Re: [PATCH 4/4] Memory controller soft limit reclaim on contention (v5)

[KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>]

> > > -	did_some_progress = try_to_free_pages(zonelist, order, gfp_mask);
> > > +	/*
> > > +	 * Try to free up some pages from the memory controllers soft
> > > +	 * limit queue.
> > > +	 */
> > > +	did_some_progress = mem_cgroup_soft_limit_reclaim(zonelist, gfp_mask);
> > > +	if (!order || !did_some_progress)
> > > +		did_some_progress += try_to_free_pages(zonelist, order,
> > > +							gfp_mask);
> > 
> > I have two objection to this.
> > 
> > - "if (!order || !did_some_progress)" mean no call try_to_free_pages()
> >   in order>0 and did_some_progress>0 case.
> >   but mem_cgroup_soft_limit_reclaim() don't have lumpy reclaim.
> >   then, it break high order reclaim.
> 
> I am sending a fix for this right away. Thanks, the check should be
> if (order || !did_some_progress)

No.

it isn't enough.
after is does, order-1 allocation case twrice reclaim (soft limit shrinking
and normal try_to_free_pages()).
then, order-1 reclaim makes slower about 2 times.

unfortunately, order-1 allocation is very frequent. it is used for
kernel stack.


> > - in global reclaim view, foreground reclaim and background reclaim's
> >   reclaim rate is about 1:9 typically.
> >   then, kswapd reclaim the pages by global lru order before proceccing
> >   this logic.
> >   IOW, this soft limit is not SOFT.
> 
> It depends on what you mean by soft. I call them soft since they are
> imposed only when there is contention. If you mean kswapd runs more
> often than direct reclaim, that is true, but it does not impact this
> code extensively since the high water mark is a very small compared to
> the pages present on the system.

No.

My point is, contention case kswapd wakeup. and kswapd reclaim by
global lru order before soft limit shrinking.
Therefore, In typical usage, mem_cgroup_soft_limit_reclaim() almost
don't call properly.

soft limit shrinking should run before processing global reclaim.


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