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Sender: Guenter Roeck <groeck7@gmail.com>
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Date:   Thu, 15 Dec 2022 06:22:16 -0800
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To:     Marcelo Tosatti <mtosatti@redhat.com>,
        Fenghua Yu <fenghua.yu@intel.com>
Cc:     linux-kernel@vger.kernel.org, linux-hwmon@vger.kernel.org,
        Frederic Weisbecker <frederic@kernel.org>
References: <Y5sWMEG0xCl9bgEi@tpad>
From:   Guenter Roeck <linux@roeck-us.net>
Subject: Re: [PATCH] hwmon: coretemp: avoid RDMSR interruptions to isolated
 CPUs
In-Reply-To: <Y5sWMEG0xCl9bgEi@tpad>
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On 12/15/22 04:42, Marcelo Tosatti wrote:
> 
> The coretemp driver uses rdmsr_on_cpu calls to read
> MSR_IA32_PACKAGE_THERM_STATUS/MSR_IA32_THERM_STATUS registers,
> which contain information about current core temperature.
> 
> For certain low latency applications, the RDMSR interruption exceeds
> the applications requirements.
> 
> So disable reading of crit_alarm and temp files via /sys, in case
> CPU isolation is enabled.
> 

That isn't really what the code is doing. It doesn't disable reading
the attributes, it returns an error when an attempt is made to read
them.

> Temperature information from the housekeeping cores should be
> sufficient to infer die temperature.
> 
> Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
> 
> diff --git a/drivers/hwmon/coretemp.c b/drivers/hwmon/coretemp.c
> index 9bee4d33fbdf..30a35f4130d5 100644
> --- a/drivers/hwmon/coretemp.c
> +++ b/drivers/hwmon/coretemp.c
> @@ -27,6 +27,7 @@
>   #include <asm/msr.h>
>   #include <asm/processor.h>
>   #include <asm/cpu_device_id.h>
> +#include <linux/sched/isolation.h>
>   
>   #define DRVNAME	"coretemp"
>   
> @@ -121,6 +122,10 @@ static ssize_t show_crit_alarm(struct device *dev,
>   	struct platform_data *pdata = dev_get_drvdata(dev);
>   	struct temp_data *tdata = pdata->core_data[attr->index];
>   
> +
> +	if (!housekeeping_cpu(tdata->cpu, HK_TYPE_MISC))
> +		return -EINVAL;


Littering the output of the "sensors" command with errors is most
definitely wrong and not acceptable. On top of that, the user didn't
do anything wrong, so -EINVAL ("Invalid Argument") is definitely
the wrong error. Maybe return -ENODATA, or if the condition is
static just don't instantiate the attribute for the affected CPUs
to start with. Also, this warrants a comment in the code and an
explanation in Documentation/hwmon/coretemp.rst.

Guenter