Re: [PATCH v8 3/6] cpufreq: powernv: Remove cpu_to_chip_id() from hot-path

[Michael Neuling]

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@xxxxxxxxx>
> Signed-off-by: Shilpasri G Bhat <shilpa.bhat@xxxxxxxxxxxxxxxxxx>
> Reviewed-by: Gautham R. Shenoy <ego@xxxxxxxxxxxxxxxxxx>
> Acked-by: Viresh Kumar <viresh.kumar@xxxxxxxxxx>
> ---
> 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;