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Tue, 28 Mar 2023 15:22:57 -0700 (PDT) MIME-Version: 1.0 References: <20230328221644.803272-1-yosryahmed@google.com> <20230328221644.803272-6-yosryahmed@google.com> In-Reply-To: <20230328221644.803272-6-yosryahmed@google.com> From: Shakeel Butt Date: Tue, 28 Mar 2023 15:22:45 -0700 Message-ID: Subject: Re: [PATCH v2 5/9] memcg: replace stats_flush_lock with an atomic To: Yosry Ahmed Cc: Tejun Heo , Josef Bacik , Jens Axboe , Zefan Li , Johannes Weiner , Michal Hocko , Roman Gushchin , Muchun Song , Andrew Morton , =?UTF-8?Q?Michal_Koutn=C3=BD?= , Vasily Averin , cgroups@vger.kernel.org, linux-block@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, bpf@vger.kernel.org Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 0F41A40007 X-Stat-Signature: q3tr1wehei89hau7a9uyhgjyrqhja9aj X-Rspam-User: X-HE-Tag: 1680042177-203877 X-HE-Meta: U2FsdGVkX194IUkmjO8BdmLifgs6UFMMAAF1T1g2U8jVrKUlhc61xatwlm9G09UfRL7MwOy11285SH/mLM5UShHYfu6xu6doFExWWZ5slQfFAbj9zBEkOUImRroZ1Gk+LsmkdWQ/D1qY2FT99RWawt8o5+g3KCNDFAFz2qi4WV5oyVX8JfiNJw4KvxMJ/U3FVf2MkDAPvaDMwa1Z/e+VgLSHQWTRJphbzH7L1hexpxi8erhAPnosMMF53YjxA4MpNxLzSslksv0ZadiZBczQKlGi+Fe/+MeV0Nd57a6GLnV5/XtiQLVKzL/fNifVqVUg3VAJorva+yvg8o5AcSrrUqJOaSw47E4b3YJHHMuBR7TRuuVFwJ2XLE+6jS4l8Ucxzqs24WRAZOmf4GC+7oFtzwkPKFmqUnIrp1Kl+aa1OTwn7RmVm1KUxtBk5GQoJN7dNS0ScLN+mOakK2Z+bzbPtPVXJDCn70AGoFo3stpTgGrf7LnmoxUcvreJFd4kALmgBwV/a5JK8JlxGYN7x0Wved/ansvqXv5V4GhRyEj+H3cpfq/c39WQlOEsOJT1S87hNwZwVf9LYSMYSUBFbRrlQtFx+bKQlpYa+yvjR7ivog2QF/gxzwo/gLWatWlSqxy4/7Zulv6tcFAFgQ/j0vUFYavaK8PsT+osSxV2Xf81xe4ffzgt7sWUQlTh6e+hyhpVKi6WEV0wa8Rg1k2Fw7B5HbXbKHsOTP/BTgweXdxnRmkee2dG4x/ap57LqORx5kgxI1maw96KntG6W+3BGVBCtypYLSVuuYrHirbAWBlfS/ZF+qHrkVEbhXC7ld0hcQqEOeSZL/6hkrPvRxGGbTI274Xti+HNlaeZiIMIjC+hBc3zDEAi/SvVZusxngVENi5WOuLKa4mnviTL4up6vipHgax+CFnjthPcu8MyNev5ehxDXKr1AaUbKcH4ZTZJBRlm3LX9KBmGqZ7+YXX7Pu8 DpvIpJ/m ZZr1rxWxbstQK5M/6sftss9widzOclcpxcoqmECmgHkWQByqH1ixuxIeY14Ynccx4PzU8n2qQOBpCRueaPNcOZWcXxwE7tBQSg6YGcVA+NJ4fcSIba5Rg6z8Axid1y4OAg8hyEhfHFfvXxMCiWMg5zKY3SeU7sd7qX99fonoR14BIdBRDIRGURbqTtYLGBulaX5rqNuPkOqQIyD8fc7zuFwFS3c1kF9Xy5zubpY9X0A7wf9vsrIS9IHv551Z48WMgthR9fdUQimBqWI40dG3M+44tFNgDY3t11FLnm3SCzlAO6IEBn+fj3vNz5kp+anmhgJpn/qKJx/KwS1qzcxTu9AHPaQ== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Tue, Mar 28, 2023 at 3:17=E2=80=AFPM Yosry Ahmed = wrote: > > As Johannes notes in [1], stats_flush_lock is currently used to: > (a) Protect updated to stats_flush_threshold. > (b) Protect updates to flush_next_time. > (c) Serializes calls to cgroup_rstat_flush() based on those ratelimits. > > However: > > 1. stats_flush_threshold is already an atomic > > 2. flush_next_time is not atomic. The writer is locked, but the reader > is lockless. If the reader races with a flush, you could see this: > > if (time_after(jiffies, flush_nex= t_time)) > spin_trylock() > flush_next_time =3D now + delay > flush() > spin_unlock() > spin_trylock() > flush_next_time =3D now + delay > flush() > spin_unlock() > > which means we already can get flushes at a higher frequency than > FLUSH_TIME during races. But it isn't really a problem. > > The reader could also see garbled partial updates, so it needs at > least READ_ONCE and WRITE_ONCE protection. > > 3. Serializing cgroup_rstat_flush() calls against the ratelimit > factors is currently broken because of the race in 2. But the race > is actually harmless, all we might get is the occasional earlier > flush. If there is no delta, the flush won't do much. And if there > is, the flush is justified. > > So the lock can be removed all together. However, the lock also served > the purpose of preventing a thundering herd problem for concurrent > flushers, see [2]. Use an atomic instead to serve the purpose of > unifying concurrent flushers. > > [1]https://lore.kernel.org/lkml/20230323172732.GE739026@cmpxchg.org/ > [2]https://lore.kernel.org/lkml/20210716212137.1391164-2-shakeelb@google.= com/ > > Signed-off-by: Yosry Ahmed > Acked-by: Johannes Weiner Acked-by: Shakeel Butt