From: Con Kolivas <kernel@kolivas.org>
To: Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH] mm: swap prefetch more improvements
Date: Tue, 15 May 2007 09:24:19 +1000
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References: <200705141050.55038.kernel@kolivas.org> <200705150843.36721.kernel@kolivas.org> <20070514160123.4b1ab108.akpm@linux-foundation.org>
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On Tuesday 15 May 2007 09:01, Andrew Morton wrote:
> On Tue, 15 May 2007 08:43:35 +1000
>
> Con Kolivas <kernel@kolivas.org> wrote:
> > On Tuesday 15 May 2007 08:00, Andrew Morton wrote:
> > > On Mon, 14 May 2007 10:50:54 +1000
> > >
> > > Con Kolivas <kernel@kolivas.org> wrote:
> > > > akpm, please queue on top of "mm: swap prefetch improvements"
> > > >
> > > > ---
> > > > Failed radix_tree_insert wasn't being handled leaving stale kmem.
> > > >
> > > > The list should be iterated over in the reverse order when
> > > > prefetching.
> > > >
> > > > Make the yield within kprefetchd stronger through the use of
> > > > cond_resched.
> > >
> > > hm.
> > >
> > > > -		might_sleep();
> > > > -		if (!prefetch_suitable())
> > > > +		/* Yield to anything else running */
> > > > +		if (cond_resched() || !prefetch_suitable())
> > > >  			goto out_unlocked;
> > >
> > > So if cond_resched() happened to schedule away, we terminate this
> > > swap-tricking attempt.  It's not possible to determine the reasons for
> > > this from the code or from the changelog (==bad).
> > >
> > > How come?
> >
> > Hmm I thought the line above that says "yield to anything else running"
> > was explicit enough. The idea is kprefetchd shouldn't run if any other
> > real activity is happening just about anywhere, and a positive
> > cond_resched would indicate likely activity so we just put kprefetchd
> > back to sleep.
>
> But kprefetchd runs as SCHED_BATCH.  Doesn't that mean that some low-prio
> background thing (seti?) will disable swap-prefetch?
>
> I mean, if swap-prefetch is actually useful, then it'll still be useful if
> the machine happens to be doing some computational work.  It's not obvious
> to me that there is linkage between "doing CPU work" and "prefetching is
> presently undesirable".

set_tsk_need_resched which is the trigger for a cond_resched occurring won't 
be set just by a cpu bound task constantly running in the background as far 
as I can see. It's only if some wakeup has triggered a set_tsk_need_resched. 
ie prefetching still happens here with setiathome or equivalent running in my 
testing. It might be overkill but from what I can see here it is no 
impediment to prefetching occurring. I'll think about it some more and do 
more testing but it seems ok to me.

-- 
-ck
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