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Message-ID: <e41ad66f-b8eb-4a17-aab0-6dc0f8fa55f8@arm.com> Date: Tue, 17 Sep 2024 11:22:11 +0100 From: Christian Loehle <christian.loehle@....com> To: Dietmar Eggemann <dietmar.eggemann@....com>, Qais Yousef <qyousef@...alina.io>, Ingo Molnar <mingo@...nel.org>, Peter Zijlstra <peterz@...radead.org>, Vincent Guittot <vincent.guittot@...aro.org>, "Rafael J. Wysocki" <rafael@...nel.org>, Viresh Kumar <viresh.kumar@...aro.org> Cc: Juri Lelli <juri.lelli@...hat.com>, Steven Rostedt <rostedt@...dmis.org>, John Stultz <jstultz@...gle.com>, linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org Subject: Re: [RFC PATCH 06/16] sched/schedutil: Add a new tunable to dictate response time On 9/16/24 23:22, Dietmar Eggemann wrote: > On 20/08/2024 18:35, Qais Yousef wrote: >> The new tunable, response_time_ms, allow us to speed up or slow down >> the response time of the policy to meet the perf, power and thermal >> characteristic desired by the user/sysadmin. There's no single universal >> trade-off that we can apply for all systems even if they use the same >> SoC. The form factor of the system, the dominant use case, and in case >> of battery powered systems, the size of the battery and presence or >> absence of active cooling can play a big role on what would be best to >> use. >> >> The new tunable provides sensible defaults, but yet gives the power to >> control the response time to the user/sysadmin, if they wish to. >> >> This tunable is applied before we apply the DVFS headroom. >> >> The default behavior of applying 1.25 headroom can be re-instated easily >> now. But we continue to keep the min required headroom to overcome >> hardware limitation in its speed to change DVFS. And any additional >> headroom to speed things up must be applied by userspace to match their >> expectation for best perf/watt as it dictates a type of policy that will >> be better for some systems, but worse for others. >> >> There's a whitespace clean up included in sugov_start(). >> >> Signed-off-by: Qais Yousef <qyousef@...alina.io> >> --- >> Documentation/admin-guide/pm/cpufreq.rst | 17 +++- >> drivers/cpufreq/cpufreq.c | 4 +- >> include/linux/cpufreq.h | 3 + >> kernel/sched/cpufreq_schedutil.c | 115 ++++++++++++++++++++++- >> 4 files changed, 132 insertions(+), 7 deletions(-) >> >> diff --git a/Documentation/admin-guide/pm/cpufreq.rst b/Documentation/admin-guide/pm/cpufreq.rst >> index 6adb7988e0eb..fa0d602a920e 100644 >> --- a/Documentation/admin-guide/pm/cpufreq.rst >> +++ b/Documentation/admin-guide/pm/cpufreq.rst >> @@ -417,7 +417,7 @@ is passed by the scheduler to the governor callback which causes the frequency >> to go up to the allowed maximum immediately and then draw back to the value >> returned by the above formula over time. >> >> -This governor exposes only one tunable: >> +This governor exposes two tunables: >> >> ``rate_limit_us`` >> Minimum time (in microseconds) that has to pass between two consecutive >> @@ -427,6 +427,21 @@ This governor exposes only one tunable: >> The purpose of this tunable is to reduce the scheduler context overhead >> of the governor which might be excessive without it. >> >> +``respone_time_ms`` s/respone/response >> + Amount of time (in milliseconds) required to ramp the policy from >> + lowest to highest frequency. Can be decreased to speed up the > ^^^^^^^^^^^^^^^^^ > > This has changed IMHO. Should be the time from lowest (or better 0) to > second highest frequency. > > https://lkml.kernel.org/r/20230827233203.1315953-6-qyousef@layalina.io > > [...] > Isn't it even more complicated than that? We have the headroom applied on top of the response_time_ms, so response_time_ms will be longer than the time it takes to reach highest cap OPP. Furthermore, applying this to a big CPU e.g. with OPP0 cap of 200, starting from 0 is (usually?) irrelevant, as we likely wouldn't be here if we were at 0. I get the intent, but conveying this in an understandable interface is hard.
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