Subject: Re: [PATCH v2 3/5] powerpc/pseries: Account for SPURR ticks on idle CPUs

From: Nathan Lynch <[email-masked]>
Date: Fri, 21 Feb 2020 10:47:41 -0600

"Gautham R. Shenoy" <[email-masked]> writes:
> +static inline void snapshot_spurr_idle_entry(void)
> +{
> +	*this_cpu_ptr(&idle_entry_spurr_snap) = mfspr(SPRN_SPURR);
> +}
> +

[...]

> +static inline void update_idle_spurr_accounting(void)
> +{
> +	u64 *idle_spurr_cycles_ptr = this_cpu_ptr(&idle_spurr_cycles);
> +	u64 in_spurr = *this_cpu_ptr(&idle_entry_spurr_snap);
> +
> +	*idle_spurr_cycles_ptr += mfspr(SPRN_SPURR) - in_spurr;
> +}

[...]

> +static inline u64 read_this_idle_spurr(void)
> +{
> +	/*
> +	 * If we are reading from an idle context, update the
> +	 * idle-spurr cycles corresponding to the last idle period.
> +	 * Since the idle context is not yet over, take a fresh
> +	 * snapshot of the idle-spurr.
> +	 */
> +	if (get_lppaca()->idle == 1) {
> +		update_idle_spurr_accounting();
> +		snapshot_spurr_idle_entry();

This samples spurr twice when it could do with just one. I don't know
the performance implications, but will the results be coherent?