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From: Nathan Lynch <nathanl@linux.ibm.com>
To: "Gautham R. Shenoy" <ego@linux.vnet.ibm.com>
Subject: Re: [PATCH v2 3/5] powerpc/pseries: Account for SPURR ticks on idle
 CPUs
In-Reply-To: <1582262314-8319-4-git-send-email-ego@linux.vnet.ibm.com>
References: <1582262314-8319-1-git-send-email-ego@linux.vnet.ibm.com>
 <1582262314-8319-4-git-send-email-ego@linux.vnet.ibm.com>
Date: Fri, 21 Feb 2020 10:47:41 -0600
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Cc: Tyrel Datwyler <tyreld@linux.ibm.com>, linux-kernel@vger.kernel.org,
 Kamalesh Babulal <kamalesh@linux.vnet.ibm.com>,
 "Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com>,
 Vaidyanathan Srinivasan <svaidy@linux.vnet.ibm.com>,
 linuxppc-dev@lists.ozlabs.org
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 <linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org>

"Gautham R. Shenoy" <ego@linux.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?