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Subject: Re: [PATCH v8 3/6] cpufreq: powernv: Remove cpu_to_chip_id() from
 hot-path
From: Michael Neuling <mikey@neuling.org>
To: Shilpasri G Bhat <shilpa.bhat@linux.vnet.ibm.com>,
 linuxppc-dev@ozlabs.org, linux-kernel@vger.kernel.org
Cc: ego@linux.vnet.ibm.com, linux-pm@vger.kernel.org,
 viresh.kumar@linaro.org,  rjw@rjwysocki.net, pc@us.ibm.com,
 shreyas@linux.vnet.ibm.com, anton@samba.org
Date: Fri, 18 Mar 2016 15:04:17 +1100
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On Wed, 2016-02-03 at 01:11 +0530, Shilpasri G Bhat wrote:

> cpu_to_chip_id() does a DT walk through to find out the chip id by
> taking a contended device tree lock. This adds an unnecessary overhead
> in a hot path. So instead of calling cpu_to_chip_id() everytime cache
> the chip ids for all cores in the array 'core_to_chip_map' and use it
> in the hotpath.
>=20
> Reported-by: Anton Blanchard <anton@samba.org>
> Signed-off-by: Shilpasri G Bhat <shilpa.bhat@linux.vnet.ibm.com>
> Reviewed-by: Gautham R. Shenoy <ego@linux.vnet.ibm.com>
> Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
> ---
> No changes from v7.

How about this instead?  It removes the linear lookup and seems a lot
less complex.

Mikey

diff --git a/drivers/cpufreq/powernv-cpufreq.c b/drivers/cpufreq/powernv-cp=
ufreq.c
index 547890f..d63d2cb 100644
--- a/drivers/cpufreq/powernv-cpufreq.c
+++ b/drivers/cpufreq/powernv-cpufreq.c
@@ -52,6 +52,7 @@ static struct chip {
 } *chips;
=20
 static int nr_chips;
+static DEFINE_PER_CPU(unsigned int, chip_id);
=20
 /*
  * Note: The set of pstates consists of contiguous integers, the
@@ -317,9 +318,7 @@ static void powernv_cpufreq_throttle_check(void *data)
=20
 	pmsr =3D get_pmspr(SPRN_PMSR);
=20
-	for (i =3D 0; i < nr_chips; i++)
-		if (chips[i].id =3D=3D cpu_to_chip_id(cpu))
-			break;
+	i =3D this_cpu_read(chip_id);
=20
 	/* Check for Pmax Capping */
 	pmsr_pmax =3D (s8)PMSR_MAX(pmsr);
@@ -560,6 +559,7 @@ static int init_chip_info(void)
 	for_each_possible_cpu(cpu) {
 		unsigned int id =3D cpu_to_chip_id(cpu);
=20
+		per_cpu(chip_id, cpu) =3D nr_chips;
 		if (prev_chip_id !=3D id) {
 			prev_chip_id =3D id;
 			chip[nr_chips++] =3D id;