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

[Andrew Morton <akpm@linux-foundation.org>]

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.

> >> @@ -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

> An additional
> thought was that no matter what container goes in the field would be
> useful for reclaim.

Am having trouble parsing that sentence ;)