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From: SeongJae Park <sj@kernel.org>
To: Hui Zhu <hui.zhu@linux.dev>
Cc: SeongJae Park <sj@kernel.org>,
	Johannes Weiner <hannes@cmpxchg.org>,
	Michal Hocko <mhocko@kernel.org>,
	Roman Gushchin <roman.gushchin@linux.dev>,
	Shakeel Butt <shakeel.butt@linux.dev>,
	Muchun Song <muchun.song@linux.dev>,
	Andrew Morton <akpm@linux-foundation.org>,
	cgroups@vger.kernel.org,
	linux-mm@kvack.org,
	linux-kernel@vger.kernel.org,
	Hui Zhu <zhuhui@kylinos.cn>
Subject: Re: [PATCH] mm/memcontrol: hoist pstatc_pcpu assignment out of CPU loop
Date: Wed, 29 Apr 2026 17:45:04 -0700
Message-ID: <20260430004504.113550-1-sj@kernel.org>
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On Wed, 29 Apr 2026 16:42:16 +0800 Hui Zhu <hui.zhu@linux.dev> wrote:

> From: Hui Zhu <zhuhui@kylinos.cn>
> 
> In mem_cgroup_alloc(), the assignment of pstatc_pcpu is invariant
> with respect to the for_each_possible_cpu() loop: both the 'parent'
> pointer and 'parent->vmstats_percpu' remain constant throughout all
> iterations.
> 
> The original code redundantly re-evaluated the 'if (parent)'
> condition and reassigned pstatc_pcpu on every CPU iteration, then
> repeated the same ternary check 'parent ? pstatc_pcpu : NULL' when
> storing into statc->parent_pcpu.
> 
> Move the single conditional assignment of pstatc_pcpu to before the
> loop, resolving both the loop-invariant placement issue and the
> duplicated null check. On systems with a large number of possible
> CPUs, this eliminates repeated branch evaluation with no functional
> change.
> 
> No functional change intended.

Makes sense and looks good to me.

> 
> Signed-off-by: Hui Zhu <zhuhui@kylinos.cn>

Reviewed-by: SeongJae Park <sj@kernel.org>


Thanks,
SJ

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