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    SubjectRe: RT sched: cpupri_vec lock contention with def_root_domain and no load balance
    Li Zefan wrote:
    > Max Krasnyansky wrote:
    >> Dimitri Sivanich wrote:
    >>> 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:
    Ah, makes sense.
    The funny part is that I did see the patch before but completely forgot
    about it :).

    >>> 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.
    The problem is that all cpus in cpusets with sched_load_balance==0 end
    up in the default root_domain which causes lock contention.
    We can fix it either in sched.c:partition_sched_domains() or in
    cpusets.c:generate_sched_domains(). I'd rather fix cpusets because
    sched.c fix will be sub-optimal. See my answer to Greg on the same
    thread. Basically the scheduler code would have to allocate a
    root_domain for each CPU even on transitional states. So I'd rather fix
    cpusets to generate domain for each non-overlapping cpuset regardless of
    the sched_load_balance flag.


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      Last update: 2008-11-24 22:49    [W:0.028 / U:0.676 seconds]
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