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Date: Tue, 10 Nov 2015 09:36:01 +0900
From: <byungchul.park@....com>
To: <mingo@...nel.org>, <peterz@...radead.org>
CC: linux-kernel@...r.kernel.org, efault@....de, tglx@...utronix.de,
fweisbec@...il.com, Byungchul Park <byungchul.park@....com>
Subject: [PATCH v5 1/2] sched: make __update_cpu_load() handle NOHZ_FULL tickless
From: Byungchul Park <byungchul.park@....com>
__update_cpu_load() assumes that a cpu is idle if the interval between
the cpu's ticks is more than 1/HZ. However the cpu can be non-idle even
though the interval is more than 1/HZ, in the NOHZ_FULL case.
Thus in the NOHZ_FULL tickless case, the current way to update cpu load
can be wrong. To update cpu load properly, __update_cpu_load() should be
changed so that it can handle NOHZ_FULL tickless case.
A decaying average can be computed by
load[i]' = (1 - 1/2^i) * load[i] + (1/2^i) * load
Because of NOHZ, it might not get called on every tick which gives need
for the @pending_updates argument.
load[i]_n = (1 - 1/2^i) * load[i]_n-1 + (1/2^i) * load_n-1
= A * load[i]_n-1 + B ; A := (1 - 1/2^i), B := (1/2^i) * load
= A * (A * load[i]_n-2 + B) + B
= A * (A * (A * load[i]_n-3 + B) + B) + B
= A^3 * load[i]_n-3 + (A^2 + A + 1) * B
= A^n * load[i]_0 + (A^(n-1) + A^(n-2) + ... + 1) * B
= A^n * load[i]_0 + ((1 - A^n) / (1 - A)) * B
= (1 - 1/2^i)^n * (load[i]_0 - load) + load
In the above we've assumed load_n := load, which is true for NOHZ_FULL as
any change in load would have resulted in the tick being turned back on.
For regular NOHZ, this reduces to:
load[i]_n = (1 - 1/2^i)^n * load[i]_0
[Commit message and comments become simpler, thanks to Peter Z.]
Signed-off-by: Byungchul Park <byungchul.park@....com>
---
kernel/sched/fair.c | 49 +++++++++++++++++++++++++++++++++++++++++--------
1 file changed, 41 insertions(+), 8 deletions(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 077076f..7760678 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4305,14 +4305,46 @@ decay_load_missed(unsigned long load, unsigned long missed_updates, int idx)
return load;
}
-/*
+/**
+ * __update_cpu_load - update the rq->cpu_load[] statistics
+ * @this_rq: The rq to update statistics for
+ * @this_load: The current load
+ * @pending_updates: The number of missed updates
+ * @active: !0 for NOHZ_FULL
+ *
* Update rq->cpu_load[] statistics. This function is usually called every
- * scheduler tick (TICK_NSEC). With tickless idle this will not be called
- * every tick. We fix it up based on jiffies.
+ * scheduler tick (TICK_NSEC).
+ *
+ * This function computes a decaying average:
+ *
+ * load[i]' = (1 - 1/2^i) * load[i] + (1/2^i) * load
+ *
+ * Because of NOHZ it might not get called on every tick which gives need for
+ * the @pending_updates argument.
+ *
+ * load[i]_n = (1 - 1/2^i) * load[i]_n-1 + (1/2^i) * load_n-1
+ * = A * load[i]_n-1 + B ; A := (1 - 1/2^i), B := (1/2^i) * load
+ * = A * (A * load[i]_n-2 + B) + B
+ * = A * (A * (A * load[i]_n-3 + B) + B) + B
+ * = A^3 * load[i]_n-3 + (A^2 + A + 1) * B
+ * = A^n * load[i]_0 + (A^(n-1) + A^(n-2) + ... + 1) * B
+ * = A^n * load[i]_0 + ((1 - A^n) / (1 - A)) * B
+ * = (1 - 1/2^i)^n * (load[i]_0 - load) + load
+ *
+ * In the above we've assumed load_n := load, which is true for NOHZ_FULL as
+ * any change in load would have resulted in the tick being turned back on.
+ *
+ * For regular NOHZ, this reduces to:
+ *
+ * load[i]_n = (1 - 1/2^i)^n * load[i]_0
+ *
+ * see decay_load_misses(). For NOHZ_FULL we get to subtract and add the extra
+ * term. See the @active paramter.
*/
static void __update_cpu_load(struct rq *this_rq, unsigned long this_load,
- unsigned long pending_updates)
+ unsigned long pending_updates, int active)
{
+ unsigned long tickless_load = active ? this_rq->cpu_load[0] : 0;
int i, scale;
this_rq->nr_load_updates++;
@@ -4324,8 +4356,9 @@ static void __update_cpu_load(struct rq *this_rq, unsigned long this_load,
/* scale is effectively 1 << i now, and >> i divides by scale */
- old_load = this_rq->cpu_load[i];
+ old_load = this_rq->cpu_load[i] - tickless_load;
old_load = decay_load_missed(old_load, pending_updates - 1, i);
+ old_load += tickless_load;
new_load = this_load;
/*
* Round up the averaging division if load is increasing. This
@@ -4380,7 +4413,7 @@ static void update_idle_cpu_load(struct rq *this_rq)
pending_updates = curr_jiffies - this_rq->last_load_update_tick;
this_rq->last_load_update_tick = curr_jiffies;
- __update_cpu_load(this_rq, load, pending_updates);
+ __update_cpu_load(this_rq, load, pending_updates, 0);
}
/*
@@ -4403,7 +4436,7 @@ void update_cpu_load_nohz(void)
* We were idle, this means load 0, the current load might be
* !0 due to remote wakeups and the sort.
*/
- __update_cpu_load(this_rq, 0, pending_updates);
+ __update_cpu_load(this_rq, 0, pending_updates, 0);
}
raw_spin_unlock(&this_rq->lock);
}
@@ -4419,7 +4452,7 @@ void update_cpu_load_active(struct rq *this_rq)
* See the mess around update_idle_cpu_load() / update_cpu_load_nohz().
*/
this_rq->last_load_update_tick = jiffies;
- __update_cpu_load(this_rq, load, 1);
+ __update_cpu_load(this_rq, load, 1, 1);
}
/*
--
1.7.9.5
--
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