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Subject: Re: [RFC PATCH 0/5] kmalloc-reclaimable caches
To:     Johannes Weiner <hannes@cmpxchg.org>
Cc:     linux-mm@kvack.org, Roman Gushchin <guro@fb.com>,
        Michal Hocko <mhocko@kernel.org>, linux-kernel@vger.kernel.org,
        linux-api@vger.kernel.org, Christoph Lameter <cl@linux.com>,
        Pekka Enberg <penberg@kernel.org>,
        David Rientjes <rientjes@google.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Mel Gorman <mgorman@techsingularity.net>,
        Vijayanand Jitta <vjitta@codeaurora.org>
References: <20180524110011.1940-1-vbabka@suse.cz>
 <20180524153225.GA7329@cmpxchg.org>
From:   Vlastimil Babka <vbabka@suse.cz>
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Message-ID: <fcdcf7be-afd1-9747-d97e-51cd071d3f5c@suse.cz>
Date:   Mon, 28 May 2018 10:15:46 +0200
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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?