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[86.146.64.97]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-40fc7e2c6b3sm28297378f8f.54.2025.10.01.03.38.16 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 01 Oct 2025 03:38:16 -0700 (PDT) Date: Wed, 1 Oct 2025 11:38:15 +0100 From: Qais Yousef To: "Rafael J. Wysocki" Cc: Linux PM , Shawn Guo , LKML , Viresh Kumar , Pierre Gondois , Mario Limonciello , Linux ACPI , Jie Zhan , rust-for-linux@vger.kernel.org, Miguel Ojeda , Alex Gaynor Subject: Re: [PATCH v3 1/4] cpufreq: Make drivers using CPUFREQ_ETERNAL specify transition latency Message-ID: <20251001103815.a57cc3wyasyhmrmt@airbuntu> References: <5069803.31r3eYUQgx@rafael.j.wysocki> <2264949.irdbgypaU6@rafael.j.wysocki> 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=utf-8 Content-Disposition: inline In-Reply-To: <2264949.irdbgypaU6@rafael.j.wysocki> On 09/26/25 12:12, Rafael J. Wysocki wrote: > From: Rafael J. Wysocki > > Commit a755d0e2d41b ("cpufreq: Honour transition_latency over > transition_delay_us") caused platforms where cpuinfo.transition_latency > is CPUFREQ_ETERNAL to get a very large transition latency whereas > previously it had been capped at 10 ms (and later at 2 ms). > > This led to a user-observable regression between 6.6 and 6.12 as > described by Shawn: > > "The dbs sampling_rate was 10000 us on 6.6 and suddently becomes > 6442450 us (4294967295 / 1000 * 1.5) on 6.12 for these platforms > because the default transition delay was dropped [...]. > > It slows down dbs governor's reacting to CPU loading change > dramatically. Also, as transition_delay_us is used by schedutil > governor as rate_limit_us, it shows a negative impact on device > idle power consumption, because the device gets slightly less time > in the lowest OPP." > > Evidently, the expectation of the drivers using CPUFREQ_ETERNAL as > cpuinfo.transition_latency was that it would be capped by the core, > but they may as well return a default transition latency value instead > of CPUFREQ_ETERNAL and the core need not do anything with it. > > Accordingly, introduce CPUFREQ_DEFAULT_TRANSITION_LATENCY_NS and make > all of the drivers in question use it instead of CPUFREQ_ETERNAL. Also > update the related Rust binding. > > Fixes: a755d0e2d41b ("cpufreq: Honour transition_latency over transition_delay_us") > Closes: https://lore.kernel.org/linux-pm/20250922125929.453444-1-shawnguo2@yeah.net/ > Reported-by: Shawn Guo > Reviewed-by: Mario Limonciello (AMD) > Reviewed-by: Jie Zhan > Cc: 6.6+ # 6.6+ > Signed-off-by: Rafael J. Wysocki The whole series LGTM. I think this is clearer now without the CPUFREQ_ETERNAL. Reviewed-by: Qais Yousef Cheers -- Qais Yousef