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

Li Zefan
2008-11-22T08:18:47Z

Max Krasnyansky wrote:
>
> Dimitri Sivanich wrote:
>> Hi Greg and Max,
>>
>> On Fri, Nov 21, 2008 at 12:04:25PM -0800, Max Krasnyansky wrote:
>>> Hi Greg,
>>>
>>> I attached debug instrumentation patch for Dmitri to try. I'll clean it up and
>>> add things you requested and will resubmit properly some time next week.
>>>
>> We added Max's debug patch to our kernel and have run Max's Trace 3 scenario, but we do not see a NULL sched-domain remain attached, see my comments below.
>>
>>
>> mount -t cgroup cpuset -ocpuset /cpusets/
>>
>> for i in 0 1 2 3; do mkdir par$i; echo $i > par$i/cpuset.cpus; done
>>
>> kernel: cpusets: rebuild ndoms 1
>> kernel: cpuset: domain 0 cpumask
>> 00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,0
>> 0000000,00000000,00000000,00000000,0
> Oops. I did not realize your NR_CPUS is so large. Unfortunately all your masks
> got truncated.
> I'll update the patch to print cpu list instead of the masks.
>
>> echo 0 > cpuset.sched_load_balance
>> kernel: cpusets: rebuild ndoms 4
>> kernel: cpuset: domain 0 cpumask
>> 00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,0
>> 0000000,00000000,00000000,00000000,0
>> kernel: cpuset: domain 1 cpumask
>> 00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,0
>> 0000000,00000000,00000000,00000000,0
>> kernel: cpuset: domain 2 cpumask
>> 00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,0
>> 0000000,00000000,00000000,00000000,0
>> kernel: cpuset: domain 3 cpumask
>> 00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,0
>> 0000000,00000000,00000000,00000000,0
>> kernel: CPU0 root domain default
>> kernel: CPU0 attaching NULL sched-domain.
>> kernel: CPU1 root domain default
>> kernel: CPU1 attaching NULL sched-domain.
>> kernel: CPU2 root domain default
>> kernel: CPU2 attaching NULL sched-domain.
>> kernel: CPU3 root domain default
>> kernel: CPU3 attaching NULL sched-domain.
>
>> kernel: CPU3 root domain e0000069ecb20000
>> kernel: CPU3 attaching sched-domain:
>> kernel: domain 0: span 3 level NODE
>> kernel: groups: 3
>> kernel: CPU2 root domain e000006884a00000
>> kernel: CPU2 attaching sched-domain:
>> kernel: domain 0: span 2 level NODE
>> kernel: groups: 2
>> kernel: CPU1 root domain e000006884a20000
>> kernel: CPU1 attaching sched-domain:
>> kernel: domain 0: span 1 level NODE
>> kernel: groups: 1
>> kernel: CPU0 root domain e000006884a40000
>> kernel: CPU0 attaching sched-domain:
>> kernel: domain 0: span 0 level NODE
>> kernel: groups: 0
>>
>> Which is the way sched_load_balance is supposed to work. You need to set
>> sched_load_balance=0 for all cpusets containing any cpu you want to disable
>> balancing on, otherwise some balancing will happen.
> It won't be much of a balancing in this case because this just one cpu per
> domain.
> In other words no that's not how it supposed to work. There is code in
> cpu_attach_domain() that is supposed to remove redundant levels
> (sd_degenerate() stuff). There is an explicit check in there for numcpus == 1.
> btw The reason you got a different result that I did is because you have a
> NUMA box where is mine is UMA. I was able to reproduce the problem though by
> enabling multi-core scheduler. In which case I also get one redundant domain
> level CPU, with a single CPU in it.
> So we definitely need to fix this. I'll try to poke around tomorrow and figure
> out why redundant level is not dropped.
>

You were not using latest kernel, were you?

There was a bug in sd degenerate code, and it has already been fixed:
http://lkml.org/lkml/2008/11/8/10

>> So in addition to the top (root) cpuset, we need to set it to '0' in the
>> parX cpusets. That will turn off load balancing to the cpus in question
>> (thereby attaching a NULL sched domain).
> As I explained above we should not have to disable load balancing in cpusets
> with a single CPU.
>

Yes, and please try the laste kernel. ;)

>> So when we do that for just par3, we get the following:
>> echo 0 > par3/cpuset.sched_load_balance
>> kernel: cpusets: rebuild ndoms 3
>> kernel: cpuset: domain 0 cpumask
>> 00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,0
>> 0000000,00000000,00000000,00000000,0
>> kernel: cpuset: domain 1 cpumask
>> 00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,0
>> 0000000,00000000,00000000,00000000,0
>> kernel: cpuset: domain 2 cpumask
>> 00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,00000000,0
>> 0000000,00000000,00000000,00000000,0
>> kernel: CPU3 root domain default
>> kernel: CPU3 attaching NULL sched-domain.
>>
>> So the def_root_domain is now attached for CPU 3. And we do have a NULL
>> sched-domain, which we expect for a cpu with load balancing turned off. If
>> we turn sched_load_balance off ('0') on each of the other cpusets (par0-2),
>> each of those cpus would also have a NULL sched-domain attached.
> Ok. This one is a bug in cpuset.c:generate_sched_domains(). Sched domain
> generator in cpusets should not drop domains with single cpu in them when
> sched_load_balance==0. I'll look at that tomorrow too.
>

Do you mean the correct behavior should be as following?
kernel: cpusets: rebuild ndoms 4

But why do you think this is a bug? In generate_sched_domains(), cpusets with
sched_load_balance==0 will be skippped:

list_add(&top_cpuset.stack_list, &q);
while (!list_empty(&q)) {
...
if (is_sched_load_balance(cp)) {
csa[csn++] = cp;
continue;
}
...
}

Correct me if I misunderstood your point.