linux-kernel - Re: [PATCH v5 3/7] sched: set initial value of runnable avg for new forked task

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Date:	Thu, 09 May 2013 16:22:32 +0800
From:	Alex Shi <alex.shi@...el.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Paul Turner <pjt@...gle.com>,
	Morten Rasmussen <morten.rasmussen@....com>,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Borislav Petkov <bp@...en8.de>,
	Namhyung Kim <namhyung@...nel.org>,
	Mike Galbraith <efault@....de>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	Michael Wang <wangyun@...ux.vnet.ibm.com>
Subject: Re: [PATCH v5 3/7] sched: set initial value of runnable avg for new
 forked task

On 05/08/2013 07:34 PM, Peter Zijlstra wrote:
>> > If we wanted to be more exacting about it we could just give them a
>> > sched_slice() worth; this would have a few obvious "nice" properties
>> > (pun intended).
> Oh I see I misunderstood again :/ Its not about the effective load but weight
> of the initial effective load wrt adjustment.
> 
> Previous schedulers didn't have this aspect at all, so no experience from me
> here. Paul would be the one, since he's ran longest with this stuff.
> 
> That said, I would tend to keep it shorter rather than longer so that it would
> adjust quicker to whatever it really wanted to be.
> 
> Morten says the load is unstable specifically on loaded systems. I would think
> this is because we'd experience scheduling latency, we're runnable more pushing
> things up. But if we're really an idle task at heart we'd not run again for a
> long while, pushing things down again.
> 
> So on that point Paul's suggestion of maybe starting with __sched_slice() might
> make sense because it increases the weight of the initial avg with nr_running.
> 
> Not sure really, we'll have to play and see what works best for a number of
> workloads.


The patch of using sched_slice for review, I am testing the benchmarks

---
>From da40ffa90ec1de520bd7e92f5653734a964e3bb2 Mon Sep 17 00:00:00 2001
From: Alex Shi <alex.shi@...el.com>
Date: Thu, 9 May 2013 15:28:34 +0800
Subject: [PATCH 4/8] sched: set initial runnable avg for new task

---
 kernel/sched/core.c  |  2 ++
 kernel/sched/fair.c  | 15 +++++++++++++++
 kernel/sched/sched.h |  2 ++
 3 files changed, 19 insertions(+)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index ecec7f1..c17925b 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1716,6 +1716,8 @@ void wake_up_new_task(struct task_struct *p)
 	set_task_cpu(p, select_task_rq(p, SD_BALANCE_FORK, 0));
 #endif
 
+	/* Give new task a start runnable time */
+	set_task_runnable_avg(p);
 	rq = __task_rq_lock(p);
 	activate_task(rq, p, 0);
 	p->on_rq = 1;
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 2881d42..4ec5f29 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -661,6 +661,21 @@ static u64 sched_vslice(struct cfs_rq *cfs_rq, struct sched_entity *se)
 	return calc_delta_fair(sched_slice(cfs_rq, se), se);
 }
 
+#ifdef CONFIG_SMP
+void set_task_runnable_avg(struct task_struct *p)
+{
+	u64 slice;
+
+	slice = sched_slice(task_cfs_rq(p), &p->se);
+	p->se.avg.runnable_avg_sum = slice;
+	p->se.avg.runnable_avg_period = slice;
+}
+#else
+void set_task_runnable_avg(struct task_struct *p)
+{
+}
+#endif
+
 /*
  * Update the current task's runtime statistics. Skip current tasks that
  * are not in our scheduling class.
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index c6634f1..518f3d8a 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -900,6 +900,8 @@ extern void init_rt_bandwidth(struct rt_bandwidth *rt_b, u64 period, u64 runtime
 
 extern void update_idle_cpu_load(struct rq *this_rq);
 
+extern void set_task_runnable_avg(struct task_struct *p);
+
 #ifdef CONFIG_CGROUP_CPUACCT
 #include <linux/cgroup.h>
 /* track cpu usage of a group of tasks and its child groups */
-- 
1.7.12

-- 
Thanks
    Alex
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