Messages in this thread Patch in this message | | | From | Valentin Schneider <> | Subject | [PATCH v2 2/2] sched/fair: Trigger nohz.next_balance updates when a CPU goes NOHZ-idle | Date | Mon, 19 Jul 2021 11:31:17 +0100 |
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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.
Use the new nohz.needs_update flag to mark the presence of newly-idle CPUs that need their rq->next_balance to be collated into nohz.next_balance. Trigger a NOHZ_NEXT_KICK when the flag is set.
Signed-off-by: Valentin Schneider <valentin.schneider@arm.com> --- kernel/sched/fair.c | 15 +++++++++++++-- 1 file changed, 13 insertions(+), 2 deletions(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 5c88698c3664..b5a4ea7715b9 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -5698,6 +5698,7 @@ static struct { cpumask_var_t idle_cpus_mask; atomic_t nr_cpus; int has_blocked; /* Idle CPUS has blocked load */ + int needs_update; /* Newly idle CPUs need their next_balance collated */ unsigned long next_balance; /* in jiffy units */ unsigned long next_blocked; /* Next update of blocked load in jiffies */ } nohz ____cacheline_aligned; @@ -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; + 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, /* * 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; + if (flags & NOHZ_NEXT_KICK) + WRITE_ONCE(nohz.needs_update, 1); goto abort; } -- 2.25.1
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