ACK/Cmnt: [SRU][jammy:linux][PATCH] mm/memcontrol.c: remove the redundant updating of stats_flush_threshold

[Andrei Gherzan]

On 24/02/09 01:34PM, Philip Cox wrote:
> From: Jiebin Sun <jiebin.sun at intel.com>
> 
> BugLink: https://bugs.launchpad.net/bugs/2052827
> 
> Remove the redundant updating of stats_flush_threshold.  If the global var
> stats_flush_threshold has exceeded the trigger value for
> __mem_cgroup_flush_stats, further increment is unnecessary.
> 
> Apply the patch and test the pts/hackbench-1.0.0 Count:4 (160 threads).
> 
> Score gain: 1.95x
> Reduce CPU cycles in __mod_memcg_lruvec_state (44.88% -> 0.12%)
> 
> CPU: ICX 8380 x 2 sockets
> Core number: 40 x 2 physical cores
> Benchmark: pts/hackbench-1.0.0 Count:4 (160 threads)
> 
> Link: https://lkml.kernel.org/r/20220722164949.47760-1-jiebin.sun@intel.com
> Signed-off-by: Jiebin Sun <jiebin.sun at intel.com>
> Acked-by: Shakeel Butt <shakeelb at google.com>
> Reviewed-by: Roman Gushchin <roman.gushchin at linux.dev>
> Reviewed-by: Tim Chen <tim.c.chen at linux.intel.com>
> Acked-by: Muchun Song <songmuchun at bytedance.com>
> Cc: Johannes Weiner <hannes at cmpxchg.org>
> Cc: Michal Hocko <mhocko at kernel.org>
> Cc: "Huang, Ying" <ying.huang at intel.com>
> Cc: Amadeusz Sawiski <amadeuszx.slawinski at linux.intel.com>
> Signed-off-by: Andrew Morton <akpm at linux-foundation.org>
> (cherry picked from commit 873f64b791a2b43c246e78b7d9fdd64ce909685b)
> Signed-off-by: Philip Cox <philip.cox at canonical.com>
> ---
>  mm/memcontrol.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 6f969ba0d688..f922613ddf8c 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -662,7 +662,14 @@ static inline void memcg_rstat_updated(struct mem_cgroup *memcg, int val)
>  
>  	x = __this_cpu_add_return(stats_updates, abs(val));
>  	if (x > MEMCG_CHARGE_BATCH) {
> -		atomic_add(x / MEMCG_CHARGE_BATCH, &stats_flush_threshold);
> +		/*
> +		 * If stats_flush_threshold exceeds the threshold
> +		 * (>num_online_cpus()), cgroup stats update will be triggered
> +		 * in __mem_cgroup_flush_stats(). Increasing this var further
> +		 * is redundant and simply adds overhead in atomic update.
> +		 */
> +		if (atomic_read(&stats_flush_threshold) <= num_online_cpus())
> +			atomic_add(x / MEMCG_CHARGE_BATCH, &stats_flush_threshold);
>  		__this_cpu_write(stats_updates, 0);
>  	}
>  }

I know that this is a clean cherry-pick, and it does look alright -
patch-wise. Even so, I still consider that a cover letter would be
minimum friction and in line with our contribution guidelines.

Again, patch looks good to me so:

Acked-by: Andrei Gherzan <andrei.gherzan at canonical.com>

-- 
Andrei Gherzan
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