Re: deferred rss update instead of sloppy rss

[Benjamin Herrenschmidt]

On Mon, 2004-11-22 at 14:13 -0800, Christoph Lameter wrote:
> On Mon, 22 Nov 2004, Andrew Morton wrote:
> 
> > hrm.  I cannot see anywhere in this patch where you update task_struct.rss.
> 
> This is just the piece around it dealing with rss. The updating of rss
> happens in the generic code. The change to that is trivial. I can repost
> the whole shebang if you want.
> 
> > > +	/* only holding mmap_sem here maybe get page_table_lock too? */
> > > +	mm->rss += tsk->rss;
> > > +	tsk->rss = 0;
> > >  	up_read(&mm->mmap_sem);
> >
> > mmap_sem needs to be held for writing, surely?
> 
> If there are no page faults occurring anymore then we would not need to
> get the lock. Q: Is it safe to assume that no faults occur
> anymore at this point?

Why wouldn't the mm take faults on other CPUs ? (other threads)

> > just to prevent transient gross inaccuracies.  For some value of "16".
> 
> The page fault code only increments rss. For larger transactions that
> increase / decrease rss significantly the page_table_lock is taken and
> mm->rss is updated directly. So no
> gross inaccuracies can result.
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-- 
Benjamin Herrenschmidt <benh@xxxxxxxxxxxxxxxxxxx>

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