linux-kernel - Re: [PATCH] ARM: timer: Shutdown clock event device when stopping local timer

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Date:	Sat, 30 Mar 2013 17:57:38 +0800
From:	Ning Jiang <ning.n.jiang@...il.com>
To:	Stephen Boyd <sboyd@...eaurora.org>
Cc:	linux@....linux.org.uk, kgene.kim@...sung.com,
	davidb@...eaurora.org, dwalker@...o99.com, bryanh@...eaurora.org,
	john.stultz@...aro.org, tglx@...utronix.de,
	linus.walleij@...aro.org, shawn.guo@...aro.org,
	rob.herring@...xeda.com, arnd@...db.de,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	linux-samsung-soc@...r.kernel.org, linux-arm-msm@...r.kernel.org
Subject: Re: [PATCH] ARM: timer: Shutdown clock event device when stopping
 local timer

2013/3/30 Stephen Boyd <sboyd@...eaurora.org>:
> On 03/29/13 02:24, ning.n.jiang@...il.com wrote:
>> From: Ning Jiang <ning.n.jiang@...il.com>
>>
>> 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.

>>
>> 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)

> --
> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
> hosted by The Linux Foundation
>
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