linux-kernel - Re: [BUGFIX][PATCH] Fix sched rt group scheduling when hierachy is enabled

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Message-ID: <AANLkTinLe8bMUCTp8Kmv98=s8D8-1qUo78Vff_-Vpgfv@mail.gmail.com>
Date:	Mon, 7 Mar 2011 15:00:24 +0800
From:	Yong Zhang <yong.zhang0@...il.com>
To:	balbir@...ux.vnet.ibm.com
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <pzijlstr@...hat.com>,
	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>,
	Bharata B Rao <bharata.rao@...ibm.com>
Subject: Re: [BUGFIX][PATCH] Fix sched rt group scheduling when hierachy is enabled

On Fri, Mar 4, 2011 at 8:11 PM, Balbir Singh <balbir@...ux.vnet.ibm.com> wrote:
> I based the changes on what I saw during my debugging/test. I
> explained it earlier,
>
> Everyone is dequeued
>
> 1. child runs first, finds parent throttled, so it does not queue
> anything on parent group. child is unthrottled and rt_time now becomes
> 0, parent's rt_nr_running is not incremented.
> 2. Parent timer runs, it is unthrottled, its group->rt_nr_running is 0
> hence enqueue is not called

I have tested with the attached(web mail will mangle it) patch with
yours applied. But I failed to trigger that WARNING.

Below is my steps:
1)mount -t cgroup -ocpu cpu /mnt
2)mkdir /mnt/test-1
3)mkdir /mnt/test-1-1
4)set rt_runtime to 100000 for test-1 and test-1-1
5)run a loop task and attach it to test-1-1

So I thought out a scenario to satisfy your description,
but it's based on the unpatched(without your patch) kernel:
Let's assume a dual-core system with test-1/test-1-1
for rt group, a loop task is running on CPU 1 and test-1
and test-1-1 are both throttled.

              CPU-0                                  CPU-1
do_sched_rt_period_timer(test-1-1)
{
  for CPU-1
    unthrottled test-1-1.rt_rq[1];
      but fail to enqueue it because
      we alway get test-1-1.rt_se[0]
      due to smp_processor_id();
      thus test-1.rt_rq[1].nr_running == 0;
      and it returned with run_time == 0;
}
do_sched_rt_period_timer(test-1)
  unthrottle test-1.rt_rt[1] but
  fail to enqueue test-1.rt_rt[1];
  because nr_running == 0;

                           So if we have your patch for issue-1, when
                           the hrtimer is running on CPU-1, test-1-1
                           and test-1 will be queued because that
                           additional check in run_timer == 0 case.

But once we have your patch for issue-2, the above
problem will be killed by it. right?

Correct me if I'm wrong :)

Thanks,
Yong


-- 
Only stand for myself

View attachment "0001.patch" of type "text/x-patch" (532 bytes)