Messages in this thread | | | From | Valentin Schneider <> | Subject | Re: [PATCH v2 2/2] sched/fair: Trigger nohz.next_balance updates when a CPU goes NOHZ-idle | Date | Mon, 19 Jul 2021 17:28:44 +0100 |
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On 19/07/21 17:24, Dietmar Eggemann wrote: > On 19/07/2021 12:31, Valentin Schneider wrote: > > [...] > >> @@ -10351,6 +10352,9 @@ static void nohz_balancer_kick(struct rq *rq) >> unlock: >> rcu_read_unlock(); >> out: >> + if (READ_ONCE(nohz.needs_update)) >> + flags |= NOHZ_NEXT_KICK; >> + > > Since NOHZ_NEXT_KICK is part of NOHZ_KICK_MASK, some conditions above > will already set it in flags. Is this intended?
So if no kick would be issued (e.g. flags == 0 because nohz.next_balance is later in the future), then this does the right thing and issues a NOHZ_NEXT_KICK one.
However you're right to point out that even if nohz.needs_update is false, we can set NOHZ_NEXT_KICK into the ilb rq's NOHZ flags due to it being included in NOHZ_KICK_MASK, which I think is a mistake. Looking at it now, it shouldn't be part of NOHZ_KICK_MASK.
> >> if (flags) >> kick_ilb(flags); >> } >> @@ -10447,12 +10451,13 @@ void nohz_balance_enter_idle(int cpu) >> /* >> * Ensures that if nohz_idle_balance() fails to observe our >> * @idle_cpus_mask store, it must observe the @has_blocked >> - * store. >> + * and @needs_update stores. >> */ >> smp_mb__after_atomic(); >> >> set_cpu_sd_state_idle(cpu); >> >> + WRITE_ONCE(nohz.needs_update, 1); >> out: >> /* >> * Each time a cpu enter idle, we assume that it has blocked load and >> @@ -10501,13 +10506,17 @@ static void _nohz_idle_balance(struct rq *this_rq, unsigned int flags, > > function header would need update to incorporate the new 'update > nohz.next_balance' functionality. It only talks about 'update of blocked > load' and 'complete load balance' so far. > >> /* >> * We assume there will be no idle load after this update and clear >> * the has_blocked flag. If a cpu enters idle in the mean time, it will >> - * set the has_blocked flag and trig another update of idle load. >> + * set the has_blocked flag and trigger another update of idle load. >> * Because a cpu that becomes idle, is added to idle_cpus_mask before >> * setting the flag, we are sure to not clear the state and not >> * check the load of an idle cpu. >> + * >> + * Same applies to idle_cpus_mask vs needs_update. >> */ >> if (flags & NOHZ_STATS_KICK) >> WRITE_ONCE(nohz.has_blocked, 0); >> + if (flags & NOHZ_NEXT_KICK) >> + WRITE_ONCE(nohz.needs_update, 0); >> >> /* >> * Ensures that if we miss the CPU, we must see the has_blocked >> @@ -10531,6 +10540,8 @@ static void _nohz_idle_balance(struct rq *this_rq, unsigned int flags, >> if (need_resched()) { >> if (flags & NOHZ_STATS_KICK) >> has_blocked_load = true; > > This looks weird now? 'has_blocked_load = true' vs > 'WRITE_ONCE(nohz.needs_update, 1)'. >
Well, has_blocked_load lets us factorize the nohz.has_blocked write (one is needed either when aborting or at the tail of the cpumask iteration), whereas there is just a single write for nohz.needs_update (when aborting).
>> + if (flags & NOHZ_NEXT_KICK) >> + WRITE_ONCE(nohz.needs_update, 1); >> goto abort; >> } >> >>
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