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SubjectRe: [PATCH] hwmon: coretemp: avoid RDMSR interruptions to isolated CPUs
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

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