Re: [PATCH 4/8] Use prepare_to_wait_exclusive() instead prepare_to_wait()

[Rik van Riel]

On 12/15/2009 12:32 AM, Mike Galbraith wrote:
> On Tue, 2009-12-15 at 09:45 +0900, KOSAKI Motohiro wrote:
> > > On 12/14/2009 07:30 AM, KOSAKI Motohiro wrote:
> > > > if we don't use exclusive queue, wake_up() function wake _all_ waited
> > > > task. This is simply cpu wasting.
> > > >
> > > > Signed-off-by: KOSAKI Motohiro<kosaki.motohiro@xxxxxxxxxxxxxx>
> > >
> > > >    		if (zone_watermark_ok(zone, sc->order, low_wmark_pages(zone),
> > > >    					0, 0)) {
> > > > -			wake_up(wq);
> > > > +			wake_up_all(wq);
> > > >    			finish_wait(wq,&wait);
> > > >    			sc->nr_reclaimed += sc->nr_to_reclaim;
> > > >    			return -ERESTARTSYS;
> > >
> > > I believe we want to wake the processes up one at a time
> > > here.

> > Actually, wake_up() and wake_up_all() aren't different so much.
> > Although we use wake_up(), the task wake up next task before
> > try to alloate memory. then, it's similar to wake_up_all().

That is a good point.  Maybe processes need to wait a little
in this if() condition, before the wake_up().  That would give
the previous process a chance to allocate memory and we can
avoid waking up too many processes.

> What happens to waiters should running tasks not allocate for a while?

When a waiter is woken up, it will either:
1) see that there is enough free memory and wake up the next guy, or
2) run shrink_zone and wake up the next guy

Either way, the processes that just got woken up will ensure that
the sleepers behind them in the queue will get woken up.

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