linux-kernel - Re: [PATCH v3] cpuidle: psci: Init cpuidle only for present CPUs

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Message-ID: <Z8mYP7AGBPeDTvXn@bogus>
Date: Thu, 6 Mar 2025 12:42:39 +0000
From: Sudeep Holla <sudeep.holla@....com>
To: Ulf Hansson <ulf.hansson@...aro.org>
Cc: Jacky Bai <ping.bai@....com>, lpieralisi@...nel.org, rafael@...nel.org,
	daniel.lezcano@...aro.org, james.morse@....com, d-gole@...com,
	linux-pm@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org, imx@...ts.linux.dev,
	khilman@...libre.com, quic_tingweiz@...cinc.com,
	quic_yuanjiey@...cinc.com
Subject: Re: [PATCH v3] cpuidle: psci: Init cpuidle only for present CPUs

On Thu, Mar 06, 2025 at 11:53:14AM +0100, Ulf Hansson wrote:
> On Thu, 6 Mar 2025 at 07:17, Jacky Bai <ping.bai@....com> wrote:
> >
> > for_each_possible_cpu() is currently used to initialize cpuidle
> > in the PSCI cpuidle driver.
> >
> > However, in cpu_dev_register_generic(), for_each_present_cpu()
> > is used to register CPU devices which means the CPU devices are
> > only registered for present CPUs and not all possible CPUs.
> >
> > With nosmp or maxcpus=0, only the boot CPU is present, leading
> > to the failure:
> >
> >   |  Failed to register cpuidle device for cpu1
> >
> > Change for_each_possible_cpu() to for_each_present_cpu() in the
> > PSCI cpuidle driver to ensure it only registers cpuidle devices
> > for CPUs that are actually present.
> >
> > Fixes: b0c69e1214bc ("drivers: base: Use present CPUs in GENERIC_CPU_DEVICES")
> > Reviewed-by: Dhruva Gole <d-gole@...com>
> > Reviewed-by: Sudeep Holla <sudeep.holla@....com>
> > Tested-by: Yuanjie Yang <quic_yuanjiey@...cinc.com>
> > Signed-off-by: Jacky Bai <ping.bai@....com>
> 
> Is this problem specific to cpuidle-psci?
> 
> Others are using for_each_possible_cpu() when registering their cpuidle drivers.
> 

Good point. I assumed as this was very old patch, only this is left without
conversion to using present_cpumask. Looks like there are many other drivers
that need similar change. May be PSCI is most commonly used ones these days
and hardly anyone tests with nosmp or maxcpus=1 on those platforms.

But yes, all the users of for_each_possible_cpu() need to move to
for_each_present_cpu() if they are relying on CPU device being registered.

-- 
Regards,
Sudeep