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SubjectRe: [PATCH v8 3/6] cpufreq: powernv: Remove cpu_to_chip_id() from hot-path
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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.
>
> 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-cpufreq.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;

static int nr_chips;
+static DEFINE_PER_CPU(unsigned int, chip_id);

/*
* Note: The set of pstates consists of contiguous integers, the
@@ -317,9 +318,7 @@ static void powernv_cpufreq_throttle_check(void *data)

pmsr = get_pmspr(SPRN_PMSR);

- for (i = 0; i < nr_chips; i++)
- if (chips[i].id == cpu_to_chip_id(cpu))
- break;
+ i = this_cpu_read(chip_id);

/* Check for Pmax Capping */
pmsr_pmax = (s8)PMSR_MAX(pmsr);
@@ -560,6 +559,7 @@ static int init_chip_info(void)
for_each_possible_cpu(cpu) {
unsigned int id = cpu_to_chip_id(cpu);

+ per_cpu(chip_id, cpu) = nr_chips;
if (prev_chip_id != id) {
prev_chip_id = id;
chip[nr_chips++] = id;
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