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Wed, 2 Jul 2025 17:25:38 -0700 (PDT) Date: Wed, 2 Jul 2025 17:25:38 -0700 (PDT) From: "Christoph Lameter (Ampere)" To: Thomas Gleixner cc: Christoph Lameter via B4 Relay , Anna-Maria Behnsen , Frederic Weisbecker , Ingo Molnar , linux-kernel@vger.kernel.org, linux-mm@kvack.org, sh@gentwo.org, Darren Hart Subject: Re: [PATCH] Skew tick for systems with a large number of processors In-Reply-To: <87sejew87r.ffs@tglx> Message-ID: References: <87sejew87r.ffs@tglx> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: D2721160004 X-Stat-Signature: e1dydo8cwbfq56op7sqrwn11x13nkqss X-Rspam-User: X-HE-Tag: 1751502339-945209 X-HE-Meta: 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 sAEOQfr5 glIKpaxdmh8VWI8CZhlEj6sW2AbSMFL1uW1eRUYdanxrTQJfT5dWhOiVQZeL8TI/ksYXWPZXHsOdcQl1E56PawEQt+HpIqb9le/K/AmTqz/wnJzBjDs2K4UVHnKHCMESZBUriHIaAWpTZCBP/OLG9vrcWN2cdxtVr2qTjEyML5sQnn8h/K95URNAJlxEglQ4jMZUfSOhbdtvoyREdYWFXZWeA485pVgDAjol4MeGojYVT9IKyqzkQ5Xq3/D7cHgib2o0+QyTg97ArCEO/RPaL3eT7shUvyCnPk/RBCIH9tFAiG27GrBpsGF3QAinCX6aAHjVA0UnbuPuj1zc= 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 Thu, 3 Jul 2025, Thomas Gleixner wrote: > The above aside. As you completely failed to provide at least the > minimal historical background in the change log, let me fill in the > blanks. > > commit 3704540b4829 ("tick management: spread timer interrupt") added the > skew unconditionally in 2007 to avoid lock contention on xtime lock. Right but that was only one reason why the timer interrupts where staggered. > commit af5ab277ded0 ("clockevents: Remove the per cpu tick skew") > removed it in 2010 because the xtime lock contention was gone and the > skew affected the power consumption of slightly loaded _large_ servers. But then the tick also executes other code that can cause contention. Why merge such an obvious problematic patch without considering the reasons for the 2007 patch? > commit 5307c9556bc1 ("tick: Add tick skew boot option") brought it back > with a command line option to address contention and jitter issues on > larger systems. And then issues resulted because the scaling issues where not considered when merging the 2010 patch. > So while you preserved the behaviour of the command line option in the > most obscure way, you did not even make an attempt to explain why this > change does not bring back the issues which caused the removal in commit > af5ab277ded0 or why they are irrelevant today. As pointed out in the patch description: The synchronized tick (aside from the jitter) also causes power spikes on large core systems which can cause system instabilities. > "Scratches my itch" does not work and you know that. This needs to be > consolidated both on the implementation side and also on the user > side. We can get to that but I at least need some direction on how to approach this and figure out the concerns that exist. Frankly my initial idea was just to remove the buggy patches since this caused a regression in performance and system stability but I guess there were power savings concerns. How can we address this issue in a better way then? The kernel should not come up all wobbly and causing power spikes every tick.