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Fri, 11 Aug 2023 19:48:26 -0700 (PDT) MIME-Version: 1.0 References: <20230809045810.1659356-1-yosryahmed@google.com> In-Reply-To: From: Shakeel Butt Date: Fri, 11 Aug 2023 19:48:14 -0700 Message-ID: Subject: Re: [PATCH] mm: memcg: provide accurate stats for userspace reads To: Yosry Ahmed Cc: Michal Hocko , Johannes Weiner , Roman Gushchin , Andrew Morton , Muchun Song , cgroups@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspam-User: X-Rspamd-Server: rspam06 X-Rspamd-Queue-Id: 5C48CA000E X-Stat-Signature: gbwhu3m5iibjmxe5go7hfx464m8rzywd X-HE-Tag: 1691808507-228477 X-HE-Meta: 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 NoVzmu6B /p4bgRH+2xUiYhKOnG6JHJVt6egNCt7YRvJ1rfmIynd6rM4aB8BpfeSFIWsmmDM4SOC5E5K5UhlBSTd7IAtHdyQ96NGAqw03wHMhuptr5EOK9289RK1vGcb+//+lkM1dyIylX0u1tqXWWkxutzx4it9Ob7BM8Jiys9ujbfIrMnHrdJAiz1qieFFRQKgohHr8pm+JshR59a6zPIqy5rWmdHdQu1wpBkQtWXSuTQOpiGbkz54DhWU3BRGA/UA== 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 Fri, Aug 11, 2023 at 7:36=E2=80=AFPM Yosry Ahmed = wrote: > > On Fri, Aug 11, 2023 at 7:29=E2=80=AFPM Shakeel Butt wrote: > > > > On Fri, Aug 11, 2023 at 7:12=E2=80=AFPM Yosry Ahmed wrote: > > > > > [...] > > > > > > I am worried that writing to a stat for flushing then reading will > > > increase the staleness window which we are trying to reduce here. > > > Would it be acceptable to add a separate interface to explicitly read > > > flushed stats without having to write first? If the distinction > > > disappears in the future we can just short-circuit both interfaces. > > > > What is the acceptable staleness time window for your case? It is hard > > to imagine that a write+read will always be worse than just a read. > > Even the proposed patch can have an unintended and larger than > > expected staleness window due to some processing on > > return-to-userspace or some scheduling delay. > > Maybe I am worrying too much, we can just go for writing to > memory.stat for explicit stats refresh. > > Do we still want to go with the mutex approach Michal suggested for > do_flush_stats() to support either waiting for ongoing flushes > (mutex_lock) or skipping (mutex_trylock)? I would say keep that as a separate patch.