Re: [PATCH v1 1/2] powercap: intel_rapl: Prepare read_raw interface for atomic-context callers

[Raag Jadav]

On Thu, Nov 20, 2025 at 04:05:38PM -0800, Kuppuswamy Sathyanarayanan wrote:
> The current read_raw() implementation of the TPMI, MMIO and MSR
> interfaces does not distinguish between atomic and non-atomic callers.
> 
> rapl_msr_read_raw() uses rdmsrq_safe_on_cpu(), which can sleep and
> issue cross CPU calls. When MSR-based RAPL PMU support is enabled, PMU
> event handlers can invoke this function from atomic context where
> sleeping or rescheduling is not allowed. In atomic context, the caller
> is already executing on the target CPU, so a direct rdmsrq() is
> sufficient.
> 
> To support such usage, introduce an atomic flag to the read_raw()
> interface to allow callers pass the context information. Modify the
> common RAPL code to propagate this flag, and set the flag to reflect
> the calling contexts.
> 
> Utilize the atomic flag in rapl_msr_read_raw() to perform direct MSR
> read with rdmsrq() when running in atomic context, and a sanity check
> to ensure target CPU matches the current CPU for such use cases.
> 
> The TPMI and MMIO implementations do not require special atomic
> handling, so the flag is ignored in those paths.
> 
> This is a preparatory patch for adding MSR-based RAPL PMU support.

...

> -static int rapl_msr_read_raw(int cpu, struct reg_action *ra)
> +static int rapl_msr_read_raw(int cpu, struct reg_action *ra, bool atomic)
>  {
> +	/*
> +	 * When called from atomic-context (eg PMU event handler)
> +	 * perform MSR read directly using rdmsrq().
> +	 */
> +	if (atomic) {
> +		if (unlikely(smp_processor_id() != cpu))
> +			return -EIO;

This series breaks[1] our application[2] in cases where the reads are
issued from any available CPU it is scheduled on. This issue is not seen on
older platforms which use the original arch/x86 RAPL implementation.

Can someone please shed some light on the change of userspace expectations?
Or did I miss any points in the documentation?

[1] https://gitlab.freedesktop.org/drm/xe/kernel/-/issues/6935
[2] https://gitlab.freedesktop.org/drm/igt-gpu-tools/-/blob/master/lib/igt_power.c

Raag

> +		rdmsrq(ra->reg.msr, ra->value);
> +		goto out;
> +	}
> +
>  	if (rdmsrq_safe_on_cpu(cpu, ra->reg.msr, &ra->value)) {
>  		pr_debug("failed to read msr 0x%x on cpu %d\n", ra->reg.msr, cpu);
>  		return -EIO;
>  	}
> +
> +out:
>  	ra->value &= ra->mask;
>  	return 0;
>  }