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Received: from 2001-1c00-8d85-5700-266e-96ff-fe07-7dcc.cable.dynamic.v6.ziggo.nl ([2001:1c00:8d85:5700:266e:96ff:fe07:7dcc] helo=noisy.programming.kicks-ass.net) by casper.infradead.org with esmtpsa (Exim 4.98.2 #2 (Red Hat Linux)) id 1vpsqL-0000000BUl8-3LcU; Tue, 10 Feb 2026 18:52:13 +0000 Received: by noisy.programming.kicks-ass.net (Postfix, from userid 1000) id D5783300754; Tue, 10 Feb 2026 19:52:12 +0100 (CET) Date: Tue, 10 Feb 2026 19:52:12 +0100 From: Peter Zijlstra To: Doug Smythies Cc: 'K Prateek Nayak' , mingo@kernel.org, juri.lelli@redhat.com, vincent.guittot@linaro.org, dietmar.eggemann@arm.com, rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de, vschneid@redhat.com, linux-kernel@vger.kernel.org, wangtao554@huawei.com, quzicheng@huawei.com, wuyun.abel@bytedance.com Subject: Re: [PATCH 0/4] sched: Various reweight_entity() fixes Message-ID: <20260210185212.GJ3016024@noisy.programming.kicks-ass.net> References: <20260130093439.803225718@infradead.org> <4c48fe59-8ff3-41fb-83cb-869409f6fbc6@amd.com> <20260203111134.GL1282955@noisy.programming.kicks-ass.net> <38ef3462-4c4e-4f40-8d63-84dd71cbd043@amd.com> <20260209154718.GW1282955@noisy.programming.kicks-ass.net> <001901dc9aa3$cbad47f0$6307d7d0$@telus.net> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <001901dc9aa3$cbad47f0$6307d7d0$@telus.net> On Tue, Feb 10, 2026 at 07:41:58AM -0800, Doug Smythies wrote: > On 2026.02.09.07:47 Peter Zijlstra wrote: > > On Wed, Feb 04, 2026 at 03:45:58PM +0530, K Prateek Nayak wrote: > > > >> # Overflow on enqueue > >> > >> <...>-102371 [255] ... : __enqueue_entity: Overflowed cfs_rq: > >> <...>-102371 [255] ... : dump_h_overflow_cfs_rq: cfs_rq: depth(0) weight(90894772) nr_queued(2) sum_w_vruntime(0) > sum_weight(0) zero_vruntime(701164930256050) sum_shift(0) avg_vruntime(701809615900788) > >> <...>-102371 [255] ... : dump_h_overflow_entity: se: weight(3508) vruntime(701809615900788) slice(2800000) > deadline(701810568648095) curr?(1) task?(1) <-------- cfs_rq->curr > >> <...>-102371 [255] ... : __enqueue_entity: Overflowed se: > >> <...>-102371 [255] ... : dump_h_overflow_entity: se: weight(90891264) vruntime(701808975077099) slice(2800000) > deadline(701808975109401) curr?(0) task?(0) <-------- new se > > > > So I spend a whole time trying to reproduce the splat, but alas. > > > > That said, I did spot something 'funny' in the above, note that > > zero_vruntime and avg_vruntime/curr->vruntime are significantly apart. > > That is not something that should happen. zero_vruntime is supposed to > > closely track avg_vruntime. > > > > That lead me to hypothesise that there is a problem tracking > > zero_vruntime when there is but a single runnable task, and sure > > enough, I could reproduce that, albeit not at such a scale as to lead to > > such problems (probably too much noise on my machine). > > > > I ended up with the below; and I've already pushed out a fresh > > queue/sched/core. Could you please test again? > > I tested this "V2". The CPU migration times test results are not good. > We expect the sample time to not deviate from the nominal 1 second > by more than 10 milliseconds for this test. The test ran for about > 13 hours and 41 minutes (49,243 samples). Histogram of times: > > It seems something has regressed over the last year. > Our threshold of 10 milliseconds was rather arbitrary. Moo.. I'll go dig out that benchmark too.