linux-kernel - [tip: sched/urgent] sched/core: Accounting forceidle time for all tasks except idle task

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Date:   Tue, 18 Jan 2022 11:18:11 -0000
From:   "tip-bot2 for Cruz Zhao" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     Cruz Zhao <CruzZhao@...ux.alibaba.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Josh Don <joshdon@...gle.com>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: [tip: sched/urgent] sched/core: Accounting forceidle time for all
 tasks except idle task

The following commit has been merged into the sched/urgent branch of tip:

Commit-ID:     b171501f258063f5c56dd2c5fdf310802d8d7dc1
Gitweb:        https://git.kernel.org/tip/b171501f258063f5c56dd2c5fdf310802d8d7dc1
Author:        Cruz Zhao <CruzZhao@...ux.alibaba.com>
AuthorDate:    Tue, 11 Jan 2022 17:55:59 +08:00
Committer:     Peter Zijlstra <peterz@...radead.org>
CommitterDate: Tue, 18 Jan 2022 12:09:59 +01:00

sched/core: Accounting forceidle time for all tasks except idle task

There are two types of forced idle time: forced idle time from cookie'd
task and forced idle time form uncookie'd task. The forced idle time from
uncookie'd task is actually caused by the cookie'd task in runqueue
indirectly, and it's more accurate to measure the capacity loss with the
sum of both.

Assuming cpu x and cpu y are a pair of SMT siblings, consider the
following scenarios:
  1.There's a cookie'd task running on cpu x, and there're 4 uncookie'd
    tasks running on cpu y. For cpu x, there will be 80% forced idle time
    (from uncookie'd task); for cpu y, there will be 20% forced idle time
    (from cookie'd task).
  2.There's a uncookie'd task running on cpu x, and there're 4 cookie'd
    tasks running on cpu y. For cpu x, there will be 80% forced idle time
    (from cookie'd task); for cpu y, there will be 20% forced idle time
    (from uncookie'd task).

The scenario1 can recurrent by stress-ng(scenario2 can recurrent similary):
    (cookie'd)taskset -c x stress-ng -c 1 -l 100
    (uncookie'd)taskset -c y stress-ng -c 4 -l 100

In the above two scenarios, the total capacity loss is 1 cpu, but in
scenario1, the cookie'd forced idle time tells us 20% cpu capacity loss, in
scenario2, the cookie'd forced idle time tells us 80% cpu capacity loss,
which are not accurate. It'll be more accurate to measure with cookie'd
forced idle time and uncookie'd forced idle time.

Signed-off-by: Cruz Zhao <CruzZhao@...ux.alibaba.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Reviewed-by: Josh Don <joshdon@...gle.com>
Link: https://lore.kernel.org/r/1641894961-9241-2-git-send-email-CruzZhao@linux.alibaba.com
---
 kernel/sched/core.c       | 3 +--
 kernel/sched/core_sched.c | 2 +-
 2 files changed, 2 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 83872f9..0d2ab2a 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -5822,8 +5822,7 @@ pick_next_task(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
 	}
 
 	if (schedstat_enabled() && rq->core->core_forceidle_count) {
-		if (cookie)
-			rq->core->core_forceidle_start = rq_clock(rq->core);
+		rq->core->core_forceidle_start = rq_clock(rq->core);
 		rq->core->core_forceidle_occupation = occ;
 	}
 
diff --git a/kernel/sched/core_sched.c b/kernel/sched/core_sched.c
index 1fb4567..c8746a9 100644
--- a/kernel/sched/core_sched.c
+++ b/kernel/sched/core_sched.c
@@ -277,7 +277,7 @@ void __sched_core_account_forceidle(struct rq *rq)
 		rq_i = cpu_rq(i);
 		p = rq_i->core_pick ?: rq_i->curr;
 
-		if (!p->core_cookie)
+		if (p == rq_i->idle)
 			continue;
 
 		__schedstat_add(p->stats.core_forceidle_sum, delta);