lkml.org 
[lkml]   [2019]   [Dec]   [17]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
Date
From
Subject[tip: sched/core] schied/fair: Skip calculating @contrib without load
The following commit has been merged into the sched/core branch of tip:

Commit-ID: d040e0734fb3dedfe24c3d94f5a32b4812eca610
Gitweb: https://git.kernel.org/tip/d040e0734fb3dedfe24c3d94f5a32b4812eca610
Author: Peng Wang <rocking@linux.alibaba.com>
AuthorDate: Fri, 13 Dec 2019 11:45:40 +08:00
Committer: Peter Zijlstra <peterz@infradead.org>
CommitterDate: Tue, 17 Dec 2019 13:32:51 +01:00

schied/fair: Skip calculating @contrib without load

Because of the:

if (!load)
runnable = running = 0;

clause in ___update_load_sum(), all the actual users of @contrib in
accumulate_sum():

if (load)
sa->load_sum += load * contrib;
if (runnable)
sa->runnable_load_sum += runnable * contrib;
if (running)
sa->util_sum += contrib << SCHED_CAPACITY_SHIFT;

don't happen, and therefore we don't care what @contrib actually is and
calculating it is pointless.

If we count the times when @load equals zero and not as below:

if (load) {
load_is_not_zero_count++;
contrib = __accumulate_pelt_segments(periods,
1024 - sa->period_contrib,delta);
} else
load_is_zero_count++;

As we can see, load_is_zero_count is much bigger than
load_is_zero_count, and the gap is gradually widening:

load_is_zero_count: 6016044 times
load_is_not_zero_count: 244316 times
19:50:43 up 1 min, 1 user, load average: 0.09, 0.06, 0.02

load_is_zero_count: 7956168 times
load_is_not_zero_count: 261472 times
19:51:42 up 2 min, 1 user, load average: 0.03, 0.05, 0.01

load_is_zero_count: 10199896 times
load_is_not_zero_count: 278364 times
19:52:51 up 3 min, 1 user, load average: 0.06, 0.05, 0.01

load_is_zero_count: 14333700 times
load_is_not_zero_count: 318424 times
19:54:53 up 5 min, 1 user, load average: 0.01, 0.03, 0.00

Perhaps we can gain some performance advantage by saving these
unnecessary calculation.

Signed-off-by: Peng Wang <rocking@linux.alibaba.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Vincent Guittot < vincent.guittot@linaro.org>
Link: https://lkml.kernel.org/r/1576208740-35609-1-git-send-email-rocking@linux.alibaba.com
---
kernel/sched/pelt.c | 20 +++++++++++++++++---
1 file changed, 17 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/pelt.c b/kernel/sched/pelt.c
index a96db50..bd006b7 100644
--- a/kernel/sched/pelt.c
+++ b/kernel/sched/pelt.c
@@ -129,8 +129,20 @@ accumulate_sum(u64 delta, struct sched_avg *sa,
* Step 2
*/
delta %= 1024;
- contrib = __accumulate_pelt_segments(periods,
- 1024 - sa->period_contrib, delta);
+ if (load) {
+ /*
+ * This relies on the:
+ *
+ * if (!load)
+ * runnable = running = 0;
+ *
+ * clause from ___update_load_sum(); this results in
+ * the below usage of @contrib to dissapear entirely,
+ * so no point in calculating it.
+ */
+ contrib = __accumulate_pelt_segments(periods,
+ 1024 - sa->period_contrib, delta);
+ }
}
sa->period_contrib = delta;

@@ -205,7 +217,9 @@ ___update_load_sum(u64 now, struct sched_avg *sa,
* This means that weight will be 0 but not running for a sched_entity
* but also for a cfs_rq if the latter becomes idle. As an example,
* this happens during idle_balance() which calls
- * update_blocked_averages()
+ * update_blocked_averages().
+ *
+ * Also see the comment in accumulate_sum().
*/
if (!load)
runnable = running = 0;
\
 
 \ /
  Last update: 2019-12-17 13:41    [W:0.113 / U:26.028 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site