linux-kernel - Re: [PATCH] mm: make allocation counters per-order

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Date:   Thu, 6 Jul 2017 14:19:41 +0100
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Roman Gushchin <guro@...com>
Cc:     linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...e.com>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Rik van Riel <riel@...hat.com>, kernel-team@...com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: make allocation counters per-order

On Thu, Jul 06, 2017 at 02:04:31PM +0100, Roman Gushchin wrote:
> High-order allocations are obviously more costly, and it's very useful
> to know how many of them happens, if there are any issues
> (or suspicions) with memory fragmentation.
> 
> This commit changes existing per-zone allocation counters to be
> per-zone per-order. These counters are displayed using a new
> procfs interface (similar to /proc/buddyinfo):
> 
> $ cat /proc/allocinfo
>      DMA          0          0          0          0          0 \
>        0          0          0          0          0          0
>    DMA32          3          0          1          0          0 \
>        0          0          0          0          0          0
>   Normal    4997056      23594      10902      23686        931 \
>       23        122        786         17          1          0
>  Movable          0          0          0          0          0 \
>        0          0          0          0          0          0
>   Device          0          0          0          0          0 \
>        0          0          0          0          0          0
> 
> The existing vmstat interface remains untouched*, and still shows
> the total number of single page allocations, so high-order allocations
> are represented as a corresponding number of order-0 allocations.
> 
> $ cat /proc/vmstat | grep alloc
> pgalloc_dma 0
> pgalloc_dma32 7
> pgalloc_normal 5461660
> pgalloc_movable 0
> pgalloc_device 0
> 
> * I've added device zone for consistency with other zones,
> and to avoid messy exclusion of this zone in the code.
> 

The alloc counter updates are themselves a surprisingly heavy cost to
the allocation path and this makes it worse for a debugging case that is
relatively rare. I'm extremely reluctant for such a patch to be added
given that the tracepoints can be used to assemble such a monitor even
if it means running a userspace daemon to keep track of it. Would such a
solution be suitable? Failing that if this is a severe issue, would it be
possible to at least make this a compile-time or static tracepoint option?
That way, only people that really need it have to take the penalty.

-- 
Mel Gorman
SUSE Labs