Cc: LKML <linux-kernel@...>, Jack Ren <jack.ren@...>, Thomas Gleixner <tglx@...>, Peter Zijlstra <a.p.zijlstra@...>, Dmitry Adamushko <dmitry.adamushko@...>
applied to tip/sched/urgent, thanks Eric.
Thomas, Peter, Dmitry, do you concur with the analysis? (commit below)
It looks a bit ugly to me in the middle of schedule() - is there no wait
to solve this within kernel/time/*.c ?
Ingo
-------------->
commit ca1b5a8a9abb3db57562a838f41cdba842f13fe8
Author: eric miao <eric.y.miao@gmail.com>
Date: Fri Jul 18 14:41:29 2008 +0800
sched: do not stop ticks when cpu is not idle
Issue: the sched tick would be stopped in some race conditions.
One of issues caused by that is:
Since there is no timer ticks any more from then, the jiffies update will be
up to other interrupt to happen. The jiffies will not be updated for a long
time, until next interrupt happens. That will cause APIs like
wait_for_completion_timeout(&complete, timeout) to return timeout by mistake,
since it is using a old jiffies as start time.
Please see comments (1)~(6) inline for how the ticks are stopped
by mistake when cpu is not idle:
void cpu_idle(void)
{
...
while (1) {
void (*idle)(void) = pm_idle;
if (!idle)
idle = default_idle;
leds_event(led_idle_start);
tick_nohz_stop_sched_tick();
while (!need_resched())
idle();
leds_event(led_idle_end);
tick_nohz_restart_sched_tick();
(1) ticks are retarted before switch to other tasks
preempt_enable_no_resched();
schedule();
preempt_disable();
}
}
asmlinkage void __sched schedule(void)
{
...
...
need_resched:
(6) the idle task will be scheduled out again and switch to next task,
with ticks stopped in (5). So the next task will be running with tick stopped.
preempt_disable();
cpu = smp_processor_id();
rq = cpu_rq(cpu);
rcu_qsctr_inc(cpu);
prev = rq->curr;
switch_count = &prev->nivcsw;
release_kernel_lock(prev);
need_resched_nonpreemptible:
schedule_debug(prev);
hrtick_clear(rq);
/*
* Do the rq-clock update outside the rq lock:
*/
local_irq_disable();
__update_rq_clock(rq);
spin_lock(&rq->lock);
clear_tsk_need_resched(prev); (2) resched flag is clear from idle task
....
context_switch(rq, prev, next); /* unlocks the rq */
(3) IRQ will be enabled at end of context_swtich( ).
...
preempt_enable_no_resched();
if (unlikely(test_thread_flag(TIF_NEED_RESCHED)))
(4) the idle task is scheduled back. If an interrupt happen here,
The irq_exit( ) will be called at end of the irq handler.
goto need_resched;
}
void irq_exit(void)
{
...
/* Make sure that timer wheel updates are propagated */
if (!in_interrupt() && idle_cpu(smp_processor_id()) && !need_resched())
tick_nohz_stop_sched_tick();
(5) The ticks will be stopped again since current
task is idle task and its resched flag is clear in (2).
rcu_irq_exit();
preempt_enable_no_resched();
}
Signed-off-by: Jack Ren <jack.ren@marvell.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
---
kernel/sched.c | 3 ++-
1 files changed, 2 insertions(+), 1 deletions(-)
diff --git a/kernel/sched.c b/kernel/sched.c
index 1ee18db..e0e0162 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -4446,7 +4446,8 @@ need_resched_nonpreemptible:
rq->nr_switches++;
rq->curr = next;
++*switch_count;
-
+ if (rq->curr != rq->idle)
+ tick_nohz_restart_sched_tick();
context_switch(rq, prev, next); /* unlocks the rq */
/*
* the context switch might have flipped the stack from under
--
