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Date: Tue, 14 Jul 2015 16:07:10 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Mike Galbraith <umgwanakikbuti@...il.com>
Cc: Josef Bacik <jbacik@...com>, riel@...hat.com, mingo@...hat.com,
linux-kernel@...r.kernel.org, morten.rasmussen@....com,
kernel-team <Kernel-team@...com>
Subject: Re: [patch] sched: beef up wake_wide()
On Tue, Jul 14, 2015 at 03:49:17PM +0200, Mike Galbraith wrote:
> On Tue, 2015-07-14 at 13:19 +0200, Peter Zijlstra wrote:
>
> > OK, how about something like the below; it tightens things up by
> > applying two rules:
> >
> > - We really should not continue looking for a balancing domain once
> > SD_LOAD_BALANCE is not set.
> >
> > - SD (balance) flags should really be set in a single contiguous range,
> > always starting at the bottom.
> >
> > The latter means what if !want_affine and the (first) sd doesn't have
> > BALANCE_WAKE set, we're done. Getting rid of (most of) that iteration
> > junk you didn't like..
> >
> > Hmm?
>
> Yeah, that's better. It's not big hairy deal either way, it just bugged
> me to knowingly toss those cycles out the window ;-)
>
> select_idle_sibling() looks kinda funny down there, but otoh when the
> day comes (hah) that we can just balance, it's closer to the exit.
Right, not too pretty, does this look beter?
---
Subject: sched: Beef up wake_wide()
From: Mike Galbraith <umgwanakikbuti@...il.com>
Date: Fri, 10 Jul 2015 07:19:26 +0200
Josef Bacik reported that Facebook sees better performance with their
1:N load (1 dispatch/node, N workers/node) when carrying an old patch
to try very hard to wake to an idle CPU. While looking at wake_wide(),
I noticed that it doesn't pay attention to the wakeup of a many partner
waker, returning 1 only when waking one of its many partners.
Correct that, letting explicit domain flags override the heuristic.
While at it, adjust task_struct bits, we don't need a 64bit counter.
Cc: mingo@...hat.com
Cc: morten.rasmussen@....com
Cc: riel@...hat.com
Signed-off-by: Mike Galbraith <umgwanakikbuti@...il.com>
Tested-by: Josef Bacik <jbacik@...com>
[peterz: frobbings]
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Link: http://lkml.kernel.org/r/1436505566.5715.50.camel@gmail.com
---
include/linux/sched.h | 4 +-
kernel/sched/fair.c | 68 +++++++++++++++++++++++---------------------------
2 files changed, 34 insertions(+), 38 deletions(-)
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1359,9 +1359,9 @@ struct task_struct {
#ifdef CONFIG_SMP
struct llist_node wake_entry;
int on_cpu;
- struct task_struct *last_wakee;
- unsigned long wakee_flips;
+ unsigned int wakee_flips;
unsigned long wakee_flip_decay_ts;
+ struct task_struct *last_wakee;
int wake_cpu;
#endif
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4726,26 +4726,29 @@ static long effective_load(struct task_g
#endif
+/*
+ * Detect M:N waker/wakee relationships via a switching-frequency heuristic.
+ * A waker of many should wake a different task than the one last awakened
+ * at a frequency roughly N times higher than one of its wakees. In order
+ * to determine whether we should let the load spread vs consolodating to
+ * shared cache, we look for a minimum 'flip' frequency of llc_size in one
+ * partner, and a factor of lls_size higher frequency in the other. With
+ * both conditions met, we can be relatively sure that the relationship is
+ * non-monogamous, with partner count exceeding socket size. Waker/wakee
+ * being client/server, worker/dispatcher, interrupt source or whatever is
+ * irrelevant, spread criteria is apparent partner count exceeds socket size.
+ */
static int wake_wide(struct task_struct *p)
{
+ unsigned int master = current->wakee_flips;
+ unsigned int slave = p->wakee_flips;
int factor = this_cpu_read(sd_llc_size);
- /*
- * Yeah, it's the switching-frequency, could means many wakee or
- * rapidly switch, use factor here will just help to automatically
- * adjust the loose-degree, so bigger node will lead to more pull.
- */
- if (p->wakee_flips > factor) {
- /*
- * wakee is somewhat hot, it needs certain amount of cpu
- * resource, so if waker is far more hot, prefer to leave
- * it alone.
- */
- if (current->wakee_flips > (factor * p->wakee_flips))
- return 1;
- }
-
- return 0;
+ if (master < slave)
+ swap(master, slave);
+ if (slave < factor || master < slave * factor)
+ return 0;
+ return 1;
}
static int wake_affine(struct sched_domain *sd, struct task_struct *p, int sync)
@@ -4757,13 +4760,6 @@ static int wake_affine(struct sched_doma
unsigned long weight;
int balanced;
- /*
- * If we wake multiple tasks be careful to not bounce
- * ourselves around too much.
- */
- if (wake_wide(p))
- return 0;
-
idx = sd->wake_idx;
this_cpu = smp_processor_id();
prev_cpu = task_cpu(p);
@@ -5015,19 +5011,19 @@ static int get_cpu_usage(int cpu)
static int
select_task_rq_fair(struct task_struct *p, int prev_cpu, int sd_flag, int wake_flags)
{
- struct sched_domain *tmp, *affine_sd = NULL, *sd = NULL;
+ struct sched_domain *tmp, affine_sd = NULL, *sd = NULL;
int cpu = smp_processor_id();
- int new_cpu = cpu;
+ int new_cpu = prev_cpu;
int want_affine = 0;
int sync = wake_flags & WF_SYNC;
if (sd_flag & SD_BALANCE_WAKE)
- want_affine = cpumask_test_cpu(cpu, tsk_cpus_allowed(p));
+ want_affine = !wake_wide(p) && cpumask_test_cpu(cpu, tsk_cpus_allowed(p));
rcu_read_lock();
for_each_domain(cpu, tmp) {
if (!(tmp->flags & SD_LOAD_BALANCE))
- continue;
+ break;
/*
* If both cpu and prev_cpu are part of this domain,
@@ -5041,17 +5037,17 @@ select_task_rq_fair(struct task_struct *
if (tmp->flags & sd_flag)
sd = tmp;
+ else if (!want_affine)
+ break;
}
- if (affine_sd && cpu != prev_cpu && wake_affine(affine_sd, p, sync))
- prev_cpu = cpu;
+ if (affine_sd) { /* Prefer affinity over any other flags */
+ if (cpu != prev_cpu && wake_affine(affine_sd, p, sync))
+ new_cpu = cpu;
- if (sd_flag & SD_BALANCE_WAKE) {
- new_cpu = select_idle_sibling(p, prev_cpu);
- goto unlock;
- }
+ new_cpu = select_idle_sibling(p, new_cpu);
- while (sd) {
+ } else while (sd) {
struct sched_group *group;
int weight;
@@ -5085,10 +5081,10 @@ select_task_rq_fair(struct task_struct *
}
/* while loop will break here if sd == NULL */
}
-unlock:
- rcu_read_unlock();
+ rcu_read_unlock();
return new_cpu;
+
}
/*
--
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