On Thu, 2015-07-09 at 15:26 +0200, Peter Zijlstra wrote:
> On Wed, Jul 08, 2015 at 08:13:46AM +0200, Mike Galbraith wrote:
> >
> > +/*
> > + * Detect 1:N waker/wakee relationship 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 we are seeing a 1:N
> > + * relationship, and that load size exceeds socket size.
> > + */
> > static int wake_wide(struct task_struct *p)
> > {
> > + unsigned int waker_flips = current->wakee_flips;
> > + unsigned int wakee_flips = p->wakee_flips;
> > int factor = this_cpu_read(sd_llc_size);
> >
> > + if (waker_flips < wakee_flips)
> > + swap(waker_flips, wakee_flips);
>
> This makes the wakee/waker names useless, the end result is more like
> wakee_flips := client_flips, waker_flips := server_flips.
True, perhaps a rename is in order.
> > + if (wakee_flips < factor || waker_flips < wakee_flips * factor)
> > + return 0;
>
> I don't get the first condition... why would the client ever flip? It
> only talks to that one server.
So I was thinking too, and I initially cemented the relationship by
flipping both. However, the thing works in virgin source, ie clients do
in fact flip, so I removed that cementing based on the hard evidence.
> > @@ -5021,14 +5015,17 @@ select_task_rq_fair(struct task_struct *
> > {
> > struct sched_domain *tmp, *affine_sd = NULL, *sd = NULL;
> > int cpu = smp_processor_id();
> > + int new_cpu = prev_cpu;
> > int want_affine = 0;
> > int sync = wake_flags & WF_SYNC;
> >
> > rcu_read_lock();
> > + if (sd_flag & SD_BALANCE_WAKE) {
> > + want_affine = !wake_wide(p) && cpumask_test_cpu(cpu,
> > tsk_cpus_allowed(p));
> > + if (!want_affine)
> > + goto select_idle;
> > + }
>
> So this preserves/makes worse the bug Morten spotted, even without
> want_affine we should still attempt SD_BALANCE_WAKE if set.
Yeah. I can redo it if you want, but it seems a shame to traverse for
nothing given we know SD_BALANCE_WAKE is so painful that nobody really
really wants to do that. One has to override the other in any case, no?
-Mike
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