Re: [Xen-devel] [PATCH] xen/sched: fix locking in sched_tick_[suspend|resume]()

["Jürgen Groß" <jgross@suse.com>]

On 04.10.19 18:09, George Dunlap wrote:
> On 10/4/19 4:40 PM, Jürgen Groß wrote:
>> On 04.10.19 17:37, George Dunlap wrote:
>>> On 10/4/19 4:03 PM, Jürgen Groß wrote:
>>>> On 04.10.19 16:56, George Dunlap wrote:
>>>>> On 10/4/19 3:43 PM, Jürgen Groß wrote:
>>>>>> On 04.10.19 16:34, George Dunlap wrote:
>>>>>>> On 10/4/19 3:24 PM, Jürgen Groß wrote:
>>>>>>>> On 04.10.19 16:08, George Dunlap wrote:
>>>>>>>>> On 10/4/19 7:40 AM, Juergen Gross wrote:
>>>>>>>>>> sched_tick_suspend() and sched_tick_resume() should not call the
>>>>>>>>>> scheduler specific timer handlers in case the cpu they are
>>>>>>>>>> running on
>>>>>>>>>> is just being moved to or from a cpupool.
>>>>>>>>>>
>>>>>>>>>> Use a new percpu lock for that purpose.
>>>>>>>>>
>>>>>>>>> Is there a reason we can't use the pcpu_schedule_lock() instead of
>>>>>>>>> introducing a new one?  Sorry if this is obvious, but it's been a
>>>>>>>>> while
>>>>>>>>> since I poked around this code.
>>>>>>>>
>>>>>>>> Lock contention would be higher especially with credit2 which is
>>>>>>>> using a
>>>>>>>> per-core or even per-socket lock. We don't care about other
>>>>>>>> scheduling
>>>>>>>> activity here, all we need is a guard against our per-cpu scheduler
>>>>>>>> data being changed beneath our feet.
>>>>>>>
>>>>>>> Is this code really being called so often that we need to worry about
>>>>>>> this level of contention?
>>>>>>
>>>>>> Its called each time idle is entered and left again.
>>>>>>
>>>>>> Especially with core scheduling there is a high probability of
>>>>>> multiple
>>>>>> cpus leaving idle at the same time and the per-scheduler lock being
>>>>>> used
>>>>>> in parallel already.
>>>>>
>>>>> Hrm, that does sound pretty bad.
>>>>>
>>>>>>> We already have a *lot* of locks; and in this case you're adding a
>>>>>>> second lock which interacts with the per-scheduler cpu lock.  This
>>>>>>> just
>>>>>>> seems like asking for trouble.
>>>>>>
>>>>>> In which way does it interact with the per-scheduler cpu lock?
>>>>>>
>>>>>>> I won't Nack the patch, but I don't think I would ack it without
>>>>>>> clear
>>>>>>> evidence that the extra lock has a performance improvement that's
>>>>>>> worth
>>>>>>> the cost of the extra complexity.
>>>>>>
>>>>>> I think complexity is lower this way. Especially considering the per-
>>>>>> scheduler lock changing with moving a cpu to or from a cpupool.
>>>>>
>>>>> The key aspect of the per-scheduler lock is that once you hold it, the
>>>>> pointer to the lock can't change.
>>>>>
>>>>> After this patch, the fact remains that sometimes you need to grab one
>>>>> lock, sometimes the other, and sometimes both.
>>>>>
>>>>> And, tick_suspend() lives in the per-scheduler code.  Each scheduler
>>>>> has
>>>>> to remember that tick_suspend and tick_resume hold a completely
>>>>> different lock to the rest of the scheduling functions.
>>>>
>>>> Is that really so critical? Today only credit1 has tick_suspend and
>>>> tick_resume hooks, and both are really very simple. I can add a
>>>> comment in sched-if.h if you like.
>>>>
>>>> And up to now there was no lock at all involved when calling them...
>>>>
>>>> If you think using the normal scheduler lock is to be preferred I'd
>>>> be happy to change the patch. But I should mention I was already
>>>> planning to revisit usage of the scheduler lock and replace it by the
>>>> new per-cpu lock where appropriate (not sure I'd find any appropriate
>>>> path for replacement).
>>>
>>> Well the really annoying thing here is that all the other schedulers --
>>> in particular, credit2, which as you say, is designed to have multiple
>>> runqueues share the same lock -- have to grab & release the lock just to
>>> find out that there's nothing to do.
>>>
>>> And even credit1 doesn't do anything particularly clever -- all it does
>>> is stop and start a timer based on a scheduler-global configuration. And
>>> the scheduling lock is grabbed to switch to idle anyway.  It seems like
>>> we should be able to do something more sensible.
>>
>> Yeah, I thought the same.
> 
> I can think of a couple of options:
> 
> 1. Have schedule.c call s->tick_* when switching to / from idle
> 
> 2. Get rid of s->tick_*, and have sched_credit.c suspend / resume ticks
> when switching to / from idle in csched_schedule()
> 
> 3. Have schedule.c suspend / resume ticks, and have an interface that
> allows schedulers to enable / disable them.
> 
> 4. Rework sched_credit to be tickless.

I'm going with 2., as it will have multiple advantages:

- not very intrusive
- keeps credit specifics in credit
- allows us to fix an existing bug in credit: if we have only one domain
   running in a cpupool with credit scheduler and this domain is capped
   today's handling will disable the credit tick in idle and there will
   be nobody unpausing the vcpus of the capped domain.

Will send patches soon.


Juergen


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