Messages in this thread Patch in this message | | | Subject | Re: [PATCH v3 1/2] sched/fair: Add NOHZ balancer flag for nohz.next_balance updates | From | Dietmar Eggemann <> | Date | Mon, 23 Aug 2021 15:53:16 +0200 |
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On 23/08/2021 14:57, Valentin Schneider wrote: > On 23/08/21 13:59, Peter Zijlstra wrote: >> On Mon, Aug 23, 2021 at 12:16:59PM +0100, Valentin Schneider wrote: >> >>> 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. >> >>> @@ -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 */ >> >> I'm a bit puzzled by this; that function has: >> >> SCHED_WARN_ON((flags & NOHZ_KICK_MASK) == NOHZ_BALANCE_KICK); >> >> Which: >> >> - isn't updated >> - implies STATS must be set when BALANCE > > Yup > >> >> the latter gives rise to my confusion; why add that gate on STATS? It >> just doesn't make sense to do a BALANCE and not update STATS. > > AFAIA that warning was only there to catch BALANCE && !STATS, so I didn't > tweak it. > > Now, you could still end up with > > flags == NOHZ_NEXT_KICK > > (e.g. nohz.next_balance is in the future, but a new CPU entered NOHZ-idle > and needs its own rq.next_balance collated into the nohz struct) > > in which case you don't do any blocked load update, hence the > gate. In v1 I had that piggyback on NOHZ_STATS_KICK, but Vincent noted > that might not be the best given blocked load updates can be time > consuming - hence the separate flag.
Maybe the confusion stems from the fact that the NOHZ_NEXT_KICK-set changes are only introduced in 2/2?
@@ -10417,6 +10418,9 @@ static void nohz_balancer_kick(struct rq *rq) unlock: rcu_read_unlock(); out: + if (READ_ONCE(nohz.needs_update)) + flags |= NOHZ_NEXT_KICK; +
@@ -10513,12 +10517,13 @@ void nohz_balance_enter_idle(int cpu)
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+ WRITE_ONCE(nohz.needs_update, 1);
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