On Thu, Jun 28, 2018 at 06:29:41PM +0200, Frederic Weisbecker wrote:
Some people have reported that the warning in sched_tick_remote() occasionally triggers, especially in favour of some RCU-Torture pressure:
WARNING: CPU: 11 PID: 906 at kernel/sched/core.c:3138 sched_tick_remote+0xb6/0xc0 Modules linked in: CPU: 11 PID: 906 Comm: kworker/u32:3 Not tainted 4.18.0-rc2+ #1 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.10.2-1 04/01/2014 Workqueue: events_unbound sched_tick_remote RIP: 0010:sched_tick_remote+0xb6/0xc0 Code: e8 0f 06 b8 00 c6 03 00 fb eb 9d 8b 43 04 85 c0 75 8d 48 8b 83 e0 0a 00 00 48 85 c0 75 81 eb 88 48 89 df e8 bc fe ff ff eb aa <0f> 0b eb +c5 66 0f 1f 44 00 00 bf 17 00 00 00 e8 b6 2e fe ff 0f b6 Call Trace: process_one_work+0x1df/0x3b0 worker_thread+0x44/0x3d0 kthread+0xf3/0x130 ? set_worker_desc+0xb0/0xb0 ? kthread_create_worker_on_cpu+0x70/0x70 ret_from_fork+0x35/0x40
This happens when the remote tick applies on an idle task. Usually the idle_cpu() check avoids that, but it is performed before we lock the runqueue and it is therefore racy. It was intended to be that way in order to prevent from useless runqueue locks since idle task tick callback is a no-op.
Now if the racy check slips out of our hands and we end up remotely ticking an idle task, the empty task_tick_idle() is harmless. Still it won't pass the WARN_ON_ONCE() test that ensures rq_clock_task() is not too far from curr->se.exec_start because update_curr_idle() doesn't update the exec_start value like other scheduler policies. Hence the reported false positive.
So let's have another check, while the rq is locked, to make sure we don't remote tick on an idle task. The lockless idle_cpu() still applies to avoid unecessary rq lock contention.
Reported-by: Jacek Tomaka jacekt@dug.com Reported-by: Paul E. McKenney paulmck@linux.vnet.ibm.com Reported-by: Anna-Maria Gleixner anna-maria@linutronix.de Cc: Thomas Gleixner tglx@linutronix.de Cc: Peter Zijlstra peterz@infradead.org Cc: Ingo Molnar mingo@kernel.org Cc: stable@vger.kernel.org Signed-off-by: Frederic Weisbecker frederic@kernel.org
Tested-by: Paul E. McKenney paulmck@linux.vnet.ibm.com
kernel/sched/core.c | 18 ++++++++++-------- 1 file changed, 10 insertions(+), 8 deletions(-)
diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 78d8fac..da8f121 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -3127,16 +3127,18 @@ static void sched_tick_remote(struct work_struct *work) u64 delta;
rq_lock_irq(rq, &rf);
update_rq_clock(rq);delta = rq_clock_task(rq) - curr->se.exec_start;
if (!is_idle_task(curr)) {update_rq_clock(rq);delta = rq_clock_task(rq) - curr->se.exec_start;
/** Make sure the next tick runs within a reasonable* amount of time.*/WARN_ON_ONCE(delta > (u64)NSEC_PER_SEC * 3);curr->sched_class->task_tick(rq, curr, 0);
/** Make sure the next tick runs within a reasonable* amount of time.*/WARN_ON_ONCE(delta > (u64)NSEC_PER_SEC * 3);curr->sched_class->task_tick(rq, curr, 0);}-- 2.7.4