linux-kernel - Re: [Query]: hrtimer: Does switch_hrtimer_base() need to check hrtimer_check

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Date:	Mon, 31 Mar 2014 15:11:56 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Viresh Kumar <viresh.kumar@...aro.org>
cc:	Frédéric Weisbecker <fweisbec@...il.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Lists linaro-kernel <linaro-kernel@...ts.linaro.org>,
	Linaro Networking <linaro-networking@...aro.org>
Subject: Re: [Query]: hrtimer: Does switch_hrtimer_base() need to check
 hrtimer_check_target()?

On Fri, 28 Mar 2014, Viresh Kumar wrote:

> Hi Thomas,
> 
> I couldn't understand one part of hrtimer (obviously there are
> others as well, but here I am sure I couldn't understood it :)).
> 
> In switch_hrtimer_base() we are calling hrtimer_check_target()
> which guarantees this:
> 
> /*
>  * With HIGHRES=y we do not migrate the timer when it is expiring
>  * before the next event on the target cpu because we cannot reprogram
>  * the target cpu hardware and we would cause it to fire late.
>  *
>  * Called with cpu_base->lock of target cpu held.
>  */
> 
> But switch_hrtimer_base() is only called from one place, i.e.
> __hrtimer_start_range_ns() and at the point (where we call
> switch_hrtimer_base()) expiration time is not yet known as
> we call this routine later:
> 
> hrtimer_set_expires_range_ns()
> 
> So, do we really need to call hrtimer_check_target() at all
> in switch_hrtimer_base()? Or do we need to move
> hrtimer_set_expires_range_ns() before calling
> switch_hrtimer_base() ??

Yes, we should move setting the expiry time before we call
switch_hrtimer_base().

Thanks,

	tglx
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