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

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Message-ID: <20230828070206.dc44mhe4qztg52kc@vireshk-i7>
Date:   Mon, 28 Aug 2023 12:32:06 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     Liao Chang <liaochang1@...wei.com>
Cc:     rafael@...nel.org, linux-pm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] cpufreq: pcc: Fix the potentinal scheduling delays in
 target_index()

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>

-- 
viresh