linux-kernel - Re: [PATCH -mm v2] vmscan: move reclaim_state handling to shrink

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Date:	Thu, 15 Jan 2015 16:25:16 +0300
From:	Vladimir Davydov <vdavydov@...allels.com>
To:	Michal Hocko <mhocko@...e.cz>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Vlastimil Babka <vbabka@...e.cz>, Mel Gorman <mgorman@...e.de>,
	Rik van Riel <riel@...hat.com>, <linux-mm@...ck.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH -mm v2] vmscan: move reclaim_state handling to shrink_slab

On Thu, Jan 15, 2015 at 01:58:20PM +0100, Michal Hocko wrote:
> On Thu 15-01-15 11:37:53, Vladimir Davydov wrote:
> > current->reclaim_state is only used to count the number of slab pages
> > reclaimed by shrink_slab(). So instead of initializing it before we are
> > 
> > Note that after this patch try_to_free_mem_cgroup_pages() will count not
> > only reclaimed user pages, but also slab pages, which is expected,
> > because it can reclaim kmem from kmem-active sub cgroups.
> 
> Except that reclaim_state counts all freed slab objects that have
> current->reclaim_state != NULL AFAIR. This includes also kfreed pages
> from interrupt context and who knows what else and those pages might be
> from a different memcgs, no?

Hmm, true, good point. Can an interrupt handler free a lot of memory
though? Does RCU free objects from irq or soft irq context?

> Besides that I am not sure this makes any difference in the end. No
> try_to_free_mem_cgroup_pages caller really cares about the exact
> number of reclaimed pages. We care only about whether there was any
> progress done - and even that not exactly (e.g. try_charge checks
> mem_cgroup_margin before retry/oom so if sufficient kmem pages were
> uncharged then we will notice that).

Frankly, I thought exactly the same initially, that's why I dropped
reclaim_state handling from the initial memcg shrinkers patch set.
However, then Hillf noticed that nr_reclaimed is checked right after
calling shrink_slab() in the memcg iteration loop in shrink_zone():


		memcg = mem_cgroup_iter(root, NULL, &reclaim);
		do {
			[...]
			if (memcg && is_classzone)
				shrink_slab(sc->gfp_mask, zone_to_nid(zone),
					    memcg, sc->nr_scanned - scanned,
					    lru_pages);

			/*
			 * Direct reclaim and kswapd have to scan all memory
			 * cgroups to fulfill the overall scan target for the
			 * zone.
			 *
			 * Limit reclaim, on the other hand, only cares about
			 * nr_to_reclaim pages to be reclaimed and it will
			 * retry with decreasing priority if one round over the
			 * whole hierarchy is not sufficient.
			 */
			if (!global_reclaim(sc) &&
					sc->nr_reclaimed >= sc->nr_to_reclaim) {
				mem_cgroup_iter_break(root, memcg);
				break;
			}
			memcg = mem_cgroup_iter(root, memcg, &reclaim);
		} while (memcg);


If we can ignore reclaimed slab pages here (?), let's drop this patch.

Thanks,
Vladimir
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