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Mon, 16 Jun 2025 04:24:18 -0700 (PDT) Date: Mon, 16 Jun 2025 13:24:18 +0200 From: Michal Hocko To: David Hildenbrand Cc: Zihuan Zhang , Peter Zijlstra , rafael@kernel.org, len.brown@intel.com, pavel@kernel.org, kees@kernel.org, mingo@redhat.com, juri.lelli@redhat.com, vincent.guittot@linaro.org, dietmar.eggemann@arm.com, rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de, vschneid@redhat.com, akpm@linux-foundation.org, lorenzo.stoakes@oracle.com, Liam.Howlett@oracle.com, vbabka@suse.cz, rppt@kernel.org, surenb@google.com, linux-pm@vger.kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: [RFC PATCH] PM: Optionally block user fork during freeze to improve performance Message-ID: References: <20250606062502.19607-1-zhangzihuan@kylinos.cn> <20250606082244.GL30486@noisy.programming.kicks-ass.net> <83513599-e007-4d07-ac28-386bc5c7552d@kylinos.cn> <775aaf10-3d19-4d5a-bf2b-703211166be4@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <775aaf10-3d19-4d5a-bf2b-703211166be4@redhat.com> X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 03AFE100006 X-Stat-Signature: 5q59ajqtezjmy495ism5ano6tu788hsc X-Rspam-User: X-HE-Tag: 1750073060-184984 X-HE-Meta: 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 nZyWfSH0 BVD59KqOCwCiIK5c1BA7mK12g24ED400nXBdArfbF//rRg0K4brzJ0tTERKMXEJpDA0/o31I4PuwYCvHLH5b29VvQrAwuDLL29fjYUt13l9VI8Jbme/adCoVj5Nk11y94ryknhdhQPMTMEE6AfZZuqeCGLJT3R0pe0meH4RVBEW826/n/d4aCnlxGad6kOTS+N0rYay8bf0CSiLZRnOQfhGgc0wsDhULHXOzVdjxZjaZcopUoTay8CzYk0zrgGeJu0qSZJoYxRIj+TlG4y3BVzz8Em8okHZku3qP1QMVWhxje9jda60YiiWz6kd7+I9h9BDj+ysLDZjNxsER0cOkv6DyAeQ== 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: List-Subscribe: List-Unsubscribe: On Mon 16-06-25 09:45:59, David Hildenbrand wrote: > > > > [...] > > In our test scenario, although new processes can indeed be created > > during the usleep_range() intervals between freeze iterations, it’s > > actually difficult to make the freezer fail outright. This is because > > user processes are forcibly frozen: when they return to user space and > > check for pending signals, they enter try_to_freeze() and transition > > into the refrigerator. > > > > However, since the scheduler is fair by design, it gives both newly > > forked tasks and yet-to-be-frozen tasks a chance to run. This > > competition for CPU time can slightly delay the overall freeze process. > > While this typically doesn’t lead to failure, it does cause more retries > > than necessary, especially under CPU pressure. > > I think that goes back to my original comment: why are we even allowing fork > children to run at all when we are currently freezing all tasks? The same should be the case for cgroup freezer as well. -- Michal Hocko SUSE Labs