Messages in this thread | | | From | Valentin Schneider <> | Subject | Re: [PATCH] sched/fair: Update nohz.next_balance for newly NOHZ-idle CPUs | Date | Thu, 15 Jul 2021 12:56:10 +0100 |
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Hi Vincent,
Thanks for taking a look.
On 15/07/21 09:42, Vincent Guittot wrote: > On Wed, 14 Jul 2021 at 13:39, Valentin Schneider > <valentin.schneider@arm.com> wrote: >> >> Consider a system with some NOHZ-idle CPUs, such that >> >> nohz.idle_cpus_mask = S >> nohz.next_balance = T >> >> When a new CPU k goes NOHZ idle (nohz_balance_enter_idle()), we end up >> with: >> >> nohz.idle_cpus_mask = S \U {k} >> nohz.next_balance = T >> >> Note that the nohz.next_balance hasn't changed - it won't be updated until >> a NOHZ balance is triggered. This is problematic if the newly NOHZ idle CPU >> has an earlier rq.next_balance than the other NOHZ idle CPUs, IOW if: >> >> cpu_rq(k).next_balance < nohz.next_balance >> >> In such scenarios, the existing nohz.next_balance will prevent any NOHZ >> balance from happening, which itself will prevent nohz.next_balance from >> being updated to this new cpu_rq(k).next_balance. Unnecessary load balance >> delays of over 12ms caused by this were observed on an arm64 RB5 board. > > How many CPUs has the arm64 RB5 ?
That's an 8 CPU DynamIQ system - 4 littles, 3 bigs and one "huge". That should give us a regular balance_interval of 8ms, but our tests have picked up CPUs staying idle for >20ms when they really have stuff to pull. In this case balance_interval increases are involved.
>> @@ -10351,6 +10354,13 @@ static void nohz_balancer_kick(struct rq *rq) >> unlock: >> rcu_read_unlock(); >> out: >> + /* >> + * Some CPUs have recently gone into NOHZ idle; kick a balance to >> + * collate the proper next balance interval. >> + */ >> + if (!cpumask_subset(nohz.idle_cpus_mask, nohz.last_balance_mask)) > > I don't really like having to manipulate a cpumask just to trigger an > ILB and force the update of nohz.next_balance. Could we use something > similar to nohz.has_blocked and introduce a nohz.force_update. > manipulating cpumask will even be more complex if we start to have a > per node idle_cpus_mask like proposed here: > https://lore.kernel.org/lkml/20210701055323.2199175-1-npiggin@gmail.com/ > > Also > > > Something like below is simpler > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index 44e44c235f1f..91c314f58982 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -10657,6 +10657,9 @@ static void nohz_newidle_balance(struct rq *this_rq) > if (this_rq->avg_idle < sysctl_sched_migration_cost) > return; > > + if (time_before(this_rq->next_balance, READ_ONCE(nohz.next_balance)) > + WRITE_ONCE(nohz.need_update, 1); > +
I think we have to do this unconditionally, as we can observe the old nohz.next_balance while a NOHZ balance is ongoing (which will update nohz.next_balance without taking into account this newly idle CPU).
> /* Don't need to update blocked load of idle CPUs*/ > if (!READ_ONCE(nohz.has_blocked) || > time_before(jiffies, READ_ONCE(nohz.next_blocked))) > > > Then we have to test nohz.need_update in nohz_balancer_kick() >
But then, when can we safely clear this new nohz.need_update? We can't do it unconditionally in nohz_idle_balance() as this could race with a new CPU going NOHZ idle.
Perhaps instead we could have a single nohz.needs_update_mask, the CPU is set in nohz_newidle_balance(), cleared when iterated over in _nohz_idle_balance(), and nohz_balancer_kick() can trigger an e.g. NOHZ_UPDATE_KICK if this new cpumask is non-empty.
>> + flags |= NOHZ_STATS_KICK; > > people complain that an update of blocked load is time consuming so we > should not kick this update unnecessarily. > We should introduce a new bit like NOHZ_NEXT_KICK that will only go > through idle cpus and update nohz.next_balance >
That sounds reasonable.
>> + >> if (flags) >> kick_ilb(flags); >> } >> @@ -10487,6 +10497,7 @@ static bool update_nohz_stats(struct rq *rq) >> static void _nohz_idle_balance(struct rq *this_rq, unsigned int flags, >> enum cpu_idle_type idle) >> { >> + struct cpumask *cpus = this_cpu_cpumask_var_ptr(nohz_balance_mask); >> /* Earliest time when we have to do rebalance again */ >> unsigned long now = jiffies; >> unsigned long next_balance = now + 60*HZ; >> @@ -10518,7 +10529,8 @@ static void _nohz_idle_balance(struct rq *this_rq, unsigned int flags, >> * Start with the next CPU after this_cpu so we will end with this_cpu and let a >> * chance for other idle cpu to pull load. >> */ >> - for_each_cpu_wrap(balance_cpu, nohz.idle_cpus_mask, this_cpu+1) { >> + cpumask_copy(cpus, nohz.idle_cpus_mask); > > we are not sure to go through all idle cpus and ilb can abort >
Right, this is missing something to re-kick an update, but I think we can get rid of that entirely...
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