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Message-ID: <CAKfTPtCJzoYNEDu4WJWs0LraL59r8ZDuavBv9rypRKJiX4ZL7g@mail.gmail.com>
Date: Fri, 26 Apr 2013 16:52:50 +0200
From: Vincent Guittot <vincent.guittot@...aro.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: linux-kernel <linux-kernel@...r.kernel.org>,
LAK <linux-arm-kernel@...ts.infradead.org>,
"linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>,
Ingo Molnar <mingo@...nel.org>,
Russell King - ARM Linux <linux@....linux.org.uk>,
Paul Turner <pjt@...gle.com>,
Santosh <santosh.shilimkar@...com>,
Morten Rasmussen <Morten.Rasmussen@....com>,
Chander Kashyap <chander.kashyap@...aro.org>,
"cmetcalf@...era.com" <cmetcalf@...era.com>,
"tony.luck@...el.com" <tony.luck@...el.com>,
Alex Shi <alex.shi@...el.com>,
Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
Paul McKenney <paulmck@...ux.vnet.ibm.com>,
Thomas Gleixner <tglx@...utronix.de>,
Len Brown <len.brown@...el.com>,
Arjan van de Ven <arjan@...ux.intel.com>,
Amit Kucheria <amit.kucheria@...aro.org>,
Jonathan Corbet <corbet@....net>,
Lukasz Majewski <l.majewski@...sung.com>
Subject: Re: [PATCH 08/14] sched: trig ILB on an idle buddy
On 26 April 2013 15:15, Peter Zijlstra <peterz@...radead.org> wrote:
> On Thu, Apr 25, 2013 at 07:23:24PM +0200, Vincent Guittot wrote:
>> If the buddy CPU is not full and currently idle, we trigger an Idle Load
>> Balance to give it the opportunity to pull more tasks.
>>
>> Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
>> ---
>> kernel/sched/fair.c | 24 ++++++++++++++++++++++++
>> 1 file changed, 24 insertions(+)
>>
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index 874f330..954adfd 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -5776,6 +5776,26 @@ end:
>> clear_bit(NOHZ_BALANCE_KICK, nohz_flags(this_cpu));
>> }
>>
>> +static int check_nohz_buddy(int cpu)
>> +{
>> + int buddy = per_cpu(sd_pack_buddy, cpu);
>> +
>> + if (sysctl_sched_packing_mode != SCHED_PACKING_FULL)
>> + return false;
>> +
>> + /* No pack buddy for this CPU */
>> + if (buddy == -1)
>> + return false;
>> +
>> + if (is_buddy_full(buddy))
>> + return false;
>> +
>> + if (cpumask_test_cpu(buddy, nohz.idle_cpus_mask))
>> + return true;
>> +
>> + return false;
>
> return cpumask_test_cpu(..); ?
right
>
>> +}
>> +
>> /*
>> * Current heuristic for kicking the idle load balancer in the presence
>> * of an idle cpu is the system.
>> @@ -5813,6 +5833,10 @@ static inline int nohz_kick_needed(struct rq *rq, int cpu)
>> if (rq->nr_running >= 2)
>> goto need_kick;
>>
>> + /* the buddy is idle and not busy so we can pack */
>> + if (check_nohz_buddy(cpu))
>> + goto need_kick;
>> +
>> rcu_read_lock();
>> for_each_domain(cpu, sd) {
>> struct sched_group *sg = sd->groups;
>
> Bah.. this code is so confusing.. nohz_balance_kick(cpu) won't actually kick
> @cpu.
>
> Again.. suppose we're a big cpu (2) and our little buddy cpu (0) is busy, we
> miss an opportunity to kick the other little cpu into gear (1) and maybe take
> the big core out.
You're right that we can miss an opportunity but it means that cpus
allocated to packing effort are full and a new buddy cpu is going to
be allocated and will pull the task a bit later.
>
>
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