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SubjectRe: [RFC][PATCH 00/32] Nohz cpusets v2 (adaptive tickless kernel)
From
commit c28c1ce3b410db9c59cc78c403bcbc0f076e25fe
Author: Gilad Ben-Yossef <gilad@benyossef.com>
Date: Mon Feb 13 15:47:24 2012 +0200

timer: make __next_timer_interrupt explicit about no future event

While playing with Frederic's adaptive tick patch set' I've noticed
that no matter what I do, even though I can get the scheduler tick
to turn itself off, I still got an interrupt from the timer once
every few seconds, although it did not run the scheduler tick code.

After poking at it for some time I believe what is happening is as
follows:

1. tick_nohz_stop_sched_tick() [kernel/time/tick-sched.c] calls
get_next_timer_interrupt() [kernel/timer.c] with last_jiffies as
parameter to get the next timer event, which in turn calls
__next_timer_interrupt()

2. next_timer_interrupt() starts with a default expiry time of
(base->timer_jiffies + NEXT_TIMER_MAX_DELTA) and searches for
the next timer event.

3. Having failed to find any, __next_timer_interrupt() returns the
default expiry time, which is returned by get_next_timer_interrupt()
to tick_nohz_stop_sched_tick()

4. tick_nohz_stop_sched_tick() now subtracts the value of last_jiffies
from the return value and checks if the delta is smaller then
NEXT_TIMER_MAX_DELTA, if it does (and multiple other things are
aligned just right...) it cancels the timer interrupt.

5. Alas, base->timer_jiffies and last_jiffies are (usually) not equal,
with the result being that tick_nohz_stop_sched_tick() thinks there
is a timer event pending sometime in the distant future, aprox.
12 days from now(!). It therefore must keep the underlying timer
firing every KTIME_MAX nsecs so that the clocksource will not wrap
around.

The end result is that we get a timer interrupt firing every KTIME_MAX
nsecs even there is no future timer event at all.

The attached patch tries to fix the above, by adding an explicit
boolean return value to __next_timer_interrupt() to indicate whether
or not it found a future timer event and having
get_next_timer_interrupt() return (last_jiffies + NEXT_TIMER_MAX_DELTA)
to indicate there is no timer event, in the same way it does for offline
CPUs.

Signed-off-by: Gilad Ben-Yossef <gilad@benyossef.com>
CC: Thomas Gleixner <tglx@linutronix.de>
Cc: Alessio Igor Bogani <abogani@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Avi Kivity <avi@redhat.com>
Cc: Chris Metcalf <cmetcalf@tilera.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Cc: Geoff Levand <geoff@infradead.org>
Cc: Gilad Ben Yossef <gilad@benyossef.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Max Krasnyansky <maxk@qualcomm.com>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephen Hemminger <shemminger@vyatta.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Sven-Thorsten Dietrich <thebigcorporation@gmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Zen Lin <zen@openhuawei.org>

diff --git a/kernel/timer.c b/kernel/timer.c
index c203297..500b484 100644
--- a/kernel/timer.c
+++ b/kernel/timer.c
@@ -1187,11 +1187,13 @@ static inline void __run_timers(struct tvec_base *base)
* is used on S/390 to stop all activity when a CPU is idle.
* This function needs to be called with interrupts disabled.
*/
-static unsigned long __next_timer_interrupt(struct tvec_base *base)
+static bool __next_timer_interrupt(struct tvec_base *base,
+ unsigned long *next_timer)
{
unsigned long timer_jiffies = base->timer_jiffies;
unsigned long expires = timer_jiffies + NEXT_TIMER_MAX_DELTA;
- int index, slot, array, found = 0;
+ int index, slot, array;
+ bool found = false;
struct timer_list *nte;
struct tvec *varray[4];

@@ -1202,12 +1204,12 @@ static unsigned long
__next_timer_interrupt(struct tvec_base *base)
if (tbase_get_deferrable(nte->base))
continue;

- found = 1;
+ found = true;
expires = nte->expires;
/* Look at the cascade bucket(s)? */
if (!index || slot < index)
goto cascade;
- return expires;
+ goto out;
}
slot = (slot + 1) & TVR_MASK;
} while (slot != index);
@@ -1233,7 +1235,7 @@ cascade:
if (tbase_get_deferrable(nte->base))
continue;

- found = 1;
+ found = true;
if (time_before(nte->expires, expires))
expires = nte->expires;
}
@@ -1245,7 +1247,7 @@ cascade:
/* Look at the cascade bucket(s)? */
if (!index || slot < index)
break;
- return expires;
+ goto out;
}
slot = (slot + 1) & TVN_MASK;
} while (slot != index);
@@ -1254,7 +1256,10 @@ cascade:
timer_jiffies += TVN_SIZE - index;
timer_jiffies >>= TVN_BITS;
}
- return expires;
+out:
+ if(found)
+ *next_timer = expires;
+ return found;
}

/*
@@ -1317,9 +1322,15 @@ unsigned long get_next_timer_interrupt(unsigned long now)
if (cpu_is_offline(smp_processor_id()))
return now + NEXT_TIMER_MAX_DELTA;
spin_lock(&base->lock);
- if (time_before_eq(base->next_timer, base->timer_jiffies))
- base->next_timer = __next_timer_interrupt(base);
- expires = base->next_timer;
+ if (time_before_eq(base->next_timer, base->timer_jiffies)) {
+
+ if(__next_timer_interrupt(base, &expires))
+ base->next_timer = expires;
+ else
+ expires = now + NEXT_TIMER_MAX_DELTA;
+ } else
+ expires = base->next_timer;
+
spin_unlock(&base->lock);

if (time_before_eq(expires, now))

--
Gilad Ben-Yossef
Chief Coffee Drinker
gilad@benyossef.com
Israel Cell: +972-52-8260388
US Cell: +1-973-8260388
http://benyossef.com

"If you take a class in large-scale robotics, can you end up in a
situation where the homework eats your dog?"
 -- Jean-Baptiste Queru
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