Commit 46c8f0b0 authored Aug 08, 2016 by Chris Metcalf Committed by Thomas Gleixner Aug 09, 2016

timers: Fix get_next_timer_interrupt() computation



The tick_nohz_stop_sched_tick() routine is not properly
canceling the sched timer when nothing is pending, because
get_next_timer_interrupt() is no longer returning KTIME_MAX in
that case.  This causes periodic interrupts when none are needed.

When determining the next interrupt time, we first use
__next_timer_interrupt() to get the first expiring timer in the
timer wheel.  If no timer is found, we return the base clock value
plus NEXT_TIMER_MAX_DELTA to indicate there is no timer in the
timer wheel.

Back in get_next_timer_interrupt(), we set the "expires" value
by converting the timer wheel expiry (in ticks) to a nsec value.
But we don't want to do this if the timer wheel expiry value
indicates no timer; we want to return KTIME_MAX.

Prior to commit 500462a9 ("timers: Switch to a non-cascading
wheel") we checked base->active_timers to see if any timers
were active, and if not, we didn't touch the expiry value and so
properly returned KTIME_MAX.  Now we don't have active_timers.

To fix this, we now just check the timer wheel expiry value to
see if it is "now + NEXT_TIMER_MAX_DELTA", and if it is, we don't
try to compute a new value based on it, but instead simply let the
KTIME_MAX value in expires remain.

Fixes: 500462a9 "timers: Switch to a non-cascading wheel"
Signed-off-by: Chris Metcalf <cmetcalf@mellanox.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: John Stultz <john.stultz@linaro.org>
Link: http://lkml.kernel.org/r/1470688147-22287-1-git-send-email-cmetcalf@mellanox.com


Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

parent f005bd7e

kernel/time/timer.c

+4 −1

Original line number	Diff line number	Diff line
		@@ -1496,6 +1496,7 @@ u64 get_next_timer_interrupt(unsigned long basej, u64 basem)
		struct timer_base *base = this_cpu_ptr(&timer_bases[BASE_STD]);
		u64 expires = KTIME_MAX;
		unsigned long nextevt;
		bool is_max_delta;

		/*
		* Pretend that there is no timer pending if the cpu is offline.
		@@ -1506,6 +1507,7 @@ u64 get_next_timer_interrupt(unsigned long basej, u64 basem)

		spin_lock(&base->lock);
		nextevt = __next_timer_interrupt(base);
		is_max_delta = (nextevt == base->clk + NEXT_TIMER_MAX_DELTA);
		base->next_expiry = nextevt;
		/*
		* We have a fresh next event. Check whether we can forward the base:
		@@ -1519,6 +1521,7 @@ u64 get_next_timer_interrupt(unsigned long basej, u64 basem)
		expires = basem;
		base->is_idle = false;
		} else {
		if (!is_max_delta)
		expires = basem + (nextevt - basej) * TICK_NSEC;
		/*
		* If we expect to sleep more than a tick, mark the base idle: