Messages in this thread | | | Subject | Re: [PATCH v7] sched: fix init NOHZ_IDLE flag | From | Peter Zijlstra <> | Date | Tue, 23 Apr 2013 10:50:06 +0200 |
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On Tue, 2013-04-23 at 09:52 +0200, Vincent Guittot wrote:
> > static inline unsigned long *rq_nohz_flags(int cpu) > > { > > return rcu_dereference(cpu_rq(cpu)->sd)->nohz_flags; > > } > > > > if (!test_bit(0, rq_nohz_flags(cpu))) > > return; > > clear_bit(0, rq_nohz_flags(cpu)); > > > > AFAICT, if we use different rcu_dereferences for modifying nohz_flags > and for updating the nr_busy_cpus, we open a time window for > desynchronization, isn't it ?
Oh right, we need to call rq_nohz_flags() once and use the pointer twice.
> >> +unlock: > >> rcu_read_unlock(); > >> } > >> > >> @@ -5409,14 +5416,21 @@ void set_cpu_sd_state_idle(void) > >> { > >> struct sched_domain *sd; > >> int cpu = smp_processor_id(); > >> - > >> - if (test_bit(NOHZ_IDLE, nohz_flags(cpu))) > >> - return; > >> - set_bit(NOHZ_IDLE, nohz_flags(cpu)); > >> + int first_nohz_idle = 1; > >> > >> rcu_read_lock(); > >> - for_each_domain(cpu, sd) > >> + for_each_domain(cpu, sd) { > >> + if (first_nohz_idle) { > >> + if (test_bit(NOHZ_IDLE, &sd->nohz_flags)) > >> + goto unlock; > >> + > >> + set_bit(NOHZ_IDLE, &sd->nohz_flags); > >> + first_nohz_idle = 0; > >> + } > >> + > >> atomic_dec(&sd->groups->sgp->nr_busy_cpus); > >> + } > >> +unlock: > > > > Same here, .. why on earth do it for every sched_domain for that > cpu? > > The update of rq_nohz_flags is done only once on the top level > sched_domain and the nr_busy_cpus is updated on each sched_domain > level in order to have a quick access to the number of busy cpu when > we check for the need to kick an idle load balance
Ah, I didn't see the whole first_nohz_idle thing.. but why did you place it inside the loop in the first place? Wouldn't GCC be able to avoid the double cpu_rq(cpu)->sd dereference using CSE? Argh no,.. its got an ACCESS_ONCE in it that defeats GCC.
Bothersome..
I'd almost write it like:
struct sched_domain *sd;
rcu_read_lock(); sd = rcu_dereference_check_sched_domain(cpu_rq(cpu)->sd);
if (sd->nohz_idle) goto unlock; xchg(&sd->nohz_idle, 1); /* do we actually need atomics here? */
for (; sd; sd = sd->parent) atomic_dec(&sd->groups->sgp->nr_busy_cpus); unlock: rcu_read_unlock();
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