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Wed, 30 Sep 2026 07:22:56 -0700 (PDT) Date: Wed, 30 Sep 2026 10:22:52 -0400 From: Johannes Weiner To: Peter Zijlstra Cc: David Stevens , Suren Baghdasaryan , Ingo Molnar , K Prateek Nayak , linux-kernel@vger.kernel.org, stable@vger.kernel.org Subject: Re: [PATCH] sched/psi: clamp negative cpu_clock() skew Message-ID: References: <20260928233743.3777102-1-stevensd@google.com> <20260929081004.GS4120091@noisy.programming.kicks-ass.net> <20260929081740.GH4121620@noisy.programming.kicks-ass.net> Precedence: bulk X-Mailing-List: stable@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: <20260929081740.GH4121620@noisy.programming.kicks-ass.net> On Tue, Sep 29, 2026 at 10:17:40AM +0200, Peter Zijlstra wrote: > On Tue, Sep 29, 2026 at 10:10:04AM +0200, Peter Zijlstra wrote: > > > > When running browser workloads on an Intel N100 with a sustained > > > psi.mem.some of 2-3%, this underflow is observed once every couple of > > > hours. > > Reading is hard. This is sad, that's a relatively modern chip :-( I'm still struggling, but I'm under the weather so bear with me. That's a cpu_clock() implementation issue, right? "The value for cpu_clock(i) is monotonic for constant i". Presumably violating that would be a problem beyond psi?