linux-kernel - Re: [RFC PATCH] cpufreq: qcom-cpufreq-hw: allow work to be done on other CPU for PREEMPT

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Date:   Thu, 16 Mar 2023 13:28:18 +0100
From:   Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
To:     Andy Gross <agross@...nel.org>,
        Bjorn Andersson <andersson@...nel.org>,
        Konrad Dybcio <konrad.dybcio@...aro.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        linux-arm-msm@...r.kernel.org, linux-pm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Cc:     Adrien Thierry <athierry@...hat.com>,
        Brian Masney <bmasney@...hat.com>,
        linux-rt-users@...r.kernel.org
Subject: Re: [RFC PATCH] cpufreq: qcom-cpufreq-hw: allow work to be done on
 other CPU for PREEMPT_RT


On 15/03/2023 17:49, Krzysztof Kozlowski wrote:
> Qualcomm cpufreq driver configures interrupts with affinity to each
> cluster, e.g.  dcvsh-irq-0, dcvsh-irq-4 and dcvsh-irq-7 on SM8250.
> Triggered interrupt will schedule delayed work, but, since workqueue
> prefers local CPUs, it might get executed on a CPU dedicated to realtime
> tasks causing unexpected latencies in realtime workload.
> 
> Use unbound workqueue for such case.  This might come with performance
> or energy penalty, e.g. because of cache miss or when other CPU is
> sleeping.
> 
> Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@...aro.org>
> ---
>  drivers/cpufreq/qcom-cpufreq-hw.c | 11 ++++++++++-

Let me also paste impact of this patch - rtla osnoise on entirely idle
system (cores 2-7 isolated for Realtime):

BEFORE:
       osnoise/7-2967    [007] d..h2..  3937.898311: irq_noise: dcvsh-irq-7:179 start 3937.898310871 duration 104 ns
 irq/179-dcvsh-i-343     [007] d..h2..  3937.898318: irq_noise: IPI:6 start 3937.898317537 duration 104 ns
 irq/179-dcvsh-i-343     [007] d...3..  3937.898321: thread_noise: irq/179-dcvsh-i:343 start 3937.898316287 duration 4740 ns
     kworker/7:0-85      [007] d..h2..  3937.898323: irq_noise: IPI:6 start 3937.898322381 duration 104 ns
     kworker/7:0-85      [007] d...3..  3937.898343: thread_noise: kworker/7:0:85 start 3937.898321339 duration 20990 ns
       osnoise/7-2967    [007] .......  3937.898343: sample_threshold: start 3937.898308475 duration 34531 ns interference 5

Noise duration: 34 us

AFTER:
       osnoise/7-2637    [007] d..h2..   530.563819: irq_noise: dcvsh-irq-7:178 start 530.563817139 duration 260 ns
       osnoise/7-2637    [007] d..h2..   530.563827: irq_noise: IPI:6 start 530.563826670 duration 156 ns
       osnoise/7-2637    [007] .......   530.563828: sample_threshold: start 530.563814587 duration 12864 ns interference 2

Noise duration: 13 us

Best regards,
Krzysztof