[ 47/47] sched: Fix nohz load accounting -- again!

Greg KH
2012-05-04T20:51:09Z

[-- Warning: decoded text below may be mangled, UTF-8 assumed --]
[-- Attachment #1: Type: text/plain, Size: 4170 bytes --]

3.0-stable review patch. If anyone has any objections, please let me know.

------------------

From: Peter Zijlstra <peterz@infradead.org>

commit c308b56b5398779cd3da0f62ab26b0453494c3d4 upstream.
[ backported to 3.0 by Kerin Millar <kerframil@gmail.com>]

Various people reported nohz load tracking still being wrecked, but Doug
spotted the actual problem. We fold the nohz remainder in too soon,
causing us to loose samples and under-account.

So instead of playing catch-up up-front, always do a single load-fold
with whatever state we encounter and only then fold the nohz remainder
and play catch-up.

Reported-by: Doug Smythies <dsmythies@telus.net>
Reported-by: LesÅ=82aw Kope=C4=87 <leslaw.kopec@nasza-klasa.pl>
Reported-by: Aman Gupta <aman@tmm1.net>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/n/tip-4v31etnhgg9kwd6ocgx3rxl8@git.kernel.org
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Cc: Kerin Millar <kerframil@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
kernel/sched.c | 53 ++++++++++++++++++++++++++---------------------------
1 file changed, 26 insertions(+), 27 deletions(-)

--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -3392,13 +3392,10 @@ calc_load_n(unsigned long load, unsigned
* Once we've updated the global active value, we need to apply the exponential
* weights adjusted to the number of cycles missed.
*/
-static void calc_global_nohz(unsigned long ticks)
+static void calc_global_nohz(void)
{
long delta, active, n;

- if (time_before(jiffies, calc_load_update))
- return;
-
/*
* If we crossed a calc_load_update boundary, make sure to fold
* any pending idle changes, the respective CPUs might have
@@ -3410,31 +3407,25 @@ static void calc_global_nohz(unsigned lo
atomic_long_add(delta, &calc_load_tasks);

/*
- * If we were idle for multiple load cycles, apply them.
+ * It could be the one fold was all it took, we done!
*/
- if (ticks >= LOAD_FREQ) {
- n = ticks / LOAD_FREQ;
+ if (time_before(jiffies, calc_load_update + 10))
+ return;

- active = atomic_long_read(&calc_load_tasks);
- active = active > 0 ? active * FIXED_1 : 0;
+ /*
+ * Catch-up, fold however many we are behind still
+ */
+ delta = jiffies - calc_load_update - 10;
+ n = 1 + (delta / LOAD_FREQ);

- avenrun[0] = calc_load_n(avenrun[0], EXP_1, active, n);
- avenrun[1] = calc_load_n(avenrun[1], EXP_5, active, n);
- avenrun[2] = calc_load_n(avenrun[2], EXP_15, active, n);
+ active = atomic_long_read(&calc_load_tasks);
+ active = active > 0 ? active * FIXED_1 : 0;

- calc_load_update += n * LOAD_FREQ;
- }
+ avenrun[0] = calc_load_n(avenrun[0], EXP_1, active, n);
+ avenrun[1] = calc_load_n(avenrun[1], EXP_5, active, n);
+ avenrun[2] = calc_load_n(avenrun[2], EXP_15, active, n);

- /*
- * Its possible the remainder of the above division also crosses
- * a LOAD_FREQ period, the regular check in calc_global_load()
- * which comes after this will take care of that.
- *
- * Consider us being 11 ticks before a cycle completion, and us
- * sleeping for 4*LOAD_FREQ + 22 ticks, then the above code will
- * age us 4 cycles, and the test in calc_global_load() will
- * pick up the final one.
- */
+ calc_load_update += n * LOAD_FREQ;
}
#else
static void calc_load_account_idle(struct rq *this_rq)
@@ -3446,7 +3437,7 @@ static inline long calc_load_fold_idle(v
return 0;
}

-static void calc_global_nohz(unsigned long ticks)
+static void calc_global_nohz(void)
{
}
#endif
@@ -3474,8 +3465,6 @@ void calc_global_load(unsigned long tick
{
long active;

- calc_global_nohz(ticks);
-
if (time_before(jiffies, calc_load_update + 10))
return;

@@ -3487,6 +3476,16 @@ void calc_global_load(unsigned long tick
avenrun[2] = calc_load(avenrun[2], EXP_15, active);

calc_load_update += LOAD_FREQ;
+
+ /*
+ * Account one period with whatever state we found before
+ * folding in the nohz state and ageing the entire idle period.
+ *
+ * This avoids loosing a sample when we go idle between
+ * calc_load_account_active() (10 ticks ago) and now and thus
+ * under-accounting.
+ */
+ calc_global_nohz();
}

/*