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Tue, 04 Apr 2023 10:21:44 -0700 (PDT) MIME-Version: 1.0 References: <20230330191801.1967435-1-yosryahmed@google.com> <20230330191801.1967435-5-yosryahmed@google.com> <20230404165258.ie6ttxobbmgn62hs@blackpad> In-Reply-To: From: Shakeel Butt Date: Tue, 4 Apr 2023 10:21:33 -0700 Message-ID: Subject: Re: [PATCH v3 4/8] memcg: replace stats_flush_lock with an atomic To: =?UTF-8?Q?Michal_Koutn=C3=BD?= Cc: Yosry Ahmed , Tejun Heo , Josef Bacik , Jens Axboe , Zefan Li , Johannes Weiner , Michal Hocko , Roman Gushchin , Muchun Song , Andrew Morton , Vasily Averin , cgroups@vger.kernel.org, linux-block@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, bpf@vger.kernel.org, Michal Hocko Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspam-User: X-Rspamd-Server: rspam03 X-Stat-Signature: j79cm5e6xt57nj54yjh4orq59zynctkp X-Rspamd-Queue-Id: 7E9CAA0004 X-HE-Tag: 1680628905-32659 X-HE-Meta: U2FsdGVkX181q1TEG0SEZf5Wj4gQtUW5H7rWLqkmBDV0suWTQKBUNohiYuGoic0Pt4RF9knYYyo+V9byoignqO3qoetH7/Txz3tQKT4NTg0comCs/MJ69eZhirLV0CCKC3y2thMixYizZ2m52vqZhozZfe8n5jsfKJ72CjpIAKwsPlqWOsIGEK9A0HFNFYyvnu95c+usTZt7gu3UkStxpQfC5EBfoYbYjPYbPby1siCZ7g1y84qJ/rXCmUx+aEACswjfjvw1m8w7blcBdWzsfo3SOjzIAvwhy/PWdxdIOQlz0MWWMUD2bII4FEnTJeGsKgDpKNs+2Iv3MUg8Zv+3JczJ/0fzT43MOUckrrp0SwM8EOdpoArC/KICQCs1uof3NXVYE4k/UbQP9nHl7yMH5npvvhpWJB/2lTtTgTvBOtAdpE06fKfxodZrH/yfKZH65yIV3mpEnUbPfCEQe168B9frLG1OQSuX7tiDZntQscQ8JKJpQJD6mf7E8O91RdO8RGicznPfcBbQfzGHX8MDt6GFTSXwS/9V+Lfur4dUuAo82+sAVAFIphhiPg5Jp1eRofNeHyKpyQLdv+A1Nb2nTQ4YF0TxeEU9ce3vzySk7BrTRcY7AD2wjHXug3AT84q+DWAu6669Ft5Ph++cpW/ATXKzdaHyuHrWs5kcGpdNYL+wyODp+8rZ1/I1Qzjc4kHmZOAxHvoFcJP/yzIhUU8iTzTETtrzZEi8FPdhQzMW8yo8/OYH+KvWmqUua5K4pMWaTttnD8s5uGlluzQ7mAXKWRO6y8YCn6Kmi58sQqNZAFhmpLZoExgU2SL1/4PhXiXiISt+g5aYXXTeAmDyX3l1qeUxvLyq81QghprBXbxNWbsgJAomlleoZSZoeVOGgRD3WKkSNU69Y3KtlvoL9Zkx5h4XayqgNqLrCkugObU0fg1B3yN8xj2XsjJC98zKgOY6ivcLcAagDfLOH4KubGj rDIGkVW2 cj6opVkxsTPDRQnSIaEPnU2d36sjce6c+qDN/TKiyfXHLI8/mOcYfWkCM8PJMSesRUA6oM37dZT8YU8C8x3x7IMiaQHWJ7k3pAkP8VnfBFbRMHb8VcyliNuJhCBS8h772+2ry08u294oW/DkgkWSaOxB8uRiTGkpINeGAOPrTsf2M632p9xr5yX9yoYhBwIiuSouTD0Zvfgm70p9kyQRhhq21UWKZjwDyqzXJYm3cw3f+oHF47B/3wj2fEb0+wzeMXkC4eNZl7kf6eExxMxH1gjzYrOVklvOK/HDydxz6cNj0z6YQLIZ3TQ6XGN+FOKOXZ/8mNqwEqjVkotD2qDPPE5zmA8uF76E6c5tTFdeqqTs4wnpLzEsknb4/qEpWcviTD6WoQ/BEEco4d88/2ozKMY4PEVkJQClvBnTsw7iKQPP+iW5pfzlaJKk60Kl3p4FBj4llLR/Y1OxSalU= 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, Apr 4, 2023 at 10:13=E2=80=AFAM Shakeel Butt = wrote: > > On Tue, Apr 4, 2023 at 9:53=E2=80=AFAM Michal Koutn=C3=BD wrote: > > > > Hello. > > > > On Thu, Mar 30, 2023 at 07:17:57PM +0000, Yosry Ahmed wrote: > > > static void __mem_cgroup_flush_stats(void) > > > { > > > - unsigned long flag; > > > - > > > - if (!spin_trylock_irqsave(&stats_flush_lock, flag)) > > > + /* > > > + * We always flush the entire tree, so concurrent flushers can = just > > > + * skip. This avoids a thundering herd problem on the rstat glo= bal lock > > > + * from memcg flushers (e.g. reclaim, refault, etc). > > > + */ > > > + if (atomic_read(&stats_flush_ongoing) || > > > + atomic_xchg(&stats_flush_ongoing, 1)) > > > return; > > > > I'm curious about why this instead of > > > > if (atomic_xchg(&stats_flush_ongoing, 1)) > > return; > > > > Is that some microarchitectural cleverness? > > > > Yes indeed it is. Basically we want to avoid unconditional cache > dirtying. This pattern is also used at other places in the kernel like > qspinlock. Oh also take a look at https://lore.kernel.org/all/20230404052228.15788-1-feng.tang@intel.com/