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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