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Message-ID: <1271934490.1776.343.camel@laptop>
Date: Thu, 22 Apr 2010 13:08:10 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Chase Douglas <chase.douglas@...onical.com>
Cc: linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
Andrew Morton <akpm@...ux-foundation.org>,
Ingo Molnar <mingo@...e.hu>, "Rafael J. Wysocki" <rjw@...k.pl>,
kernel-team <kernel-team@...ts.ubuntu.com>
Subject: Re: [REGRESSION 2.6.30][PATCH v3] sched: update load count only
once per cpu in 10 tick update window
On Tue, 2010-04-13 at 16:19 -0700, Chase Douglas wrote:
>
> 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.
>
Ok, so delaying the whole ILB angle for now, the below is a similar
approach to yours but with a more explicit code flow.
Does that work for you?
---
kernel/sched.c | 80 +++++++++++++++++++++++++++++++++++++++-------
kernel/sched_idletask.c | 3 +-
2 files changed, 68 insertions(+), 15 deletions(-)
diff --git a/kernel/sched.c b/kernel/sched.c
index 95eaecc..8ac02a9 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -1815,7 +1815,7 @@ static void cfs_rq_set_shares(struct cfs_rq *cfs_rq, unsigned long shares)
}
#endif
-static void calc_load_account_active(struct rq *this_rq);
+static void calc_load_account_idle(struct rq *this_rq);
static void update_sysctl(void);
static int get_update_sysctl_factor(void);
@@ -2907,6 +2907,61 @@ static unsigned long calc_load_update;
unsigned long avenrun[3];
EXPORT_SYMBOL(avenrun);
+static long calc_load_fold_active(struct rq *this_rq)
+{
+ long nr_active, delta = 0;
+
+ nr_active = this_rq->nr_running;
+ nr_active += (long) this_rq->nr_uninterruptible;
+
+ if (nr_active != this_rq->calc_load_active) {
+ delta = nr_active - this_rq->calc_load_active;
+ this_rq->calc_load_active = nr_active;
+ }
+
+ return delta;
+}
+
+#ifdef CONFIG_NO_HZ
+/*
+ * For NO_HZ we delay the active fold to the next LOAD_FREQ update.
+ *
+ * When making the ILB scale, we should try to pull this in as well.
+ */
+static atomic_long_t calc_load_tasks_idle;
+
+static void calc_load_account_idle(struct rq *this_rq)
+{
+ long delta;
+
+ delta = calc_load_fold_active(this_rq);
+ if (delta)
+ atomic_long_add(delta, &calc_load_tasks_idle);
+}
+
+static long calc_load_fold_idle(void)
+{
+ long delta = 0;
+
+ /*
+ * Its got a race, we don't care...
+ */
+ if (atomic_long_read(&calc_load_tasks_idle))
+ delta = atomic_long_xchg(&calc_load_tasks_idle, 0);
+
+ return delta;
+}
+#else
+static void calc_load_account_idle(struct rq *this_rq)
+{
+}
+
+static inline long calc_load_fold_idle(void)
+{
+ return 0;
+}
+#endif
+
/**
* get_avenrun - get the load average array
* @loads: pointer to dest load array
@@ -2953,20 +3008,22 @@ void calc_global_load(void)
}
/*
- * Either called from update_cpu_load() or from a cpu going idle
+ * Called from update_cpu_load() to periodically update this CPU's
+ * active count.
*/
static void calc_load_account_active(struct rq *this_rq)
{
- long nr_active, delta;
+ long delta;
- nr_active = this_rq->nr_running;
- nr_active += (long) this_rq->nr_uninterruptible;
+ if (time_before(jiffies, this_rq->calc_load_update))
+ return;
- if (nr_active != this_rq->calc_load_active) {
- delta = nr_active - this_rq->calc_load_active;
- this_rq->calc_load_active = nr_active;
+ delta = calc_load_fold_active(this_rq);
+ delta += calc_load_fold_idle();
+ if (delta)
atomic_long_add(delta, &calc_load_tasks);
- }
+
+ this_rq->calc_load_update += LOAD_FREQ;
}
/*
@@ -2998,10 +3055,7 @@ static void update_cpu_load(struct rq *this_rq)
this_rq->cpu_load[i] = (old_load*(scale-1) + new_load) >> i;
}
- if (time_after_eq(jiffies, this_rq->calc_load_update)) {
- this_rq->calc_load_update += LOAD_FREQ;
- calc_load_account_active(this_rq);
- }
+ calc_load_account_active(this_rq);
}
#ifdef CONFIG_SMP
diff --git a/kernel/sched_idletask.c b/kernel/sched_idletask.c
index bea2b8f..9fa0f40 100644
--- a/kernel/sched_idletask.c
+++ b/kernel/sched_idletask.c
@@ -23,8 +23,7 @@ static void check_preempt_curr_idle(struct rq *rq, struct task_struct *p, int fl
static struct task_struct *pick_next_task_idle(struct rq *rq)
{
schedstat_inc(rq, sched_goidle);
- /* adjust the active tasks as we might go into a long sleep */
- calc_load_account_active(rq);
+ calc_load_account_idle(rq);
return rq->idle;
}
--
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