Re: scheduler scalability - cgroups, cpusets and load-balancing

["Gregory Haskins" <ghaskins@novell.com>]

>>> On Tue, Jan 29, 2008 at  6:30 AM, in message
<20080129053005.bc7a11d7.pj@sgi.com>, Paul Jackson <pj@sgi.com> wrote: 
> Peter wrote, in reply to Peter ;):
>> > [ It looks to me it balances a group over the largest SD the current cpu
>> >   has access to, even though that might be larger than the SD associated
>> >   with the cpuset of that particular cgroup. ]
>> 
>> Hmm, with a bit more thought I think that does indeed DTRT. Because, if
>> the cpu belongs to a disjoint cpuset, the highest sd (with
>> load-balancing enabled) would be that. Right?
> 
> The code that defines sched domains, kernel/sched.c 
> partition_sched_domains(),
> as called from the cpuset code in kernel/cpuset.c rebuild_sched_domains(),
> does not make use of the full range of sched_domain possibilities.
> 
> In particular, it only sets up some non-overlapping set of sched domains.
> Every CPU ends up in at most a single sched domain.
> 
> The original reason that one can't define overlapping sched domains via
> this cpuset interface (based off the cpuset 'sched_load_balance' flag)
> is that I didn't realize it was even possible to overlap sched domains
> when I wrote the cpuset code defining sched domains.  And then when I
> later realized one could overlap sched domains, I (a) didn't see a need
> to do so, and (b) couldn't see how to do so via the cpuset interface
> without causing my brain to explode.
> 
> Now, back to Peter's question, being a bit pedantic, CPUs don't belong
> to disjoint cpusets, except in the most minimal situation that there is
> only one cpuset covering all CPUs.
> 
> Rather what happens, when you have need for some realtime CPUs, is that:
>  1) you turn off sched_load_balance on the top cpuset,
>  2) you setup your realtime cpuset as a child cpuset of the top cpuset
>     such that its CPUs doesn't overlap any of its siblings, and
>  3) you turn off sched_load_balance in that realtime cpuset.
> 
> At that point, sched domains are rebuilt, including providing a
> sched domain that just contains the CPUs in that realtime cpuset, and
> normal scheduler load balancing ceases on the CPUs in that realtime
> cpuset.

Hi Paul,
  I am a bit confused as to why you disable load-balancing in the RT cpuset?  It shouldn't be strictly necessary in order for the RT scheduler to do its job (unless I am misunderstanding what you are trying to accomplish?).  Do you do this because you *have* to in order to make real-time deadlines, or because its just a further optimization?

-Greg


> 
>> [ Just a bit of a shame we have all cgroups represented on each cpu. ]
> 
> Could you restate this -- I suspect it's obvious, but I'm oblivious ;).