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Date: Wed, 04 Jun 2008 08:28:53 -0600
From: "Gregory Haskins" <ghaskins@...ell.com>
To: "Peter Zijlstra" <peterz@...radead.org>
Cc: "Ingo Molnar" <mingo@...e.hu>,
"Steven Rostedt" <rostedt@...dmis.org>,
"Thomas Gleixner" <tglx@...utonix.de>,
"Arnaldo Carvalho de Melo" <acme@...hat.com>,
<linux-kernel@...r.kernel.org>, <linux-rt-users@...r.kernel.org>
Subject: Re: [PATCH] sched: fix cpupri hotplug support
>>> On Wed, Jun 4, 2008 at 7:59 AM, in message <1212580796.23439.10.camel@...ns>,
Peter Zijlstra <peterz@...radead.org> wrote:
> On Tue, 2008-06-03 at 23:24 -0400, Gregory Haskins wrote:
>> Hi Ingo, Steven, Thomas,
>> This patch is critical to fix a snafu in the root-domain/cpupri code. It
>> applies to 25.4-rt4, but as mentioned below it affects most recent -rt
>> kernels as well as sched-devel. If you accept this patch, and you have
>> non-trivial merge issues with other flavors just let me know and I will
> port
>> it to those kernels as well.
>>
>> Regards
>> -Greg
>>
>> --------------------------------
>> sched: fix cpupri hotplug support
>>
>> The RT folks over at RedHat found an issue w.r.t. hotplug support which
>> was traced to problems with the cpupri infrastructure in the scheduler:
>>
>> https://bugzilla.redhat.com/show_bug.cgi?id=449676
>>
>> This bug affects 23-rt12+, 24-rtX, 25-rtX, and sched-devel. This patch
>> applies to 25.4-rt4, though it should trivially apply to most cpupri enabled
>> kernels mentioned above.
>>
>> It turned out that the issue was that offline cpus could get inadvertently
>> registered with cpupri so that they were erroneously selected during
>> migration decisions. The end result would be an OOPS as the offline cpu
>> had tasks routed to it.
>
> This is not what I saw, I traced it to select_task_rq_rt() selecting a
> cpu that never was online. In my case cpu 2 on a dual core.
First off, Peter, I didnt realize you were involved too. So I apologize for omitting you in the credits. :(
Note that despite the reproduction case difference, I think we are still talking about the same basic problem here...and I think your observation would also be fixed by this patch.
The issue is that a cpu that is not online is being set to something other than INVALID in cpupri, so it comes up as a valid target during the various migration decisions. This should now be fixed.
>
>> This patch adds a new per-sched-class pair of interfaces:
>> online/offline. They allow for the sched-class to register for
>> notification when a run-queue is taken online or offline. Additionally,
>> the existing join/leave-domain code has been unified behind the new
>> online/offline handlers so that they do the right thing w.r.t. the online
>> status of a particular CPU.
>>
>> I was able to easily reproduce the issue prior to this patch, and am no
>> longer able to reproduce it after this patch. I can offline cpus
>> indefinately and everything seems to be in working order.
>>
>> Thanks to Arnaldo (acme) and Thomas (tglx) for doing the legwork to point
>> me in the right direction.
>>
>> Signed-off-by: Gregory Haskins <ghaskins@...ell.com>
>> CC: Ingo Molnar <mingo@...e.hu>
>> CC: Thomas Gleixner <tglx@...utonix.de>
>> CC: Steven Rostedt <rostedt@...dmis.org>
>> CC: Arnaldo Carvalho de Melo <acme@...hat.com>
>> ---
>>
>> include/linux/sched.h | 2 ++
>> kernel/sched.c | 19 +++++++++++++++++++
>> kernel/sched_rt.c | 16 ++++++++++++----
>> 3 files changed, 33 insertions(+), 4 deletions(-)
>>
>> diff --git a/include/linux/sched.h b/include/linux/sched.h
>> index bb71371..9c5b8c9 100644
>> --- a/include/linux/sched.h
>> +++ b/include/linux/sched.h
>> @@ -970,6 +970,8 @@ struct sched_class {
>>
>> void (*join_domain)(struct rq *rq);
>> void (*leave_domain)(struct rq *rq);
>> + void (*online)(struct rq *rq);
>> + void (*offline)(struct rq *rq);
>
> Rather unfortunate to add yet another two callbacks, can't this be done
> with a creative use of leave/join?
