Messages in this thread | | | From | Valentin Schneider <> | Subject | Re: [PATCH v3 1/2] sched/fair: Add NOHZ balancer flag for nohz.next_balance updates | Date | Mon, 23 Aug 2021 13:57:46 +0100 |
| |
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.
| |