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[82.69.66.36]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-480424aa344sm51662315e9.3.2026.01.22.03.20.02 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 22 Jan 2026 03:20:02 -0800 (PST) Date: Thu, 22 Jan 2026 11:19:59 +0000 From: David Laight To: "Ionut Nechita (Sunlight Linux)" Cc: rafael@kernel.org, daniel.lezcano@linaro.org, christian.loehle@arm.com, linux-pm@vger.kernel.org, linux-kernel@vger.kernel.org, yumpusamongus@gmail.com, Ionut Nechita , stable@vger.kernel.org Subject: Re: [PATCH v2 1/1] cpuidle: menu: Use min() to prevent deep C-states when tick is stopped Message-ID: <20260122111959.14e8fb3e@pumpkin> In-Reply-To: <20260122080937.22347-4-sunlightlinux@gmail.com> References: <20260122080937.22347-2-sunlightlinux@gmail.com> <20260122080937.22347-4-sunlightlinux@gmail.com> X-Mailer: Claws Mail 4.1.1 (GTK 3.24.38; arm-unknown-linux-gnueabihf) Precedence: bulk X-Mailing-List: linux-pm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit On Thu, 22 Jan 2026 10:09:39 +0200 "Ionut Nechita (Sunlight Linux)" wrote: > From: Ionut Nechita > > When the tick is already stopped and the predicted idle duration is short > (< TICK_NSEC), the original code uses next_timer_ns directly. This can > lead to selecting excessively deep C-states when the actual idle duration > is much shorter than the next timer event. > > On modern Intel server platforms (Sapphire Rapids and newer), deep package > C-states can have exit latencies of 150-190us due to: > - Tile-based architecture with per-tile power gating > - DDR5 and CXL power management overhead > - Complex mesh interconnect resynchronization > > When a network packet arrives after 500us but the governor selected a deep > C-state (PC6) based on a 10ms timer, the high exit latency (150us+) > dominates the response time. .... We had to disable the deep sleep states on much older Intel -7 cpus. The problem was that we needed to wake up multiple cpu and they tended to get woken in turn - so it was far too long before they were all running. I suspect that pretty much anything that cares about latency has always needed to disable them. David