Re: [RFC][PATCH][3/4] Add reclaim support

[Balbir Singh <balbir@in.ibm.com>]

Andrew Morton wrote:
> On Mon, 19 Feb 2007 16:20:53 +0530 Balbir Singh <balbir@in.ibm.com> wrote:
>
> > > > + * so, is the container over it's limit. Returns 1 if the container is above
> > > > + * its limit.
> > > > + */
> > > > +int memctlr_mm_overlimit(struct mm_struct *mm, void *sc_cont)
> > > > +{
> > > > +	struct container *cont;
> > > > +	struct memctlr *mem;
> > > > +	long usage, limit;
> > > > +	int ret = 1;
> > > > +
> > > > +	if (!sc_cont)
> > > > +		goto out;
> > > > +
> > > > +	read_lock(&mm->container_lock);
> > > > +	cont = mm->container;
> > > > +
> > > > +	/*
> > > > + 	 * Regular reclaim, let it proceed as usual
> > > > + 	 */
> > > > +	if (!sc_cont)
> > > > +		goto out;
> > > > +
> > > > +	ret = 0;
> > > > +	if (cont != sc_cont)
> > > > +		goto out;
> > > > +
> > > > +	mem = memctlr_from_cont(cont);
> > > > +	usage = atomic_long_read(&mem->counter.usage);
> > > > +	limit = atomic_long_read(&mem->counter.limit);
> > > > +	if (limit && (usage > limit))
> > > > +		ret = 1;
> > > > +out:
> > > > +	read_unlock(&mm->container_lock);
> > > > +	return ret;
> > > > +}
> > > hm, I wonder how much additional lock traffic all this adds.
> > It's a read_lock() and most of the locks are read_locks
> > which allow for concurrent access, until the container
> > changes or goes away
>
> read_lock isn't free, and I suspect we're calling this function pretty
> often (every pagefault?) It'll be measurable on some workloads, on some
> hardware.
>
> It probably won't be terribly bad because each lock-taking is associated
> with a clear_page().  But still, if there's any possibility of lightening
> the locking up, now is the time to think about it.

Yes, good point. I'll revisit to see if barriers can replace the locking
or if the locking is required at all?

> > > > @@ -66,6 +67,9 @@ struct scan_control {
> > > >  	int swappiness;
> > > >  
> > > >  	int all_unreclaimable;
> > > > +
> > > > +	void *container;		/* Used by containers for reclaiming */
> > > > +					/* pages when the limit is exceeded  */
> > > >  };
> > > eww.  Why void*?
> > I did not want to expose struct container in mm/vmscan.c.
>
> It's already there, via rmap.h

Yes, true

> > An additional
> > thought was that no matter what container goes in the field would be
> > useful for reclaim.
>
> Am having trouble parsing that sentence ;)

The thought was that irrespective of the infrastructure that goes in
having an entry for reclaim in scan_control would be useful. I guess
the name exposes what the type tries to hide :-)

--
	Warm Regards,
	Balbir Singh
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