Messages in this thread | | | Date | Mon, 12 Feb 2018 16:38:05 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH v3 3/3] sched: update blocked load when newly idle |
| |
On Mon, Feb 12, 2018 at 03:34:44PM +0100, Vincent Guittot wrote: > Le Monday 12 Feb 2018 à 13:04:11 (+0100), Peter Zijlstra a écrit : > > On Mon, Feb 12, 2018 at 09:07:54AM +0100, Vincent Guittot wrote:
> > So I really hate this one, also I suspect its broken, because we do this > > check before dropping rq->lock and _nohz_idle_balance() will take > > rq->lock. > > yes. it will take both newly idle rq and idle rq lock
Right, can't do that, there's ordering rules for multiple RQ locks etc..
> > > > > > > Aside from the above being an unreadable mess, I dislike that it breaks > > the various isolation crud, we should not touch CPUs outside of our > > domain. > > > > > > Maybe something like the below? (unfinished) > > > > good catch. I completely miss the isolation stuff. > But isn't already the case when kicking ilb ? I mean that an idle CPU touches > all idle CPUs and some can be outside its domain during ilb.
> Shouldn't we test housekeeping_cpu(cpu, HK_FLAG_SCHED) instead if we want to > make sure that an isolated/full nohz CPU will not be used for updating blocked > load of CPUs outside its domain ?
I _thought_ we had some 'housekeeping' crud in the ilb selection logic, but now I can't find it. Frederic?
> Is something below more readable: > > /* > + * This CPU doesn't want to be disturbed by scheduler > + * houskeeping > */ > + if (!housekeeping_cpu(cpu, HK_FLAG_SCHED)) > + goto out; > + > + /* Will wake up very soon. No time for doing anything else*/ > + if (this_rq->avg_idle < sysctl_sched_migration_cost) > + goto out; > + > + /* Don't need to update blocked load of idle CPUs*/ > + if (!has_blocked || time_after_eq(jiffies, next_blocked) > + goto out; > + > + raw_spin_unlock(&this_rq->lock); > + /* > + * This CPU is going to be idle and blocked load of idle CPUs > + * need to be updated. Run the ilb locally as it is a good > + * candidate for ilb instead of waking up another idle CPU. > + * Kick an normal ilb if we failed to do the update. > + */ > + if !_nohz_idle_balance(this_rq, NOHZ_STATS_KICK, CPU_NEWLY_IDLE)) > kick_ilb(NOHZ_STATS_KICK); > + raw_spin_lock(&this_rq->lock); > > goto out;
It is, but I think you're still doing that avg_idle thing twice now, right?
> > @@ -7850,7 +7850,7 @@ static bool update_nohz_stats(struct rq > > if (!cpumask_test_cpu(cpu, nohz.idle_cpus_mask)) > > return false; > > > > - if (!time_after(jiffies, rq->last_blocked_load_update_tick)) > > + if (!force && !time_after(jiffies, rq->last_blocked_load_update_tick)) > > This fix the concern raised on the other thread, isn't it ?
Yes.
> > +static int nohz_age(struct sched_domain *sd) > > +{ > > + struct cpumask *cpus = this_cpu_cpumask_var_ptr(load_balance_mask); > > + bool has_blocked_load; > > + > > + WRITE_ONCE(nohz.has_blocked, 0); > > + > > + smp_mb(); > > + > > + cpumask_and(cpus, sched_domain_span(sd), nohz.idle_cpus_mask); > > + > > + has_blocked_load = cpumask_subset(nohz.idle_cpus_mask, sched_domain_span(sd)); > > + > > + for_each_cpu(cpu, cpus) { > > + struct rq *rq = cpu_rq(cpu); > > + > > + has_blocked_load |= update_nohz_stats(rq, true); > > + } > > + > > + if (has_blocked_load) > > + WRITE_ONCE(nohz.has_blocked, 1); > > +} > > + > > we duplicate what is done in nohe_idle_balance
In parts yes.. I was too lazy to combine :-)
> > @@ -8919,9 +8955,13 @@ static int idle_balance(struct rq *this_ > > if (sd->flags & SD_BALANCE_NEWIDLE) { > > t0 = sched_clock_cpu(this_cpu); > > > > - pulled_task = load_balance(this_cpu, this_rq, > > - sd, CPU_NEWLY_IDLE, > > - &continue_balancing); > > + if (nohz_blocked) { > > + nohz_age(sd); > > Do we really need to loop all sched_domain of newly idle CPU and call > nohz_age for each level ? > Can't we only call nohz_age with the widest/last sched_domain level ?
Yeah, dunno. I went back and forth on that a bit. The largest is rq->rd->span. The reason I settled on this variant in the end is that it keeps locality. When short idle, it will only scan nearby CPUs instead of reaching half-way across the machine.
> Furthermore, we use sd->max_newidle_lb_cost to decide to abort the loop. > But this is updated with full load balancing which is longer than just > updating blocked load. > This will increase the chance to abort before reaching the last level.
Yes.. I figured we'd take that hit :/
> > + } else { > > + pulled_task = load_balance(this_cpu, this_rq, > > + sd, CPU_NEWLY_IDLE, > > + &continue_balancing); > > + } > > > > domain_cost = sched_clock_cpu(this_cpu) - t0; > > if (domain_cost > sd->max_newidle_lb_cost) > > We have to kick an ilb if we must abort before looping all levels and all > idle CPUs otherwise we can have situation where the load of some idle CPus > could stay blocked
Yes, like said, was unfinished, I gave up before I got to that.
| |