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Date:   Thu, 21 May 2020 14:14:14 -0700
From:   Roman Gushchin <guro@...com>
To:     Vlastimil Babka <vbabka@...e.cz>
CC:     Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>, <linux-mm@...ck.org>,
        <kernel-team@...com>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 03/19] mm: memcg: convert vmstat slab counters to bytes

On Thu, May 21, 2020 at 11:57:12AM +0200, Vlastimil Babka wrote:
> On 5/20/20 9:26 PM, Roman Gushchin wrote:
> > On Wed, May 20, 2020 at 02:25:22PM +0200, Vlastimil Babka wrote:
> >> 
> >> However __mod_node_page_state() and mode_node_state() will now branch always. I
> >> wonder if the "API clean" goal is worth it...
> > 
> > You mean just adding a special write-side helper which will perform a conversion
> > and put VM_WARN_ON_ONCE() into generic write-side helpers?
> 
> What I mean is that maybe node/global helpers should assume page granularity,
> and lruvec/memcg helpers do the check is they should convert from bytes to pages
> when calling node/global helpers. Then there would be no extra branches in
> node/global helpers. But maybe it's not worth saving those branches, dunno.

The problem is with helpers like mod_lruvec_state(), which do modify both global
and memcg-level counters. Also memcg- and global counters share idxes, so
it will be confusing to have NR_SLAB_RECLAIMABLE in bytes on one level and
in pages on the other.

So, idk, maybe there is a better way of organizing these counters in a less
complicated manner, but I've no ideas at the moment. But if you do, I'll appreciate it.

Thanks!

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