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Message-ID: <alpine.LFD.2.00.0905022138010.3375@localhost.localdomain>
Date: Sat, 2 May 2009 21:54:49 +0200 (CEST)
From: Thomas Gleixner <tglx@...utronix.de>
To: Andrew Morton <akpm@...ux-foundation.org>
cc: LKML <linux-kernel@...r.kernel.org>,
Dimitri Sivanich <sivanich@....com>,
Ingo Molnar <mingo@...e.hu>,
Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [patch 2/3] timer: move calc_load to softirq
On Sat, 2 May 2009, Andrew Morton wrote:
> > + spin_lock(&avenrun_lock);
> > + ticks = atomic_read(&avenrun_ticks);
> > + if (ticks >= LOAD_FREQ) {
> > + atomic_sub(LOAD_FREQ, &avenrun_ticks);
> > + calc_global_load();
> > }
> > + spin_unlock(&avenrun_lock);
> > + *calc = 0;
> > +}
>
> I wonder if we really really need avenrun_lock. Various bits of code
> (eg net/sched/em_meta.c) cheerily read avenrun[] without locking.
I don't care about the reader side anyway, the lock is just there to
protect the calc_load update from two cpus, but that's probably
paranoia.
Though, there is a theoretical race between 2 cpus which might want to
update avenrun_ticks in the NOHZ case, but thinking more about it we
can just prevent this by clever usage of the atomic ops on
avenrun_ticks.
Thanks,
tglx
----------->
Subject: timer: move calc_load to softirq
From: Thomas Gleixner <tglx@...utronix.de>
Date: Sat, 2 May 2009 19:43:41 +0200
xtime_lock is held write locked across calc_load() which iterates over
all online CPUs. That can cause long latencies for xtime_lock readers
on large SMP systems. The load average calculation is an rough
estimate anyway so there is no real need to protect the readers
vs. the update. It's not a problem when the avenrun array is updated
while a reader copies the values.
Move the calculation to the softirq and reduce the xtime_lock write
locked section. This also reduces the interrupts off section.
Inspired by an inital patch from Dimitri Sivanich.
Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
---
kernel/time/timekeeping.c | 2 -
kernel/timer.c | 57 ++++++++++++++++++++++++++++++++--------------
2 files changed, 41 insertions(+), 18 deletions(-)
Index: linux-2.6/kernel/time/timekeeping.c
===================================================================
--- linux-2.6.orig/kernel/time/timekeeping.c
+++ linux-2.6/kernel/time/timekeeping.c
@@ -22,7 +22,7 @@
/*
* This read-write spinlock protects us from races in SMP while
- * playing with xtime and avenrun.
+ * playing with xtime.
*/
__cacheline_aligned_in_smp DEFINE_SEQLOCK(xtime_lock);
Index: linux-2.6/kernel/timer.c
===================================================================
--- linux-2.6.orig/kernel/timer.c
+++ linux-2.6/kernel/timer.c
@@ -1127,12 +1127,13 @@ void update_process_times(int user_tick)
* imply that avenrun[] is the standard name for this kind of thing.
* Nothing else seems to be standardized: the fractional size etc
* all seem to differ on different machines.
- *
- * Requires xtime_lock to access.
*/
unsigned long avenrun[3];
EXPORT_SYMBOL(avenrun);
+static atomic_t avenrun_ticks;
+static DEFINE_PER_CPU(int, avenrun_calculate);
+
static unsigned long
calc_load(unsigned long load, unsigned long exp, unsigned long active)
{
@@ -1143,23 +1144,44 @@ calc_load(unsigned long load, unsigned l
/*
* calc_load - given tick count, update the avenrun load estimates.
- * This is called while holding a write_lock on xtime_lock.
*/
-static void calc_global_load(unsigned long ticks)
+static void calc_global_load(void)
{
- unsigned long active_tasks; /* fixed-point */
- static int count = LOAD_FREQ;
+ unsigned long active_tasks = nr_active() * FIXED_1;
- count -= ticks;
- if (unlikely(count < 0)) {
- active_tasks = nr_active() * FIXED_1;
- do {
- avenrun[0] = calc_load(avenrun[0], EXP_1, active_tasks);
- avenrun[1] = calc_load(avenrun[1], EXP_5, active_tasks);
- avenrun[2] = calc_load(avenrun[2], EXP_15, active_tasks);
- count += LOAD_FREQ;
- } while (count < 0);
- }
+ avenrun[0] = calc_load(avenrun[0], EXP_1, active_tasks);
+ avenrun[1] = calc_load(avenrun[1], EXP_5, active_tasks);
+ avenrun[2] = calc_load(avenrun[2], EXP_15, active_tasks);
+}
+
+/*
+ * Check whether do_timer has set avenrun_calculate. The variable is
+ * cpu local so we avoid cache line bouncing of avenrun_ticks.
+ */
+static void check_calc_load(void)
+{
+ int ticks, *calc = &__get_cpu_var(avenrun_calculate);
+
+ if (!*calc)
+ return;
+
+ ticks = atomic_sub_return(LOAD_FREQ, &avenrun_ticks);
+ if (ticks >= 0)
+ calc_global_load();
+ else
+ atomic_add(LOAD_FREQ, &avenrun_ticks);
+ *calc = 0;
+}
+
+/*
+ * Update avenrun_ticks and trigger the load calculation when the
+ * result is >= LOAD_FREQ.
+ */
+static void calc_load_update(unsigned long ticks)
+{
+ ticks = atomic_add_return(ticks, &avenrun_ticks);
+ if (ticks >= LOAD_FREQ)
+ __get_cpu_var(avenrun_calculate) = 1;
}
/*
@@ -1169,6 +1191,7 @@ static void run_timer_softirq(struct sof
{
struct tvec_base *base = __get_cpu_var(tvec_bases);
+ check_calc_load();
hrtimer_run_pending();
if (time_after_eq(jiffies, base->timer_jiffies))
@@ -1192,7 +1215,7 @@ void run_local_timers(void)
static inline void update_times(unsigned long ticks)
{
update_wall_time();
- calc_global_load(ticks);
+ calc_load_update(ticks);
}
/*
--
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