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Date:   Tue, 22 Oct 2019 17:42:49 -0400
From:   Johannes Weiner <hannes@cmpxchg.org>
To:     Roman Gushchin <guro@fb.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Michal Hocko <mhocko@suse.com>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        "cgroups@vger.kernel.org" <cgroups@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Kernel Team <Kernel-team@fb.com>
Subject: Re: [PATCH 5/8] mm: vmscan: replace shrink_node() loop with a retry
 jump
Message-ID: <20191022214249.GB361040@cmpxchg.org>
References: <20191022144803.302233-1-hannes@cmpxchg.org>
 <20191022144803.302233-6-hannes@cmpxchg.org>
 <20191022195629.GA24142@tower.DHCP.thefacebook.com>
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On Tue, Oct 22, 2019 at 07:56:33PM +0000, Roman Gushchin wrote:
> On Tue, Oct 22, 2019 at 10:48:00AM -0400, Johannes Weiner wrote:
> > -			/* Record the group's reclaim efficiency */
> > -			vmpressure(sc->gfp_mask, memcg, false,
> > -				   sc->nr_scanned - scanned,
> > -				   sc->nr_reclaimed - reclaimed);
> > -
> > -		} while ((memcg = mem_cgroup_iter(root, memcg, NULL)));
> > +		reclaimed = sc->nr_reclaimed;
> > +		scanned = sc->nr_scanned;
> > +		shrink_node_memcg(pgdat, memcg, sc);
> >  
> > -		if (reclaim_state) {
> > -			sc->nr_reclaimed += reclaim_state->reclaimed_slab;
> > -			reclaim_state->reclaimed_slab = 0;
> > -		}
> > +		shrink_slab(sc->gfp_mask, pgdat->node_id, memcg,
> > +			    sc->priority);
> >  
> > -		/* Record the subtree's reclaim efficiency */
> > -		vmpressure(sc->gfp_mask, sc->target_mem_cgroup, true,
> > -			   sc->nr_scanned - nr_scanned,
> > -			   sc->nr_reclaimed - nr_reclaimed);
> > +		/* Record the group's reclaim efficiency */
> > +		vmpressure(sc->gfp_mask, memcg, false,
> > +			   sc->nr_scanned - scanned,
> > +			   sc->nr_reclaimed - reclaimed);
> 
> It doesn't look as a trivial change. I'd add some comments to the commit message
> why it's safe to do.

It's an equivalent change - it's just really misleading because the
+++ lines are not the counter-part of the --- lines here!

There are two vmpressure calls in this function: one against the
individual cgroups, and one against the tree. The diff puts them
adjacent here, but the counter-part for the --- lines is here:

> > +	/* Record the subtree's reclaim efficiency */
> > +	vmpressure(sc->gfp_mask, sc->target_mem_cgroup, true,
> > +		   sc->nr_scanned - nr_scanned,
> > +		   sc->nr_reclaimed - nr_reclaimed);

And the counter-part to the +++ lines is further up (beginning of the
quoted diff).