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

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Date:	Fri, 18 Mar 2016 14:13:59 +0100
From:	"Rafael J. Wysocki" <rafael@...nel.org>
To:	Michael Neuling <mikey@...ling.org>
Cc:	Shilpasri G Bhat <shilpa.bhat@...ux.vnet.ibm.com>,
	linuxppc-dev@...abs.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"Gautham R. Shenoy" <ego@...ux.vnet.ibm.com>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>, pc@...ibm.com,
	shreyas@...ux.vnet.ibm.com, Anton Blanchard <anton@...ba.org>
Subject: Re: [PATCH v8 3/6] cpufreq: powernv: Remove cpu_to_chip_id() from hot-path

On Fri, Mar 18, 2016 at 5:11 AM, Michael Neuling <mikey@...ling.org> wrote:
> 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.

This has gone in already.  Can you please send a patch on top of it?

> 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);
>
>

Including this part too maybe?

Thanks,
Rafael