Re: [RFC PATCH 1/2] tracing/osnoise: Sample IPI counts

[Crystal Wood]

On Wed, 2026-06-10 at 15:04 +0200, Valentin Schneider wrote:
> Osnoise already implictly accounts IPIs via its IRQ tracking,

Does it?  It seems that IPIs bypass the kernel/irq subsystem on some
arches (including x86, but not ARM).

It would be nice to solve this properly by adding generic ipi
entry/exit tracing (similar to what ARM already has).

> however it
> can be interesting to distiguish between the two: undesired IPIs usually
> imply a software configuration issue (e.g. wrong/incomplete CPU isolation)
> whereas undesired (non-IPI) IRQs usually imply a hardware configuration
> issue.
> 
> Signed-off-by: Valentin Schneider <vschneid@xxxxxxxxxx>
> ---
> Note that this is modifying the osnoise:osnoise_entry Ftrace entry; I know
> trace events are sort of supposed to be stable, but I'm not sure about
> ftrace entries.

I think old rtla will be OK with this since it looks up fields by name
rather than assuming a fixed layout.

> Alternatively I can have this be purely supported in userspace osnoise by
> hooking into the IPI events and counting IPIs separately from the osnoise
> events.

One benefit I could see of doing this in kernel osnoise would be if you
could atomically correlate the count with the particular noise
interval, but this patch doesn't do that.

> +static void ipi_emission(struct osnoise_variables *osn_var, unsigned int dst_cpu)
> +{
> +	if (!osn_var->sampling)
> +		return;
> +
> +	osn_var->ipi.count++;
> +}
> +
> +static void trace_ipi_send_cpu_callback(void *data, unsigned int cpu,
> +					unsigned long callsite, void *callback)
> +{
> +	struct osnoise_variables *osn_var;
> +
> +	osn_var = per_cpu_ptr(&per_cpu_osnoise_var, cpu);
> +	ipi_emission(osn_var, cpu);
> +}
> +
> +static void trace_ipi_send_cpumask_callback(void *data, const struct cpumask *cpumask,
> +					    unsigned long callsite, void *callback)
> +{
> +	struct osnoise_variables *osn_var;
> +	int cpu;
> +
> +	for_each_cpu_and(cpu, cpumask, &osnoise_cpumask) {
> +		osn_var = per_cpu_ptr(&per_cpu_osnoise_var, cpu);
> +		ipi_emission(osn_var, cpu);
> +	}
> +}

Isn't this racy to do from a different CPU?  Both in terms of the
counter, and the timing of the increment relative to when the IPI is
actually received.  Not necessarily a huge deal if you only care about
zero versus bignum, but still.  At least worth a comment, if we go with
this approach.

-Crystal