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Date:   Tue, 19 Apr 2022 14:56:06 +0200
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Subject: Re: [PATCH] mm: do not call add_nr_deferred() with zero deferred
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To:     Roman Gushchin <roman.gushchin@linux.dev>, linux-mm@kvack.org
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Dave Chinner <dchinner@redhat.com>,
        linux-kernel@vger.kernel.org, Johannes Weiner <hannes@cmpxchg.org>,
        Michal Hocko <mhocko@kernel.org>,
        Shakeel Butt <shakeelb@google.com>,
        Yang Shi <shy828301@gmail.com>
References: <20220416004104.4089743-1-roman.gushchin@linux.dev>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
In-Reply-To: <20220416004104.4089743-1-roman.gushchin@linux.dev>
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On 16.04.22 02:41, Roman Gushchin wrote:
> add_nr_deferred() is often called with next_deferred equal to 0.
> For instance, it's happening under low memory pressure for any
> shrinkers with a low number of cached objects. A corresponding trace
> looks like:
>   <...>-619914 [005] .... 467456.345160: mm_shrink_slab_end: \
>   super_cache_scan+0x0/0x1a0 0000000087027f06: nid: 1	     \
>   unused scan count 0 new scan count 0 total_scan 0	     \
>   last shrinker return val 0
> 
>   <...>-619914 [005] .... 467456.345371: mm_shrink_slab_end: \
>   super_cache_scan+0x0/0x1a0 0000000087027f06: nid: 1	     \
>   unused scan count 0 new scan count 0 total_scan 0	     \
>   last shrinker return val 0
> 
>   <...>-619914 [005] .... 467456.345380: mm_shrink_slab_end: \
>   super_cache_scan+0x0/0x1a0 0000000087027f06: nid: 1 unused \
>   scan count 0 new scan count 0 total_scan 0	             \
>   last shrinker return val 0
> 
> This lead to unnecessary checks and atomic operations, which can be
> avoided by checking next_deferred for not being zero before calling
> add_nr_deferred(). In this case the mm_shrink_slab_end trace point
> will get a potentially slightly outdated "new scan count" value, but
> it's totally fine.

Sufficient improvement to justify added complexity for anybody reading
that code?

> 
> Signed-off-by: Roman Gushchin <roman.gushchin@linux.dev>
> ---
>  mm/vmscan.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/vmscan.c b/mm/vmscan.c
> index d4a7d2bd276d..19d3d4fa1aad 100644
> --- a/mm/vmscan.c
> +++ b/mm/vmscan.c
> @@ -808,7 +808,10 @@ static unsigned long do_shrink_slab(struct shrink_control *shrinkctl,
>  	 * move the unused scan count back into the shrinker in a
>  	 * manner that handles concurrent updates.
>  	 */
> -	new_nr = add_nr_deferred(next_deferred, shrinker, shrinkctl);
> +	if (next_deferred)
> +		new_nr = add_nr_deferred(next_deferred, shrinker, shrinkctl);
> +	else
> +		new_nr = nr;
>  
>  	trace_mm_shrink_slab_end(shrinker, shrinkctl->nid, freed, nr, new_nr, total_scan);
>  	return freed;

And if we still want to do this optimization, why not put it into
add_nr_deferred()?

-- 
Thanks,

David / dhildenb