linux-kernel - Re: [PATCH v8 3/6] cpufreq: powernv: Remove cpu_to_chip

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Message-ID: <1458274308.6622.79.camel@neuling.org>
Date:	Fri, 18 Mar 2016 15:11:48 +1100
From:	Michael Neuling <mikey@...ling.org>
To:	Shilpasri G Bhat <shilpa.bhat@...ux.vnet.ibm.com>,
	linuxppc-dev@...abs.org, linux-kernel@...r.kernel.org
Cc:	ego@...ux.vnet.ibm.com, linux-pm@...r.kernel.org,
	viresh.kumar@...aro.org, rjw@...ysocki.net, pc@...ibm.com,
	shreyas@...ux.vnet.ibm.com, anton@...ba.org
Subject: Re: [PATCH v8 3/6] cpufreq: powernv: Remove cpu_to_chip_id() from
 hot-path

On Fri, 2016-03-18 at 15:04 +1100, Michael Neuling wrote:

> 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.
> > 
> > Reported-by: Anton Blanchard <anton@...ba.org>
> > Signed-off-by: Shilpasri G Bhat <shilpa.bhat@...ux.vnet.ibm.com>
> > Reviewed-by: Gautham R. Shenoy <ego@...ux.vnet.ibm.com>
> > Acked-by: Viresh Kumar <viresh.kumar@...aro.org>
> > ---
> > No changes from v7.
> 
> How about this instead?  It removes the linear lookup and seems a lot
> less complex.

BTW we never init nr_chips before using it.  We also need something
like.

diff --git a/drivers/cpufreq/powernv-cpufreq.c b/drivers/cpufreq/powernv-cpufreq.c
index d63d2cb..c819ed4 100644
--- a/drivers/cpufreq/powernv-cpufreq.c
+++ b/drivers/cpufreq/powernv-cpufreq.c
@@ -556,6 +556,8 @@ static int init_chip_info(void)
 	unsigned int cpu, i;
 	unsigned int prev_chip_id = UINT_MAX;
 
+	nr_chips = 0;
+
 	for_each_possible_cpu(cpu) {
 		unsigned int id = cpu_to_chip_id(cpu);