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Tue, 03 Oct 2023 01:04:33 -0700 (PDT) MIME-Version: 1.0 References: <20230922175741.635002-1-yosryahmed@google.com> In-Reply-To: From: Yosry Ahmed Date: Tue, 3 Oct 2023 01:03:53 -0700 Message-ID: Subject: Re: [PATCH v2 0/2] mm: memcg: fix tracking of pending stats updates values To: Michal Hocko Cc: Andrew Morton , Shakeel Butt , Johannes Weiner , Roman Gushchin , Muchun Song , =?UTF-8?Q?Michal_Koutn=C3=BD?= , linux-mm@kvack.org, cgroups@vger.kernel.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: C15281A0021 X-Stat-Signature: qsqhqmne6fi4ahzx1swj3kq3gsjsqqiy X-HE-Tag: 1696320274-989874 X-HE-Meta: U2FsdGVkX18Vo0I4C6yswgnD9IDQERoYvsc8fFiVn7jrDr+ix3sOtrB0fK548xXiFJpz6VI8k59XrDQfCZJbAif484Vd3CZF9DQy8MVqPTkrStiFGS1tGdZQ01lwj3ehdXkh8VGeZm2KZN0YYhE8rd7m94R22IpeyLmvHP0GKIZOoxBIL3xuVoJtU0iCTcwng8ltVno1XO/a8gblQ6J74v+t9zDcjCShiyEL1NSddxECFuhI1+pewHG2YO+9/ABQVf8oHJvRBVqq1IQ2xcURFlXx5LsU/sZNveUkIcxqVS74h2pCxSbP6WiTBOSYOXD0OSwXAyfHZdrQh0/NwDXoGcz+uR7tMXQFqUeWa7nZg9i5ALXeEBfcEgZwfdRVTWKP+z/xIYHF+gEtYmjlqpttykUVZFnJKma3DA9t/y97YCEth4GwiouVc3lgVzD4Mc3mZv1khfwNud7XdBH7SKPz0EsDsGfF/ZPR5P4Nw9FTt8hnZ5iqDNGg2Zxg5G+sJC4F/xbHwAor7F4X9WlFtiYMQM7pg47cApkNWc0vGcn6sb71s9lggE0qb1Xvi5rlLEoO5trKa3fXOMiAapI2fHu5r947V3Pe7w0lpZL53DWgCJqqvXKrCbzNRvqhOwHytMA7f4w8hFmq5tMPol9ycG64phz2MSIw4diFM80azQF3EHaopbxZew26vtOVV2pe/fCsHwsAye/Y0EbziDyjKNMao+RrFlZGaVRe9+CQ/N6u/lAEru5yEixiZusxEHsZpVXZXyMpUx682VGUVDgEWgCuG7HS8GjJZvomgviX6a689mh3NRwkRWamrulMldCqOi31LJpWXF5V3RJuYuAlCW1h4U30LWn3m/M5RxR8TButkYSInzFdEtN+fQVQiRhJuAfQ8sqJYUj4YpEnC24GCI8NDZm4u/4G11yN+MhCYKeh8TIuwAZmHXWFUofcFEKlakUihWUPsU4GvmLMymtToxt 8r435O1s Z0JtHJx4gSHVu2RcLaIUNBnuTeBIrmxcdHsbFjuWWJMWjtCf3noPUmq2vfypunbpphUzRCzVVpVBepHQW2FiVGPq+6E5fmbYRV2F98DaXDYZpzM6Lt0FyrhSQgqXPvLhgL5eSyCtW6VjqmX8Ty0gPsCbR3APESrjoZwD3MEAGUufAOhQbQD7arHWB1o2BbSdgBcw7KIz3FddyPB/w5HRnleeH3HER1gwUfCR6WWvFkLurziSZHCN2XL5h0PA5iAnrfWjlPOqgH+9NSZtHEAECdr5o1WAIRGrz2N9M 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, Oct 3, 2023 at 12:57=E2=80=AFAM Michal Hocko wrot= e: > > On Mon 25-09-23 10:11:05, Yosry Ahmed wrote: > > On Mon, Sep 25, 2023 at 6:50=E2=80=AFAM Michal Hocko = wrote: > > > > > > On Fri 22-09-23 17:57:38, Yosry Ahmed wrote: > > > > While working on adjacent code [1], I realized that the values pass= ed > > > > into memcg_rstat_updated() to keep track of the magnitude of pendin= g > > > > updates is consistent. It is mostly in pages, but sometimes it can = be in > > > > bytes or KBs. Fix that. > > > > > > What kind of practical difference does this change make? Is it worth > > > additional code? > > > > As explained in patch 2's commit message, the value passed into > > memcg_rstat_updated() is used for the "flush only if not worth it" > > heuristic. As we have discussed in different threads in the past few > > weeks, unnecessary flushes can cause increased global lock contention > > and/or latency. > > > > Byte-sized paths (percpu, slab, zswap, ..) feed bytes into the > > heuristic, but those are interpreted as pages, which means we will > > flush earlier than we should. This was noticed by code inspection. How > > much does this matter in practice? I would say it depends on the > > workload: how many percpu/slab allocations are being made vs. how many > > flushes are requested. > > > > On a system with 100 cpus, 25M of stat updates are needed for a flush > > usually, but ~6K of slab/percpu updates will also (mistakenly) cause a > > flush. > > This surely depends on workload and that is understandable. But it would > be really nice to provide some numbers for typical workloads which > exercise slab heavily. If you have a workload in mind I can run it and see how many flushes we get with/without this patch. The first thing that pops into my head is creating a bunch of empty files but I don't know if that's the best thing to get numbers from. > -- > Michal Hocko > SUSE Labs