[RFC 0/7] hrtimer: drop active hrtimer checks after adding it

Viresh Kumar
2014-07-09T06:55:50Z

hrtimer_start*() family never fails to enqueue a hrtimer to a clock-base. The
only special case is when the hrtimer was in past. If it is getting enqueued to
local CPUs's clock-base, we raise a softirq and exit, else we handle that on
next interrupt on remote CPU.

At several places in the kernel, we try to make sure if hrtimer was added
properly or not by calling hrtimer_active(), like:

hrtimer_start(timer, expires, mode);
if (hrtimer_active(timer)) {
/* Added successfully */
} else {
/* Was added in the past */
}

As hrtimer_start*() never fails, hrtimer_active() is guaranteed to return '1'.
So, there is no point calling hrtimer_active().

First patch adds a WARN_ON_ONCE() to __hrtimer_start_range_ns() to make sure
hrtimers are always enqueued from it. Next 6 patches update several parts of
kernel to drop calls to hrtimer_active() after starting a hrtimer.

Rebased over 3.16-rc4 and pushed here:
git://git.kernel.org/pub/scm/linux/kernel/git/vireshk/linux.git hrtimer/drop-hrtimer-active-calls

Cc: Darren Hart <dvhart@linux.intel.com>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: netdev@vger.kernel.org
Cc: Peter Zijlstra <peterz@infradead.org>

Viresh Kumar (7):
hrtimer: Warn if hrtimer_start*() failed to enqueue hrtimer
hrtimer: don't check for active hrtimer after adding it
tick: don't check for active hrtimer after adding it
sched: don't check for active hrtimer after adding it
futex: don't check for active hrtimer after adding it
rtmutex: don't check for active hrtimer after adding it
net: don't check for active hrtimer after adding it

kernel/futex.c | 5 +----
kernel/hrtimer.c | 6 ++----
kernel/locking/rtmutex.c | 5 +----
kernel/sched/core.c | 20 +++++++++-----------
kernel/sched/deadline.c | 2 +-
kernel/time/tick-sched.c | 45 ++++++++++++++++++---------------------------
net/core/pktgen.c | 2 --
7 files changed, 32 insertions(+), 53 deletions(-)

--
2.0.0.rc2