linux-kernel - Re: [PATCH] sched: next buddy hint on sleep and preempt path

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Date:	Mon, 7 Mar 2011 18:33:00 -0800
From:	Venkatesh Pallipadi <venki@...gle.com>
To:	Paul Turner <pjt@...gle.com>
Cc:	Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...e.hu>,
	linux-kernel@...r.kernel.org, Mike Galbraith <efault@....de>,
	Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH] sched: next buddy hint on sleep and preempt path - v1

On Mon, Mar 7, 2011 at 5:29 PM, Paul Turner <pjt@...gle.com> wrote:
> On Mon, Mar 7, 2011 at 4:59 PM, Venkatesh Pallipadi <venki@...gle.com> wrote:
>> When a task in a taskgroup sleeps, pick_next_task starts all the way back at
>> the root and picks the task/taskgroup with the min vruntime across all
>> runnable tasks. But, when there are many frequently sleeping tasks
>> across different taskgroups, it makes better sense to stay with same taskgroup
>> for its slice period (or until all tasks in the taskgroup sleeps) instead of
>> switching cross taskgroup on each sleep after a short runtime.
>> This helps specifically where taskgroups corresponds to a process with
>> multiple threads. The change reduces the number of CR3 switches in this case.
>>
>> Example:
>> Two taskgroups with 2 threads each which are running for 2ms and
>> sleeping for 1ms. Looking at sched:sched_switch shows -
>>
>> BEFORE: taskgroup_1 threads [5004, 5005], taskgroup_2 threads [5016, 5017]
>>      cpu-soaker-5004  [003]  3683.391089
>>      cpu-soaker-5016  [003]  3683.393106
>>      cpu-soaker-5005  [003]  3683.395119
>>      cpu-soaker-5017  [003]  3683.397130
>>      cpu-soaker-5004  [003]  3683.399143
>>      cpu-soaker-5016  [003]  3683.401155
>>      cpu-soaker-5005  [003]  3683.403168
>>      cpu-soaker-5017  [003]  3683.405170
>>
>> AFTER: taskgroup_1 threads [21890, 21891], taskgroup_2 threads [21934, 21935]
>>      cpu-soaker-21890 [003]   865.895494
>>      cpu-soaker-21935 [003]   865.897506
>>      cpu-soaker-21934 [003]   865.899520
>>      cpu-soaker-21935 [003]   865.901532
>>      cpu-soaker-21934 [003]   865.903543
>>      cpu-soaker-21935 [003]   865.905546
>>      cpu-soaker-21891 [003]   865.907548
>>      cpu-soaker-21890 [003]   865.909560
>>      cpu-soaker-21891 [003]   865.911571
>>      cpu-soaker-21890 [003]   865.913582
>>      cpu-soaker-21891 [003]   865.915594
>>      cpu-soaker-21934 [003]   865.917606
>>
>> Similar problem is there when there are multiple taskgroups and say a task A
>> preempts currently running task B of taskgroup_1. On schedule, pick_next_task
>> can pick an unrelated task on taskgroup_2. Here it would be better to give some
>> preference to task B on pick_next_task.
>>
>> A simple (may be extreme case) benchmark I tried was tbench with 2 tbench
>> client processes with 2 threads each running on a single CPU. Avg throughput
>> across 5 50 sec runs was -
>> BEFORE: 105.84 MB/sec
>> AFTER: 112.42 MB/sec
>>
>> Changes from v0:
>> * Always pass task se to set_next_buddy
>> * Avoid repeated set_next_buddy in check_preempt_wakeup
>> * Minor flag cleanup in dequeue_task_fair
>>
>> Signed-off-by: Venkatesh Pallipadi <venki@...gle.com>
>> ---
>>  kernel/sched_fair.c |   41 ++++++++++++++++++++++++++++++++++++++---
>>  1 files changed, 38 insertions(+), 3 deletions(-)
>>
>> diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
>> index 3a88dee..cbe442e 100644
>> --- a/kernel/sched_fair.c
>> +++ b/kernel/sched_fair.c
>> @@ -1339,6 +1339,20 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
>>        hrtick_update(rq);
>>  }
>>
>> +static struct sched_entity *pick_next_taskse_on_cfsrq(struct cfs_rq *cfs_rq)
>> +{
>> +       struct sched_entity *se;
>> +
>> +       do {
>> +               se = pick_next_entity(cfs_rq);
>> +               cfs_rq = group_cfs_rq(se);
>> +       } while (cfs_rq);
>> +
>> +       return se;
>> +}
>> +
>
> I think the original approach was much cleaner; the notion of a
> SCHED_IDLE task is only relative versus siblings in group scheduling

Looking at the related code,
static void set_skip_buddy(struct sched_entity *se)
{
	if (likely(task_of(se)->policy != SCHED_IDLE)) {
		for_each_sched_entity(se)
			cfs_rq_of(se)->skip = se;
	}
}

Shouldn't it be always set skip se irrespective of current task's
SCHED_IDLE setting.

Thanks,
Venki
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