Messages in this thread | | | From | Vincent Guittot <> | Date | Tue, 24 Aug 2021 11:08:17 +0200 | Subject | Re: [PATCH v3 1/2] sched/fair: Add NOHZ balancer flag for nohz.next_balance updates |
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On Mon, 23 Aug 2021 at 13:17, Valentin Schneider <valentin.schneider@arm.com> wrote: > > A following patch will trigger NOHZ idle balances as a means to update > nohz.next_balance. Vincent noted that blocked load updates can have > non-negligible overhead, which should be avoided if the intent is to only > update nohz.next_balance. > > Add a new NOHZ balance kick flag, NOHZ_NEXT_KICK. Gate NOHZ blocked load > update by the presence of NOHZ_STATS_KICK - currently all NOHZ balance > kicks will have the NOHZ_STATS_KICK flag set, so no change in behaviour is > expected. > > Suggested-by: Vincent Guittot <vincent.guittot@linaro.org> > Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
> --- > kernel/sched/fair.c | 24 ++++++++++++++---------- > kernel/sched/sched.h | 8 +++++++- > 2 files changed, 21 insertions(+), 11 deletions(-) > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index 6cd05f1d77ef..4a91f3027c92 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -10342,7 +10342,7 @@ static void nohz_balancer_kick(struct rq *rq) > goto out; > > if (rq->nr_running >= 2) { > - flags = NOHZ_KICK_MASK; > + flags = NOHZ_STATS_KICK | NOHZ_BALANCE_KICK; > goto out; > } > > @@ -10356,7 +10356,7 @@ static void nohz_balancer_kick(struct rq *rq) > * on. > */ > if (rq->cfs.h_nr_running >= 1 && check_cpu_capacity(rq, sd)) { > - flags = NOHZ_KICK_MASK; > + flags = NOHZ_STATS_KICK | NOHZ_BALANCE_KICK; > goto unlock; > } > } > @@ -10370,7 +10370,7 @@ static void nohz_balancer_kick(struct rq *rq) > */ > for_each_cpu_and(i, sched_domain_span(sd), nohz.idle_cpus_mask) { > if (sched_asym_prefer(i, cpu)) { > - flags = NOHZ_KICK_MASK; > + flags = NOHZ_STATS_KICK | NOHZ_BALANCE_KICK; > goto unlock; > } > } > @@ -10383,7 +10383,7 @@ static void nohz_balancer_kick(struct rq *rq) > * to run the misfit task on. > */ > if (check_misfit_status(rq, sd)) { > - flags = NOHZ_KICK_MASK; > + flags = NOHZ_STATS_KICK | NOHZ_BALANCE_KICK; > goto unlock; > } > > @@ -10410,7 +10410,7 @@ static void nohz_balancer_kick(struct rq *rq) > */ > nr_busy = atomic_read(&sds->nr_busy_cpus); > if (nr_busy > 1) { > - flags = NOHZ_KICK_MASK; > + flags = NOHZ_STATS_KICK | NOHZ_BALANCE_KICK; > goto unlock; > } > } > @@ -10572,7 +10572,8 @@ static void _nohz_idle_balance(struct rq *this_rq, unsigned int flags, > * setting the flag, we are sure to not clear the state and not > * check the load of an idle cpu. > */ > - WRITE_ONCE(nohz.has_blocked, 0); > + if (flags & NOHZ_STATS_KICK) > + WRITE_ONCE(nohz.has_blocked, 0); > > /* > * Ensures that if we miss the CPU, we must see the has_blocked > @@ -10594,13 +10595,15 @@ static void _nohz_idle_balance(struct rq *this_rq, unsigned int flags, > * balancing owner will pick it up. > */ > if (need_resched()) { > - has_blocked_load = true; > + if (flags & NOHZ_STATS_KICK) > + has_blocked_load = true; > goto abort; > } > > rq = cpu_rq(balance_cpu); > > - has_blocked_load |= update_nohz_stats(rq); > + if (flags & NOHZ_STATS_KICK) > + has_blocked_load |= update_nohz_stats(rq); > > /* > * If time for next balance is due, > @@ -10631,8 +10634,9 @@ static void _nohz_idle_balance(struct rq *this_rq, unsigned int flags, > if (likely(update_next_balance)) > nohz.next_balance = next_balance; > > - WRITE_ONCE(nohz.next_blocked, > - now + msecs_to_jiffies(LOAD_AVG_PERIOD)); > + if (flags & NOHZ_STATS_KICK) > + WRITE_ONCE(nohz.next_blocked, > + now + msecs_to_jiffies(LOAD_AVG_PERIOD)); > > abort: > /* There is still blocked load, enable periodic update */ > diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h > index e7e2bba5b520..30b7bd2ef25d 100644 > --- a/kernel/sched/sched.h > +++ b/kernel/sched/sched.h > @@ -2706,12 +2706,18 @@ extern void cfs_bandwidth_usage_dec(void); > #define NOHZ_BALANCE_KICK_BIT 0 > #define NOHZ_STATS_KICK_BIT 1 > #define NOHZ_NEWILB_KICK_BIT 2 > +#define NOHZ_NEXT_KICK_BIT 3 > > +/* Run rebalance_domains() */ > #define NOHZ_BALANCE_KICK BIT(NOHZ_BALANCE_KICK_BIT) > +/* Update blocked load */ > #define NOHZ_STATS_KICK BIT(NOHZ_STATS_KICK_BIT) > +/* Update blocked load when entering idle */ > #define NOHZ_NEWILB_KICK BIT(NOHZ_NEWILB_KICK_BIT) > +/* Update nohz.next_balance */ > +#define NOHZ_NEXT_KICK BIT(NOHZ_NEXT_KICK_BIT) > > -#define NOHZ_KICK_MASK (NOHZ_BALANCE_KICK | NOHZ_STATS_KICK) > +#define NOHZ_KICK_MASK (NOHZ_BALANCE_KICK | NOHZ_STATS_KICK | NOHZ_NEXT_KICK) > > #define nohz_flags(cpu) (&cpu_rq(cpu)->nohz_flags) > > -- > 2.25.1 >
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