linux-kernel - [PATCH 1/1] sched: cfs_rq h_load might not update due to irq disable

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Message-ID: <1574325009-10846-1-git-send-email-yt.chang@mediatek.com>
Date:   Thu, 21 Nov 2019 16:30:09 +0800
From:   YT Chang <yt.chang@...iatek.com>
To:     YT Chang <yt.chang@...iatek.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Matthias Brugger <matthias.bgg@...il.com>
CC:     <wsd_upstream@...iatek.com>, <linux-kernel@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-mediatek@...ts.infradead.org>
Subject: [PATCH 1/1] sched: cfs_rq h_load might not update due to irq disable

Syndrome:

Two CPUs might do idle balance in the same time.
One CPU does idle balance and pulls some tasks.
However before pick next task, ALL task are pulled back to other CPU.
That results in infinite loop in both CPUs.

=========================================
code flow:

in pick_next_task_fair()

again:

if nr_running == 0
	goto idle
pick next task
	return

idle:
	idle_balance
       /* pull some tasks from other CPU,
        * However other CPU are also do idle balance,
	* and pull back these task */

	go to again

=========================================
The result to pull ALL tasks back when the task_h_load
is incorrect and too low.

static unsigned long task_h_load(struct task_struct *p)
{
        struct cfs_rq *cfs_rq = task_cfs_rq(p);

	update_cfs_rq_h_load(cfs_rq);
	return div64_ul(p->se.avg.load_avg_contrib * cfs_rq->h_load,
			cfs_rq->runnable_load_avg + 1);
}

The cfs_rq->h_load is incorrect and might too small.
The original idea of cfs_rq::last_h_load_update will not
update cfs_rq::h_load more than once a jiffies.
When the Two CPUs pull each other in the pick_next_task_fair,
the irq disabled and result in jiffie not update.
(Other CPUs wait for runqueue lock locked by the two CPUs.
So, ALL CPUs are irq disabled.)

Solution:
cfs_rq h_load might not update due to irq disable
use sched_clock instead jiffies

Signed-off-by: YT Chang <yt.chang@...iatek.com>
---
 kernel/sched/fair.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 83ab35e..231c53f 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7578,9 +7578,11 @@ static void update_cfs_rq_h_load(struct cfs_rq *cfs_rq)
 {
 	struct rq *rq = rq_of(cfs_rq);
 	struct sched_entity *se = cfs_rq->tg->se[cpu_of(rq)];
-	unsigned long now = jiffies;
+	u64 now = sched_clock_cpu(cpu_of(rq));
 	unsigned long load;
 
+	now = now * HZ >> 30;
+
 	if (cfs_rq->last_h_load_update == now)
 		return;
 
-- 
1.9.1