linux-kernel - Re: [PATCH] cpufreq: pcc: Fix the potentinal scheduling delays in target

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Message-ID: <CAJZ5v0hSNC1f9vBHYF6SP73jxXhW3EM8WdC4ObZhHk1OSx_xQA@mail.gmail.com>
Date:   Tue, 29 Aug 2023 20:43:04 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Viresh Kumar <viresh.kumar@...aro.org>,
        Liao Chang <liaochang1@...wei.com>
Cc:     linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] cpufreq: pcc: Fix the potentinal scheduling delays in target_index()

On Mon, Aug 28, 2023 at 9:02 AM Viresh Kumar <viresh.kumar@...aro.org> wrote:
>
> On 26-08-23, 09:57, Liao Chang wrote:
> > pcc_cpufreq_target():
> >       cpufreq_freq_transition_begin();
> >       spin_lock(&pcc_lock);
> >       [critical section]
> >       cpufreq_freq_transition_end();
> >       spin_unlock(&pcc_lock);
> >
> > Above code has a performance issue, consider that Task0 executes
> > 'cpufreq_freq_transition_end()' to wake Task1 and preempted imediatedly
> > without releasing 'pcc_lock', then Task1 needs to wait for Task0 to
> > release 'pcc_lock'. In the worst case, this locking order can result in
> > Task1 wasting two scheduling rounds before it can enter the critical
> > section.
> >
> > Signed-off-by: Liao Chang <liaochang1@...wei.com>
> > ---
> >  drivers/cpufreq/pcc-cpufreq.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/drivers/cpufreq/pcc-cpufreq.c b/drivers/cpufreq/pcc-cpufreq.c
> > index 73efbcf5513b..9d732a00e2a5 100644
> > --- a/drivers/cpufreq/pcc-cpufreq.c
> > +++ b/drivers/cpufreq/pcc-cpufreq.c
> > @@ -232,8 +232,8 @@ static int pcc_cpufreq_target(struct cpufreq_policy *policy,
> >       status = ioread16(&pcch_hdr->status);
> >       iowrite16(0, &pcch_hdr->status);
> >
> > -     cpufreq_freq_transition_end(policy, &freqs, status != CMD_COMPLETE);
> >       spin_unlock(&pcc_lock);
> > +     cpufreq_freq_transition_end(policy, &freqs, status != CMD_COMPLETE);
> >
> >       if (status != CMD_COMPLETE) {
> >               pr_debug("target: FAILED for cpu %d, with status: 0x%x\n",
>
> Acked-by: Viresh Kumar <viresh.kumar@...aro.org>

Applied as 6.6-rc material, thanks!