2013/3/30 Stephen Boyd [off-list ref]:

On 03/29/13 02:24, ning.n.jiang at gmail.com wrote:

quoted

From: Ning Jiang <redacted>

Currently there are two problems when we try to stop local timer.
First, it calls set_mode function directly so mode state is not
updated for the clock event device. Second, it makes the device
unused instead of shutdown.

What device is this a problem on? I believe this only matters to drivers
which enable their timer in their set_next_event() callback? But even
then, does anything actually happen because the interrupt should have
been disabled in the local timer stop callback.

Right. Drivers which enable timer in set_next_event() will have this problem.
It will not have functional problem in my case. But my device cannot enter
low power mode with a pending interrupt even if it is disabled.

quoted

A subtle error will happen because of it. When a cpu is plugged out
it will stop the local timer. It will call tick_nohz_idle_enter()
in idle thread afterwards. It will cancel the sched timer and try
to reprogram the next event. This is wrong since the local timer
is supposed to be stopped.

The right way to stop the local timer is to shutdown it by calling
clockevents_set_mode(). Thus when we try to reprogram the clock
event device, it will return directly without doing anything since
the clock mode is CLOCK_EVT_MODE_SHUTDOWN.

While this prevents the set_next_event() callback from being called on a
dying CPU, wouldn't it be better to fix this problem in the core code
once instead of fixing it many times in each local timer driver? It
doesn't seem to make much sense to program an event on a CPU that is
about to die, so why do we do that?

Actually, I was trying to fix it in the core code like this, but I
thought it is not
that good and we still need to fix the local timer driver problem even
with this fix.

diff --git a/kernel/time/clockevents.c b/kernel/time/clockevents.c
index c6d6400..e22e268 100644
--- a/kernel/time/clockevents.c
+++ b/kernel/time/clockevents.c

@@ -210,6 +210,9 @@ int clockevents_program_event(struct

clock_event_device *dev, ktime_t expires,
                return -ETIME;
        }

+       if (cpu_is_offline(smp_processor_id()))
+               return 0;
+
        dev->next_event = expires;

        if (dev->mode == CLOCK_EVT_MODE_SHUTDOWN)

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