Re: RT sched: cpupri_vec lock contention with def_root_domain and no load balance

Nish Aravamudan
2008-11-06T09:14:16Z

On Tue, Nov 4, 2008 at 6:36 AM, Peter Zijlstra <peterz@infradead.org> wrote:
> On Tue, 2008-11-04 at 09:34 -0500, Gregory Haskins wrote:
>> Gregory Haskins wrote:
>> > Peter Zijlstra wrote:
>> >
>> >> On Mon, 2008-11-03 at 15:07 -0600, Dimitri Sivanich wrote:
>> >>
>> >>
>> >>> When load balancing gets switched off for a set of cpus via the
>> >>> sched_load_balance flag in cpusets, those cpus wind up with the
>> >>> globally defined def_root_domain attached. The def_root_domain is
>> >>> attached when partition_sched_domains calls detach_destroy_domains().
>> >>> A new root_domain is never allocated or attached as a sched domain
>> >>> will never be attached by __build_sched_domains() for the non-load
>> >>> balanced processors.
>> >>>
>> >>> The problem with this scenario is that on systems with a large number
>> >>> of processors with load balancing switched off, we start to see the
>> >>> cpupri->pri_to_cpu->lock in the def_root_domain becoming contended.
>> >>> This starts to become much more apparent above 8 waking RT threads
>> >>> (with each RT thread running on it's own cpu, blocking and waking up
>> >>> continuously).
>> >>>
>> >>> I'm wondering if this is, in fact, the way things were meant to work,
>> >>> or should we have a root domain allocated for each cpu that is not to
>> >>> be part of a sched domain? Note the the def_root_domain spans all of
>> >>> the non-load-balanced cpus in this case. Having it attached to cpus
>> >>> that should not be load balancing doesn't quite make sense to me.
>> >>>
>> >>>
>> >> It shouldn't be like that, each load-balance domain (in your case a
>> >> single cpu) should get its own root domain. Gregory?
>> >>
>> >>
>> >
>> > Yeah, this sounds broken. I know that the root-domain code was being
>> > developed coincident to some upheaval with the cpuset code, so I suspect
>> > something may have been broken from the original intent. I will take a
>> > look.
>> >
>> > -Greg
>> >
>> >
>>
>> After thinking about it some more, I am not quite sure what to do here.
>> The root-domain code was really designed to be 1:1 with a disjoint
>> cpuset. In this case, it sounds like all the non-balanced cpus are
>> still in one default cpuset. In that case, the code is correct to place
>> all those cores in the singleton def_root_domain. The question really
>> is: How do we support the sched_load_balance flag better?
>>
>> I suppose we could go through the scheduler code and have it check that
>> flag before consulting the root-domain. Another alternative is to have
>> the sched_load_balance=false flag create a disjoint cpuset. Any thoughts?
>
> Hmm, but you cannot disable load-balance on a cpu without placing it in
> an cpuset first, right?
>
> Or are folks disabling load-balance bottom-up, instead of top-down?
>
> In that case, I think we should dis-allow that.

I don't have a lot of insight into the technical discussion, but will
say that (if I understand you right), the "bottom-up" approach was
recommended on LKML by Max K. in the (long) thread from earlier this
year with Subject "Inquiry: Should we remove "isolcpus= kernel boot
option? (may have realtime uses)":

"Just to complete the example above. Lets say you want to isolate cpu2
(assuming that cpusets are already mounted).

# Bring cpu2 offline
echo 0 > /sys/devices/system/cpu/cpu2/online

# Disable system wide load balancing
echo 0 > /dev/cpuset/cpuset.sched_load_banace

# Bring cpu2 online
echo 1 > /sys/devices/system/cpu/cpu2/online

Now if you want to un-isolate cpu2 you do

# Disable system wide load balancing
echo 1 > /dev/cpuset/cpuset.sched_load_banace

Of course this is not a complete isolation. There are also irqs (see my
"default irq affinity" patch), workqueues and the stop machine. I'm working on
those too and will release .25 base cpuisol tree when I'm done."

Would you recommend instead, then, that a new cpuset be created with
only cpu 2 in it (should one set cpuset.cpu_exclusive then?) and then
disabling load balancing in that cpuset?

Thanks,
Nish