From: Shakeel Butt <shakeel.butt@linux.dev>
To: Kuniyuki Iwashima <kuniyu@google.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
Johannes Weiner <hannes@cmpxchg.org>,
Michal Hocko <mhocko@kernel.org>,
Roman Gushchin <roman.gushchin@linux.dev>,
Muchun Song <muchun.song@linux.dev>,
Kuniyuki Iwashima <kuni1840@gmail.com>,
linux-mm@kvack.org, cgroups@vger.kernel.org
Subject: Re: [PATCH v2 1/6] memcg: Remove unnecessary READ_ONCE() under memcg_stock.lock.
Date: Wed, 28 Oct 2026 17:48:26 -0700 [thread overview]
Message-ID: <auKW_UkDicyxx6fQ@linux.dev> (raw)
In-Reply-To: <20261028185034.3607746-2-kuniyu@google.com>
On Wed, Oct 28, 2026 at 06:50:13PM +0000, Kuniyuki Iwashima wrote:
> this_cpu_ptr(&memcg_stock) is always modified under
> local_lock(&memcg_stock.lock).
You need to be more precise here. Mainly the cached[] and nr_pages[] are only
modified under the local lock. There is flags field which is being modified
without it.
>
> Unlike is_memcg_drain_needed() under RCU, consume_stock(),
> drain_stock(), and refill_stock() do not need READ_ONCE().
>
> Especially, in refill_stock(), READ_ONCE() is used twice for
> stock->cached[i], which forces the compiler to emit two volatile
> loads from stock->cached[i].
>
> Let's remove the unnecessary READ_ONCE() and eliminate the
> redundant loads in refill_stock():
What are we gaining with that? Is this measurable? With READ_ONCE() at least we
are very explicit that this field can read and modified concurrently by
different CPUs i.e. better readability.
next prev parent reply other threads:[~2026-10-29 0:48 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-10-28 18:50 [PATCH v2 0/6] memcg: Batch mod_memcg_state() for MEMCG_SOCK Kuniyuki Iwashima
2026-10-28 18:50 ` [PATCH v2 1/6] memcg: Remove unnecessary READ_ONCE() under memcg_stock.lock Kuniyuki Iwashima
2026-10-29 0:48 ` Shakeel Butt [this message]
2026-10-29 1:44 ` Kuniyuki Iwashima
2026-10-28 18:50 ` [PATCH v2 2/6] memcg: Reduce nesting in consume_stock() and refill_stock() Kuniyuki Iwashima
2026-10-29 0:50 ` Shakeel Butt
2026-10-28 18:50 ` [PATCH v2 3/6] memcg: Annotate slow paths " Kuniyuki Iwashima
2026-10-29 0:55 ` Shakeel Butt
2026-10-29 1:49 ` Kuniyuki Iwashima
2026-10-28 18:50 ` [PATCH v2 4/6] memcg: Batch mod_memcg_state() for MEMCG_SOCK Kuniyuki Iwashima
2026-10-28 18:50 ` [PATCH v2 5/6] memcg: Uncharge pages outside memcg_stock.lock in __refill_stock() Kuniyuki Iwashima
2026-10-28 18:50 ` [PATCH v2 6/6] selftests: cgroup: Deflake test_memcg_sock() in test_memcontrol.c Kuniyuki Iwashima
2026-10-29 0:41 ` [PATCH v2 0/6] memcg: Batch mod_memcg_state() for MEMCG_SOCK Shakeel Butt
2026-10-29 0:52 ` Shakeel Butt
2026-10-29 1:35 ` Kuniyuki Iwashima
2026-10-29 2:32 ` Kuniyuki Iwashima
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