linux-kernel - Re: [PATCH memcg] mm/page_alloc.c: avoid statistic update with 0

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Message-ID: <20211012113804.GT3959@techsingularity.net>
Date:   Tue, 12 Oct 2021 12:38:04 +0100
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Vasily Averin <vvs@...tuozzo.com>
Cc:     Vlastimil Babka <vbabka@...e.cz>, Michal Hocko <mhocko@...nel.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        cgroups@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, kernel@...nvz.org,
        Uladzislau Rezki <urezki@...il.com>
Subject: Re: [PATCH memcg] mm/page_alloc.c: avoid statistic update with 0

On Tue, Oct 12, 2021 at 01:42:41PM +0300, Vasily Averin wrote:
> On 08.10.2021 14:47, Vlastimil Babka wrote:
> > On 10/8/21 11:24, Vasily Averin wrote:
> >> __alloc_pages_bulk can call __count_zid_vm_events and zone_statistics
> >> with nr_account = 0.
> > 
> > But that's not a bug, right? Just an effective no-op that's not commonly
> > happening, so is it worth the check?
> 
> Why not?
> 
> Yes, it's not a bug, it just makes the kernel a bit more efficient in a very unlikely case.

At the cost of an additional branch in the likely case that may be
mispredicted.

> However, it looks strange and makes uninformed code reviewers like me worry about possible
> problems inside the affected functions. No one else calls these functions from 0.
>  

The accounting functions should still work with a 0 delta.

-- 
Mel Gorman
SUSE Labs