linux-kernel - Re: [PATCH v2 2/6] cpufreq/sched: Explicitly synchronize limits

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Message-ID: <b0c863d8-b1bd-4b69-b5ec-18544608448c@arm.com>
Date: Wed, 16 Apr 2025 13:01:46 +0100
From: Christian Loehle <christian.loehle@....com>
To: "Rafael J. Wysocki" <rjw@...ysocki.net>,
 Linux PM <linux-pm@...r.kernel.org>
Cc: LKML <linux-kernel@...r.kernel.org>,
 Viresh Kumar <viresh.kumar@...aro.org>,
 Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
 Mario Limonciello <mario.limonciello@....com>,
 Vincent Guittot <vincent.guittot@...aro.org>,
 Sultan Alsawaf <sultan@...neltoast.com>,
 Peter Zijlstra <peterz@...radead.org>,
 Valentin Schneider <vschneid@...hat.com>, Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH v2 2/6] cpufreq/sched: Explicitly synchronize
 limits_changed flag handling

On 4/15/25 10:59, Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> 
> The handling of the limits_changed flag in struct sugov_policy needs to
> be explicitly synchronized to ensure that cpufreq policy limits updates
> will not be missed in some cases.
> 
> Without that synchronization it is theoretically possible that
> the limits_changed update in sugov_should_update_freq() will be
> reordered with respect to the reads of the policy limits in
> cpufreq_driver_resolve_freq() and in that case, if the limits_changed
> update in sugov_limits() clobbers the one in sugov_should_update_freq(),
> the new policy limits may not take effect for a long time.
> 
> Likewise, the limits_changed update in sugov_limits() may theoretically
> get reordered with respect to the updates of the policy limits in
> cpufreq_set_policy() and if sugov_should_update_freq() runs between
> them, the policy limits change may be missed.
> 
> To ensure that the above situations will not take place, add memory
> barriers preventing the reordering in question from taking place and
> add READ_ONCE() and WRITE_ONCE() annotations around all of the
> limits_changed flag updates to prevent the compiler from messing up
> with that code.
> 
> Fixes: 600f5badb78c ("cpufreq: schedutil: Don't skip freq update when limits change")
> Cc: 5.3+ <stable@...r.nernel.org> # 5.3+

typo in the address here.
I don't fully understand why we wouldn't want this in 6.15-rc already,
even if the actual impact may be limited?

> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>

Reviewed-by: Christian Loehle <christian.loehle@....com>

>[snip]