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Date: Tue, 14 Mar 2017 23:34:06 +0200
From: Jarmo Tiitto <jarmo.tiitto@...il.com>
To: Ingo Molnar <mingo@...hat.com>,
Peter Zijlstra <peterz@...radead.org>
Cc: linux-kernel@...r.kernel.org
Subject: [PATCH v4.9] sched/fair: Overload-On-Wakeup fix.
Try improve multi-core scaling.
The changes are experimental - the fix works
but possible regressions are unknown.
Please read the orginal author's paper to understand
the overall problem this patch tries to solve.
See the orignal patches at:
https://github.com/jplozi/wastedcores/tree/master/patches
And the related paper at:
http://www.i3s.unice.fr/~jplozi/wastedcores/
v1: the patch was re-written for v4.9 to remove goto statement.
v2: improved version from the wastedcores initial code. try take into
account the cpu wake-up latency and choose most recent idle cpu
instead.
I have tested the code and it really does improve
multi-core performance on my machine.
Reported-by: <jplozi@...ce.fr>
Signed-off-by: <jarmo.tiitto@...il.com>
---
kernel/sched/fair.c | 138
++++++++++++++++++++++++++++++++--------------------
1 file changed, 86 insertions(+), 52 deletions(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index c242944f5cbd..6569e76294d4 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5630,6 +5630,10 @@ select_task_rq_fair(struct task_struct *p, int
prev_cpu, int sd_flag, int wake_f
int new_cpu = prev_cpu;
int want_affine = 0;
int sync = wake_flags & WF_SYNC;
+ int chk_cpu;
+ u64 idle_stamp = 0;
+ unsigned int idle_exit_lat = UINT_MAX;
+ int idle_stamp_cpu = -1;
if (sd_flag & SD_BALANCE_WAKE) {
record_wakee(p);
@@ -5638,69 +5642,99 @@ select_task_rq_fair(struct task_struct *p, int
prev_cpu, int sd_flag, int wake_f
}
rcu_read_lock();
- for_each_domain(cpu, tmp) {
- if (!(tmp->flags & SD_LOAD_BALANCE))
- break;
+ /* fast wake up on current idle cpu */
+ if (!cpu_rq(prev_cpu)->nr_running) {
+ new_cpu = prev_cpu;
+ } else {
+ /* Check for suitable unused cores */
+ for_each_cpu_and(chk_cpu, cpu_online_mask, tsk_cpus_allowed(p)) {
+ struct rq *rq = cpu_rq(chk_cpu);
+ struct cpuidle_state *idle = idle_get_state(rq);
- /*
- * If both cpu and prev_cpu are part of this domain,
- * cpu is a valid SD_WAKE_AFFINE target.
- */
- if (want_affine && (tmp->flags & SD_WAKE_AFFINE) &&
- cpumask_test_cpu(prev_cpu, sched_domain_span(tmp))) {
- affine_sd = tmp;
- break;
+ if (!rq->nr_running) {
+ if (idle && idle->exit_latency < idle_exit_lat) {
+ /* Always pick lower latency idle cpu */
+ idle_exit_lat = idle->exit_latency;
+ idle_stamp = rq->idle_stamp;
+ idle_stamp_cpu = chk_cpu;
+ } else if ((!idle || idle->exit_latency == idle_exit_lat) &&
+ rq->idle_stamp > idle_stamp) {
+ /* Pick most recent idle cpu. */
+ idle_stamp = rq->idle_stamp;
+ idle_stamp_cpu = chk_cpu;
+ }
+ }
}
- if (tmp->flags & sd_flag)
- sd = tmp;
- else if (!want_affine)
- break;
- }
+ if (idle_stamp_cpu != -1) {
+ /* Found suitable unused cpu */
+ new_cpu = idle_stamp_cpu;
+ } else {
+ /* There was no idle cpus available, must load balance. */
+ for_each_domain(cpu, tmp) {
+ if (!(tmp->flags & SD_LOAD_BALANCE))
+ break;
- if (affine_sd) {
- sd = NULL; /* Prefer wake_affine over balance flags */
- if (cpu != prev_cpu && wake_affine(affine_sd, p, prev_cpu, sync))
- new_cpu = cpu;
- }
+ /* If both cpu and prev_cpu are part of this domain,
+ * cpu is a valid SD_WAKE_AFFINE target.
+ */
+ if (want_affine && (tmp->flags & SD_WAKE_AFFINE) &&
+ cpumask_test_cpu(prev_cpu, sched_domain_span(tmp))) {
+ affine_sd = tmp;
+ break;
+ }
+
+ if (tmp->flags & sd_flag)
+ sd = tmp;
+ else if (!want_affine)
+ break;
+ }
- if (!sd) {
- if (sd_flag & SD_BALANCE_WAKE) /* XXX always ? */
- new_cpu = select_idle_sibling(p, prev_cpu, new_cpu);
+ if (affine_sd) {
+ sd = NULL; /* Prefer wake_affine over balance flags */
+ if (cpu != prev_cpu && wake_affine(affine_sd, p, prev_cpu, sync))
+ new_cpu = cpu;
+ }
- } else while (sd) {
- struct sched_group *group;
- int weight;
+ if (!sd) {
+ if (sd_flag & SD_BALANCE_WAKE) /* XXX always ? */
+ new_cpu = select_idle_sibling(p, prev_cpu, new_cpu);
- if (!(sd->flags & sd_flag)) {
- sd = sd->child;
- continue;
- }
+ } else while (sd) {
+ struct sched_group *group;
+ int weight;
- group = find_idlest_group(sd, p, cpu, sd_flag);
- if (!group) {
- sd = sd->child;
- continue;
- }
+ if (!(sd->flags & sd_flag)) {
+ sd = sd->child;
+ continue;
+ }
- new_cpu = find_idlest_cpu(group, p, cpu);
- if (new_cpu == -1 || new_cpu == cpu) {
- /* Now try balancing at a lower domain level of cpu */
- sd = sd->child;
- continue;
- }
+ group = find_idlest_group(sd, p, cpu, sd_flag);
+ if (!group) {
+ sd = sd->child;
+ continue;
+ }
- /* Now try balancing at a lower domain level of new_cpu */
- cpu = new_cpu;
- weight = sd->span_weight;
- sd = NULL;
- for_each_domain(cpu, tmp) {
- if (weight <= tmp->span_weight)
- break;
- if (tmp->flags & sd_flag)
- sd = tmp;
+ new_cpu = find_idlest_cpu(group, p, cpu);
+ if (new_cpu == -1 || new_cpu == cpu) {
+ /* Now try balancing at a lower domain level of cpu */
+ sd = sd->child;
+ continue;
+ }
+
+ /* Now try balancing at a lower domain level of new_cpu */
+ cpu = new_cpu;
+ weight = sd->span_weight;
+ sd = NULL;
+ for_each_domain(cpu, tmp) {
+ if (weight <= tmp->span_weight)
+ break;
+ if (tmp->flags & sd_flag)
+ sd = tmp;
+ }
+ /* while loop will break here if sd == NULL */
+ }
}
- /* while loop will break here if sd == NULL */
}
rcu_read_unlock();
--
2.12.0
^---
I'm faily newbie to the kernel development. please be nice.
I hope this email and the attached patch is useful. I did not write the
orginal
code but I have tinkered the code as an experiment quite lot.
The attached patch is the result of this tinkering.
The code most probaly is incorrect and has side-effects/regressions.
However, I have ran multiple kernels with this applied and it never
caused any severe problems.
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