linux-kernel - [PATCH 02/25] sched/vtime: Protect idle accounting under vtime seqcount

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Message-Id: <1542163569-20047-3-git-send-email-frederic@kernel.org>
Date:   Wed, 14 Nov 2018 03:45:46 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     LKML <linux-kernel@...r.kernel.org>
Cc:     Frederic Weisbecker <frederic@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Wanpeng Li <wanpengli@...cent.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Yauheni Kaliuta <yauheni.kaliuta@...hat.com>,
        Ingo Molnar <mingo@...nel.org>, Rik van Riel <riel@...hat.com>
Subject: [PATCH 02/25] sched/vtime: Protect idle accounting under vtime seqcount

Locking the seqcount on idle vtime accounting wasn't thought to be
necessary because the readers of idle cputime don't use vtime (yet).

Now updating vtime expect the related seqcount to be locked so do it
for locking coherency purposes.

Also idle cputime updates use vtime, but idle cputime readers use the
traditional ad-hoc nohz time delta. We may want to consider moving
readers to use vtime to consolidate the overall accounting scheme. The
seqcount will be a functional requirement for it.

Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
Cc: Yauheni Kaliuta <yauheni.kaliuta@...hat.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Rik van Riel <riel@...hat.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Wanpeng Li <wanpengli@...cent.com>
Cc: Ingo Molnar <mingo@...nel.org>
---
 kernel/sched/cputime.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
index 54eb945..6e74ec2 100644
--- a/kernel/sched/cputime.c
+++ b/kernel/sched/cputime.c
@@ -800,7 +800,11 @@ EXPORT_SYMBOL_GPL(vtime_guest_exit);
 
 void vtime_account_idle(struct task_struct *tsk)
 {
+	struct vtime *vtime = &tsk->vtime;
+
+	write_seqcount_begin(&vtime->seqcount);
 	account_idle_time(get_vtime_delta(&tsk->vtime));
+	write_seqcount_end(&vtime->seqcount);
 }
 
 void arch_vtime_task_switch(struct task_struct *prev)
-- 
2.7.4