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

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Message-ID: <CAKohpo=L8ZiR67OR0Sz0LeGdRxn92rfbJbsHd-ybt==MwfQJ3Q@mail.gmail.com>
Date:	Fri, 28 Mar 2014 16:13:18 +0530
From:	Viresh Kumar <viresh.kumar@...aro.org>
To:	Thomas Gleixner <tglx@...utronix.de>
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: [Query]: hrtimer: Does switch_hrtimer_base() need to check hrtimer_check_target()?

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() ??

--
viresh
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