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b=Vp/8tqwLmUV0dpY+5G8PSbp0C94GyCfYCdhzDxYX5hLTjrHz+2FEunJDuJpg7uhQJ 8VQDPRLapN3ArwFx1H/jXPizTFxCFwGfRWadUVAS9H10eMP5gNsir/WtXSTd1eVIsy Na6/IXVRnU+JdIwyWxjzFVjeyIxScxsnmdvkhIaUo3k/2Mlu/mwccIv4AVb+g03bNq TdCUBm8QR0fnv0zME3/1Z08lHbnlkbv2V5T28Tg6bz+p0znXTpdssQUQEmfVRWLaQW c6rNlSCfONB+d+pm/up857qhaWDrpo5MUuLlXp8dJ3CuP4ZuCxt4o32GQ7dmCMBtK3 V4y+7OQz4PDXw== Message-ID: <43732a44-1f90-4119-9e52-000b5a6a2f99@kernel.org> Date: Wed, 26 Jun 2024 23:35:07 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH V2] cgroup/rstat: Avoid thundering herd problem by kswapd across NUMA nodes To: Yosry Ahmed , "Christoph Lameter (Ampere)" Cc: Shakeel Butt , tj@kernel.org, cgroups@vger.kernel.org, hannes@cmpxchg.org, lizefan.x@bytedance.com, longman@redhat.com, kernel-team@cloudflare.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: Content-Language: en-US From: Jesper Dangaard Brouer In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam03 X-Rspam-User: X-Rspamd-Queue-Id: 47E91140011 X-Stat-Signature: afmnhcsqram1hzznh3gqs48cdr1bgjqg X-HE-Tag: 1719437713-326698 X-HE-Meta: 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 6euJ3nVS Hg/eNjU03ekDQLTynOBkHDmHE5o7kd7OUe2E1IwQ3/demIAi3uFjmL8TCMbUBca9sb5tdzolSRpd2NcjouRe3zJOO3eO68fetXD2/iopnpD1AgHA+bPNQmui1oCnTKIt9cT521IJcXMN0JSXdplir8u8nbDh6anVp7D1LxbNlcF5g035zLMDjlmwIYIszrvIR/SSFPxBV4bSqnCDebzuC3syIehmFUu0haN6Xy8Ro7XkEu4kPqjCvudriddNPz7XGqUtWl6WX/CwOIuMcwuVaBWQABs7f+vVttiaju/kOmGYO8OHUr2xE3nWtRFeuy/XkTx8Y+yYdXhWXQF7Rh4+XEnEJrtEtlQU+wXofhXnnAtPIol/3zjrfIDaC6Q== X-Bogosity: Ham, tests=bogofilter, spamicity=0.015814, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 26/06/2024 00.59, Yosry Ahmed wrote: > On Tue, Jun 25, 2024 at 3:35 PM Christoph Lameter (Ampere) wrote: >> >> On Tue, 25 Jun 2024, Yosry Ahmed wrote: >> >>>> In my reply above, I am not arguing to go back to the older >>>> stats_flush_ongoing situation. Rather I am discussing what should be the >>>> best eventual solution. From the vmstats infra, we can learn that >>>> frequent async flushes along with no sync flush, users are fine with the >>>> 'non-determinism'. Of course cgroup stats are different from vmstats >>>> i.e. are hierarchical but I think we can try out this approach and see >>>> if this works or not. >>> >>> If we do not do sync flushing, then the same problem that happened >>> with stats_flush_ongoing could occur again, right? Userspace could >>> read the stats after an event, and get a snapshot of the system before >>> that event. >>> >>> Perhaps this is fine for vmstats if it has always been like that (I >>> have no idea), or if no users make assumptions about this. But for >>> cgroup stats, we have use cases that rely on this behavior. >> >> vmstat updates are triggered initially as needed by the shepherd task and >> there is no requirement that this is triggered simultaenously. We >> could actually randomize the intervals in vmstat_update() a bit if this >> will help. > > The problem is that for cgroup stats, the behavior has been that a > userspace read will trigger a flush (i.e. propagating updates). We > have use cases that depend on this. If we switch to the vmstat model > where updates are triggered independently from user reads, it > constitutes a behavioral change. I implemented a variant using completions as Yosry asked for: https://lore.kernel.org/all/171943668946.1638606.1320095353103578332.stgit@firesoul/ --Jesper