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

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Date:   Mon, 28 May 2018 10:15:46 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Johannes Weiner <hannes@...xchg.org>
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 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?