already_ have a way of doing stack-switching. It's called
> "schedule()".
>
Sure.

> The only reason I can see for implementing an in-place stack switch
> would be if schedule() is too expensive. And if we were to see
> excessive context-switch overheads in this code path (and we won't)
> then we should get in there and try to reduce the contect switch rate
> first.
>

Maybe a concern of in-place stack exchange lovers is that it's difficult
to guarantee when the pageout() will be issued.

I'd like to try to add a call as

- pageout_request(page) .... request to pageout a page to a daemon(kswapd).
- pageout_barrier(zone? node?) .... wait until all writebacks ends.
Implementation dilemmna:

Because page->lru is very useful link to implement calls like above, but
there is a concern that using it will hide pages from vmscan unnecessarily.
Avoding to use of page->lru means to use another structure like pagevec,
but it means page_count()+1 and pins pages unnecessarily. I'm now considering
how to implement safe&scalable way to pageout-in-another-stack(thread)...
I wonder it will require some throttling method for pageout, anyway.

And my own problem is that I should add per-memcg threads or using some
thread-pool ;( But it will be an another topic.

Thanks,
-Kame





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