linux-kernel - Re: [PATCH] thermal: intel_powerclamp: Use first online CPU as control

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Message-ID: <Y0gOI9hiP9NXsuJP@chenyu5-mobl1>
Date:   Thu, 13 Oct 2022 21:09:55 +0800
From:   Chen Yu <yu.c.chen@...el.com>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>
CC:     Linux PM <linux-pm@...r.kernel.org>,
        Bjorn Helgaas <helgaas@...nel.org>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        Pavel Machek <pavel@....cz>
Subject: Re: [PATCH] thermal: intel_powerclamp: Use first online CPU as
 control_cpu

On 2022-10-13 at 14:50:28 +0200, Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> 
> Commit 68b99e94a4a2 ("thermal: intel_powerclamp: Use get_cpu() instead
> of smp_processor_id() to avoid crash") fixed an issue related to using
> smp_processor_id() in preemptible context by replacing it with a pair
> of get_cpu()/put_cpu(), but what is needed there really is any online
> CPU and not necessarily the one currently running the code.  Arguably,
> getting the one that's running the code in there is confusing.
> 
> For this reason, simply give the control CPU role to the first online
> one which automatically will be CPU0 if it is online, so one check
> can be dropped from the code for an added benefit.
> 
> Link: https://lore.kernel.org/linux-pm/20221011113646.GA12080@duo.ucw.cz/
> Fixes: 68b99e94a4a2 ("thermal: intel_powerclamp: Use get_cpu() instead of smp_processor_id() to avoid crash")
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> ---
>  drivers/thermal/intel/intel_powerclamp.c |    6 +-----
>  1 file changed, 1 insertion(+), 5 deletions(-)
> 
> Index: linux-pm/drivers/thermal/intel/intel_powerclamp.c
> ===================================================================
> --- linux-pm.orig/drivers/thermal/intel/intel_powerclamp.c
> +++ linux-pm/drivers/thermal/intel/intel_powerclamp.c
> @@ -516,11 +516,7 @@ static int start_power_clamp(void)
>  	cpus_read_lock();
>  
>  	/* prefer BSP */
Above comment line is not true any more, might delete it as well?

Reviewed-by: Chen Yu <yu.c.chen@...el.com>

thanks,
Chenyu
> -	control_cpu = 0;
> -	if (!cpu_online(control_cpu)) {
> -		control_cpu = get_cpu();
> -		put_cpu();
> -	}
> +	control_cpu = cpumask_first(cpu_online_mask);
>  
>  	clamping = true;
>  	schedule_delayed_work(&poll_pkg_cstate_work, 0);
> 
> 
>