linux-kernel - Re: [PATCH 4.12 178/196] sched/cputime: Accumulate vtime on top of nsec clocksource

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Message-ID: <20170726142120.bbdfhykzlcujv6pv@techsingularity.net>
Date:   Wed, 26 Jul 2017 15:21:20 +0100
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Luiz Capitulino <lcapitulino@...hat.com>,
        Wanpeng Li <wanpeng.li@...mail.com>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Rik van Riel <riel@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Wanpeng Li <kernellwp@...il.com>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH 4.12 178/196] sched/cputime: Accumulate vtime on top of
 nsec clocksource

I noticed relatively late and it hadn't occured on my own testing. I can
send it as a separate patch but I wanted to highlight that it would affect
a 4.12.4 release.

---8<---
sched/cputime: Don't use smp_processor_id() in preemptible context

commit 0e4097c3354e2f5a5ad8affd9dc7f7f7d00bb6b9 upstream

Recent kernels trigger this warning:

 BUG: using smp_processor_id() in preemptible [00000000] code: 99-trinity/181
 caller is debug_smp_processor_id+0x17/0x19
 CPU: 0 PID: 181 Comm: 99-trinity Not tainted 4.12.0-01059-g2a42eb9 #1
 Call Trace:
  dump_stack+0x82/0xb8
  check_preemption_disabled()
  debug_smp_processor_id()
  vtime_delta()
  task_cputime()
  thread_group_cputime()
  thread_group_cputime_adjusted()
  wait_consider_task()
  do_wait()
  SYSC_wait4()
  do_syscall_64()
  entry_SYSCALL64_slow_path()

As Frederic pointed out:

| Although those sched_clock_cpu() things seem to only matter when the
| sched_clock() is unstable. And that stability is a condition for nohz_full
| to work anyway. So probably sched_clock() alone would be enough.

This patch fixes it by replacing sched_clock_cpu() with sched_clock() to
avoid calling smp_processor_id() in a preemptible context.

Reported-by: Xiaolong Ye <xiaolong.ye@...el.com>
Signed-off-by: Wanpeng Li <wanpeng.li@...mail.com>
Cc: Frederic Weisbecker <fweisbec@...il.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Luiz Capitulino <lcapitulino@...hat.com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Rik van Riel <riel@...hat.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Link: http://lkml.kernel.org/r/1499586028-7402-1-git-send-email-wanpeng.li@hotmail.com
[ Prettified the changelog. ]
Signed-off-by: Ingo Molnar <mingo@...nel.org>
Signed-off-by: Mel Gorman <mgorman@...e.de>

diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
index 3dafea579263..ad275b25e3f9 100644
--- a/kernel/sched/cputime.c
+++ b/kernel/sched/cputime.c
@@ -683,7 +683,7 @@ static u64 vtime_delta(struct vtime *vtime)
 {
 	unsigned long long clock;

-	clock = sched_clock_cpu(smp_processor_id());
+	clock = sched_clock();
 	if (clock < vtime->starttime)
 		return 0;

@@ -814,7 +814,7 @@ void arch_vtime_task_switch(struct task_struct *prev)

 	write_seqcount_begin(&vtime->seqcount);
 	vtime->state = VTIME_SYS;
-	vtime->starttime = sched_clock_cpu(smp_processor_id());
+	vtime->starttime = sched_clock();
 	write_seqcount_end(&vtime->seqcount);
 }

@@ -826,7 +826,7 @@ void vtime_init_idle(struct task_struct *t, int cpu)
 	local_irq_save(flags);
 	write_seqcount_begin(&vtime->seqcount);
 	vtime->state = VTIME_SYS;
-	vtime->starttime = sched_clock_cpu(cpu);
+	vtime->starttime = sched_clock();
 	write_seqcount_end(&vtime->seqcount);
 	local_irq_restore(flags);
 }