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From: Chase Douglas on 13 Apr 2010 19:20
There's a period of 10 ticks where calc_load_tasks is updated by all the
cpus for the load avg. Usually all the cpus do this during the first
tick. If any cpus go idle, calc_load_tasks is decremented accordingly.
However, if they wake up calc_load_tasks is not incremented. Thus, if
cpus go idle during the 10 tick period, calc_load_tasks may be
decremented to a non-representative value. This issue can lead to
systems having a load avg of exactly 0, even though the real load avg
could theoretically be up to NR_CPUS.

This change defers calc_load_tasks accounting after each cpu updates the
count until after the 10 tick update window.

A few points:

* A global atomic deferral counter, and not per-cpu vars, is needed
because a cpu may go NOHZ idle and not be able to update the global
calc_load_tasks variable for subsequent load calculations.
* It is not enough to add calls to account for the load when a cpu is
awakened:
- Load avg calculation must be independent of cpu load.
- If a cpu is awakend by one tasks, but then has more scheduled before
the end of the update window, only the first task will be accounted.

BugLink: http://bugs.launchpad.net/bugs/513848

Signed-off-by: Chase Douglas <chase.douglas(a)canonical.com>
Acked-by: Colin King <colin.king(a)canonical.com>
Acked-by: Andy Whitcroft <apw(a)canonical.com>
---
kernel/sched.c | 24 ++++++++++++++++++++++--
1 files changed, 22 insertions(+), 2 deletions(-)

diff --git a/kernel/sched.c b/kernel/sched.c
index abb36b1..be348cd 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -3010,6 +3010,7 @@ unsigned long this_cpu_load(void)

/* Variables and functions for calc_load */
static atomic_long_t calc_load_tasks;
+static atomic_long_t calc_load_tasks_deferred;
static unsigned long calc_load_update;
unsigned long avenrun[3];
EXPORT_SYMBOL(avenrun);
@@ -3064,7 +3065,7 @@ void calc_global_load(void)
*/
static void calc_load_account_active(struct rq *this_rq)
{
- long nr_active, delta;
+ long nr_active, delta, deferred;

nr_active = this_rq->nr_running;
nr_active += (long) this_rq->nr_uninterruptible;
@@ -3072,6 +3073,25 @@ static void calc_load_account_active(struct rq *this_rq)
if (nr_active != this_rq->calc_load_active) {
delta = nr_active - this_rq->calc_load_active;
this_rq->calc_load_active = nr_active;
+
+ /*
+ * Update calc_load_tasks only once per cpu in 10 tick update
+ * window.
+ */
+ if (unlikely(time_before(jiffies, this_rq->calc_load_update) &&
+ time_after_eq(jiffies, calc_load_update))) {
+ if (delta)
+ atomic_long_add(delta,
+ &calc_load_tasks_deferred);
+ return;
+ }
+
+ if (atomic_long_read(&calc_load_tasks_deferred)) {
+ deferred = atomic_long_xchg(&calc_load_tasks_deferred,
+ 0);
+ delta += deferred;
+ }
+
atomic_long_add(delta, &calc_load_tasks);
}
}
@@ -3106,8 +3126,8 @@ static void update_cpu_load(struct rq *this_rq)
}

if (time_after_eq(jiffies, this_rq->calc_load_update)) {
- this_rq->calc_load_update += LOAD_FREQ;
calc_load_account_active(this_rq);
+ this_rq->calc_load_update += LOAD_FREQ;
}
}

--
1.6.3.3

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