Yeah, I was debating how to do this. In fact, as you can see the implementation of join/online and leave/offline is identical. I wasnt sure if putting a join/leave in the hotplug code made sense, so I added a new callback that maps to the same logic. Perhaps what I should have done was to s/join/online and s/leave/offline and fixed the root-domain code to use the online/offline variant. I like this better so I will put out a v2 patch in a few minutes.
>
>> void (*switched_from) (struct rq *this_rq, struct task_struct *task,
>> int running);
>> diff --git a/kernel/sched.c b/kernel/sched.c
>> index 0399411..fe96984 100644
>> --- a/kernel/sched.c
>> +++ b/kernel/sched.c
>> @@ -6278,6 +6278,9 @@ static void unregister_sched_domain_sysctl(void)
>> }
>> #endif
>>
>> +#define for_each_class(class) \
>> + for (class = sched_class_highest; class; class = class->next)
>> +
>> /*
>> * migration_call - callback that gets triggered when a CPU is added.
>> * Here we can start up the necessary migration thread for the new CPU.
>> @@ -6314,8 +6317,15 @@ migration_call(struct notifier_block *nfb, unsigned
> long action, void *hcpu)
>> rq = cpu_rq(cpu);
>> spin_lock_irqsave(&rq->lock, flags);
>> if (rq->rd) {
>> + const struct sched_class *class;
>> +
>> BUG_ON(!cpu_isset(cpu, rq->rd->span));
>> cpu_set(cpu, rq->rd->online);
>> +
>> + for_each_class(class) {
>> + if (class->online)
>> + class->online(rq);
>> + }
>> }
>> spin_unlock_irqrestore(&rq->lock, flags);
>> break;
>> @@ -6375,8 +6385,17 @@ migration_call(struct notifier_block *nfb, unsigned
> long action, void *hcpu)
>> rq = cpu_rq(cpu);
>> spin_lock_irqsave(&rq->lock, flags);
>> if (rq->rd) {
>> + const struct sched_class *class;
>> +
>> BUG_ON(!cpu_isset(cpu, rq->rd->span));
>> +
>> + for_each_class(class) {
>> + if (class->offline)
>> + class->offline(rq);
>> + }
>> +
>> cpu_clear(cpu, rq->rd->online);
>> +
>> }
>> spin_unlock_irqrestore(&rq->lock, flags);
>> break;
>
> I'm thinking this should be done in update_sched_domains().
Hmm...I think I agree with you. I will add this to v2 as well.
>
>> diff --git a/kernel/sched_rt.c b/kernel/sched_rt.c
>> index 425cf01..8baf2e8 100644
>> --- a/kernel/sched_rt.c
>> +++ b/kernel/sched_rt.c
>> @@ -1101,8 +1101,11 @@ static void set_cpus_allowed_rt(struct task_struct *p,
> cpumask_t *new_mask)
>> }
>>
>> /* Assumes rq->lock is held */
>> -static void join_domain_rt(struct rq *rq)
>> +static void rq_online_rt(struct rq *rq)
>> {
>> + if (!cpu_isset(rq->cpu, rq->rd->online))
>> + return;
>> +
>> if (rq->rt.overloaded)
>> rt_set_overload(rq);
>>
>> @@ -1110,8 +1113,11 @@ static void join_domain_rt(struct rq *rq)
>> }
>>
>> /* Assumes rq->lock is held */
>> -static void leave_domain_rt(struct rq *rq)
>> +static void rq_offline_rt(struct rq *rq)
>> {
>> + if (!cpu_isset(rq->cpu, rq->rd->online))
>> + return;
>> +
>> if (rq->rt.overloaded)
>> rt_clear_overload(rq);
>
> So you now fully remove the leave/join calls because switching between
> root domains doesn't need to migrate the overload status between those
> domains? - seems unlikely.
I think you are misinterpreting that part. leave/join is fully functional and intact. I just merged the two implementations to call one function (see the sched_class assignment below)
>
>> @@ -1278,8 +1284,10 @@ const struct sched_class rt_sched_class = {
>> .load_balance = load_balance_rt,
>> .move_one_task = move_one_task_rt,
>> .set_cpus_allowed = set_cpus_allowed_rt,
>> - .join_domain = join_domain_rt,
>> - .leave_domain = leave_domain_rt,
>> + .join_domain = rq_online_rt,
>> + .leave_domain = rq_offline_rt,
>> + .online = rq_online_rt,
>> + .offline = rq_offline_rt,
>> .pre_schedule = pre_schedule_rt,
>> .post_schedule = post_schedule_rt,
>> .task_wake_up = task_wake_up_rt,
--
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