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

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Date:   Fri, 21 Feb 2020 10:47:41 -0600
From:   Nathan Lynch <nathanl@...ux.ibm.com>
To:     "Gautham R. Shenoy" <ego@...ux.vnet.ibm.com>
Cc:     linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
        Michael Ellerman <mpe@...erman.id.au>,
        Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>,
        Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>,
        "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>,
        Tyrel Datwyler <tyreld@...ux.ibm.com>
Subject: Re: [PATCH v2 3/5] powerpc/pseries: Account for SPURR ticks on idle CPUs

"Gautham R. Shenoy" <ego@...ux.vnet.ibm.com> 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?