linux-kernel - Re: [RFC PATCH 0/5] kmalloc-reclaimable caches

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Date:   Tue, 29 May 2018 13:58:06 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     linux-mm@...ck.org, Roman Gushchin <guro@...com>,
        Michal Hocko <mhocko@...nel.org>, linux-kernel@...r.kernel.org,
        linux-api@...r.kernel.org, Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        David Rientjes <rientjes@...gle.com>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Vijayanand Jitta <vjitta@...eaurora.org>
Subject: Re: [RFC PATCH 0/5] kmalloc-reclaimable caches

On Mon, May 28, 2018 at 10:15:46AM +0200, Vlastimil Babka wrote:
> On 05/24/2018 05:32 PM, Johannes Weiner wrote:
> > On Thu, May 24, 2018 at 01:00:06PM +0200, Vlastimil Babka wrote:
> >> - the vmstat/meminfo counter name is rather general and might suggest it also
> >>   includes reclaimable page caches, which it doesn't
> >>
> >> Suggestions welcome for all three points. For the last one, we might also keep
> >> the counter separate from nr_slab_reclaimable, not superset. I did a superset
> >> as IIRC somebody suggested that in the older threads or at LSF.
> > 
> > Yeah, the "reclaimable" name is too generic. How about KReclaimable?
> > 
> > The counter being a superset sounds good to me. We use this info for
> > both load balancing and manual debugging. For load balancing code it's
> > nice not having to worry about finding all the counters that hold
> > reclaimable memory depending on kernel version; it's always simply
> > user cache + user anon + kernel reclaimable. And for debugging, we can
> > always add more specific subset counters later on if we need them.
> 
> Hm, Christoph in his reply to patch 4/5 expressed a different opinion.
> It's true that updating two counters has extra overhead, especially if
> there are two separate critical sections:
> 
> mod_lruvec_page_state(page, NR_SLAB_RECLAIMABLE, nr_pages);
> mod_node_page_state(page_pgdat(page), NR_RECLAIMABLE, nr_pages);
> 
> The first disables irq for CONFIG_MEMCG or defers to
> mod_node_page_state() otherwise.
> mod_node_page_state() is different depending on CONFIG_SMP and
> CONFIG_HAVE_CMPXCHG_LOCAL.
> 
> I don't see an easy way to make this optimal? Different counter would be
> indeed simpler. /proc/vmstat would then print separate counters, but we
> could have both separate and summary counter in /proc/meminfo. Would
> that be enough?

Yeah, that works just as well.