Re: [PATCH v2 1/3] sched/fair: Disable runtime_enabled on dying rq

From: Kirill Tkhai
Date: Tue Jun 24 2014 - 15:02:16 EST


On 24.06.2014 21:03, bsegall@xxxxxxxxxx wrote:
> Kirill Tkhai <ktkhai@xxxxxxxxxxxxx> writes:
>
>> We kill rq->rd on the CPU_DOWN_PREPARE stage:
>>
>> cpuset_cpu_inactive -> cpuset_update_active_cpus -> partition_sched_domains ->
>> -> cpu_attach_domain -> rq_attach_root -> set_rq_offline
>>
>> This unthrottles all throttled cfs_rqs.
>>
>> But the cpu is still able to call schedule() till
>>
>> take_cpu_down->__cpu_disable()
>>
>> is called from stop_machine.
>>
>> This case the tasks from just unthrottled cfs_rqs are pickable
>> in a standard scheduler way, and they are picked by dying cpu.
>> The cfs_rqs becomes throttled again, and migrate_tasks()
>> in migration_call skips their tasks (one more unthrottle
>> in migrate_tasks()->CPU_DYING does not happen, because rq->rd
>> is already NULL).
>>
>> Patch sets runtime_enabled to zero. This guarantees, the runtime
>> is not accounted, and the cfs_rqs won't exceed given
>> cfs_rq->runtime_remaining = 1, and tasks will be pickable
>> in migrate_tasks(). runtime_enabled is recalculated again
>> when rq becomes online again.
>>
>> Ben Segall also noticed, we always enable runtime in
>> tg_set_cfs_bandwidth(). Actually, we should do that for online
>> cpus only. To fix that, we check if a cpu is online when
>> its rq is locked. This guarantees we do not have races with
>> set_rq_offline(), which also requires rq->lock.
>>
>> v2: Fix race with tg_set_cfs_bandwidth().
>> Move cfs_rq->runtime_enabled=0 above unthrottle_cfs_rq().
>>
>> Signed-off-by: Kirill Tkhai <ktkhai@xxxxxxxxxxxxx>
>> CC: Konstantin Khorenko <khorenko@xxxxxxxxxxxxx>
>> CC: Ben Segall <bsegall@xxxxxxxxxx>
>> CC: Paul Turner <pjt@xxxxxxxxxx>
>> CC: Srikar Dronamraju <srikar@xxxxxxxxxxxxxxxxxx>
>> CC: Mike Galbraith <umgwanakikbuti@xxxxxxxxx>
>> CC: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
>> CC: Ingo Molnar <mingo@xxxxxxxxxx>
>> ---
>> kernel/sched/core.c | 15 +++++++++++----
>> kernel/sched/fair.c | 22 ++++++++++++++++++++++
>> 2 files changed, 33 insertions(+), 4 deletions(-)
>>
>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>> index 7f3063c..707a3c5 100644
>> --- a/kernel/sched/core.c
>> +++ b/kernel/sched/core.c
>> @@ -7842,11 +7842,18 @@ static int tg_set_cfs_bandwidth(struct task_group *tg, u64 period, u64 quota)
>> struct rq *rq = cfs_rq->rq;
>>
>> raw_spin_lock_irq(&rq->lock);
>> - cfs_rq->runtime_enabled = runtime_enabled;
>> - cfs_rq->runtime_remaining = 0;
>> + /*
>> + * Do not enable runtime on offline runqueues. We specially
>> + * make it disabled in unthrottle_offline_cfs_rqs().
>> + */
>> + if (cpu_online(i)) {
>> + cfs_rq->runtime_enabled = runtime_enabled;
>> + cfs_rq->runtime_remaining = 0;
>> +
>> + if (cfs_rq->throttled)
>> + unthrottle_cfs_rq(cfs_rq);
>> + }
>
> We can just do for_each_online_cpu, yes? Also we probably need
> get_online_cpus/put_online_cpus, and/or want cpu_active_mask instead
> right?
>

Yes, we can use for_each_online_cpu/for_each_active_cpu with
get_online_cpus() taken. But it adds one more lock dependence.
This looks worse for me.
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