Messages in this thread Patch in this message | | | Subject | Re: sched: Improve load balancing in the presence of idle CPUs | From | Tim Chen <> | Date | Fri, 03 Apr 2015 15:35:18 -0700 |
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On Wed, 2015-04-01 at 22:59 -0700, Jason Low wrote: > On Wed, 2015-04-01 at 18:04 +0100, Morten Rasmussen wrote: > > On Wed, Apr 01, 2015 at 07:49:56AM +0100, Preeti U Murthy wrote: > > > > I am sorry I don't quite get this. Can you please elaborate? > > > > I think the scenario is that we are in nohz_idle_balance() and decide to > > bail out because we have pulled some tasks, but before leaving > > nohz_idle_balance() we want to check if more balancing is necessary > > using nohz_kick_needed() and potentially kick somebody to continue. > > Also, below is an example patch. > > (Without the conversion to idle_cpu(), the check for rq->idle_balance > would not be accurate anymore) > > --- > kernel/sched/fair.c | 17 ++++++++++------- > 1 files changed, 10 insertions(+), 7 deletions(-) > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index fdae26e..7749a14 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -7620,6 +7620,8 @@ out: > } > > #ifdef CONFIG_NO_HZ_COMMON > +static inline bool nohz_kick_needed(struct rq *rq); > + > /* > * In CONFIG_NO_HZ_COMMON case, the idle balance kickee will do the > * rebalancing for all the cpus for whom scheduler ticks are stopped. > @@ -7629,6 +7631,7 @@ static void nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle) > int this_cpu = this_rq->cpu; > struct rq *rq; > int balance_cpu; > + bool done_balancing = false; > > if (idle != CPU_IDLE || > !test_bit(NOHZ_BALANCE_KICK, nohz_flags(this_cpu))) > @@ -7644,7 +7647,7 @@ static void nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle) > * balancing owner will pick it up. > */ > if (need_resched()) > - break; > + goto end; > > rq = cpu_rq(balance_cpu); > > @@ -7663,9 +7666,12 @@ static void nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle) > if (time_after(this_rq->next_balance, rq->next_balance)) > this_rq->next_balance = rq->next_balance; > } > + done_balancing = true; > nohz.next_balance = this_rq->next_balance; > end: > clear_bit(NOHZ_BALANCE_KICK, nohz_flags(this_cpu)); > + if (!done_balancing && nohz_kick_needed(this_rq)) > + nohz_balancer_kick(); > } > >
I think we can get rid of the done_balancing boolean and make it a bit easier to read if we change the above code to
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index bcfe320..08317dc 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -7557,8 +7557,13 @@ static void nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle) * work being done for other cpus. Next load * balancing owner will pick it up. */ - if (need_resched()) - break; + if (need_resched()) { + /* preparing to bail, kicking other cpu to continue */ + clear_bit(NOHZ_BALANCE_KICK, nohz_flags(this_cpu)); + if (nohz_kick_needed(this_rq)) + nohz_balance_kick(); + return; + } rq = cpu_rq(balance_cpu);
Thanks. Tim
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