powerpc/pseries: lparcfg calculate PURR on demand

For SPLPAR, lparcfg provides a sum of PURR registers for all CPUs.
Currently this is done by reading PURR in context switch and timer
interrupt, and storing that into a per-CPU variable. These are summed
to provide the value.

This does not work with all timer schemes (e.g., NO_HZ_FULL), and it
is sub-optimal for performance because it reads the PURR register on
every context switch, although that's been difficult to distinguish
from noise in the contxt_switch microbenchmark.

This patch implements the sum by calling a function on each CPU, to
read and add PURR values of each CPU.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

1 files changed, 10 insertions, 8 deletions

diff --git a/arch/powerpc/platforms/pseries/lparcfg.c b/arch/powerpc/platforms/pseries/lparcfg.c
index c508c938dc71..7c872dc01bdb 100644
--- a/arch/powerpc/platforms/pseries/lparcfg.c
+++ b/arch/powerpc/platforms/pseries/lparcfg.c
@@ -52,18 +52,20 @@
  * Track sum of all purrs across all processors. This is used to further
  * calculate usage values by different applications
  */
+static void cpu_get_purr(void *arg)
+{
+	atomic64_t *sum = arg;
+
+	atomic64_add(mfspr(SPRN_PURR), sum);
+}
+
 static unsigned long get_purr(void)
 {
-	unsigned long sum_purr = 0;
-	int cpu;
+	atomic64_t purr = ATOMIC64_INIT(0);
 
-	for_each_possible_cpu(cpu) {
-		struct cpu_usage *cu;
+	on_each_cpu(cpu_get_purr, &purr, 1);
 
-		cu = &per_cpu(cpu_usage_array, cpu);
-		sum_purr += cu->current_tb;
-	}
-	return sum_purr;
+	return atomic64_read(&purr);
 }
 
 /*