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    SubjectRe: [PATCH] sched_groups are expected to be circular linked list, make it so right after allocation
    ----- Original Message -----
    > From: "Peter Zijlstra" <peterz@infradead.org>
    > To: "Igor Mammedov" <imammedo@redhat.com>
    > Cc: "Jiang Liu" <liuj97@gmail.com>, linux-kernel@vger.kernel.org, mingo@kernel.org, pjt@google.com,
    > tglx@linutronix.de, "seto hidetoshi" <seto.hidetoshi@jp.fujitsu.com>
    > Sent: Thursday, May 10, 2012 3:45:44 PM
    > Subject: Re: [PATCH] sched_groups are expected to be circular linked list, make it so right after allocation
    >
    > On Thu, 2012-05-10 at 15:26 +0200, Igor Mammedov wrote:
    > > [ 141.699854] sched: Bonkers domain doesn't include its own cpu: 3
    > > 0-1,3
    > > [ 141.725038] sched: Bonkers domain doesn't include its own cpu: 3
    > > 0-1
    >
    > Whee!! so cpu_mask (active_mask) does include 3, but the tl->mask()
    > doesn't.
    >
    > > [ 141.775040] sched: Topology is hosed for CPU-3!!
    > > [ 141.775596] sched: domain: NODE 0-1
    > > [ 141.776004] sched: group: 0-1
    > >
    > This seems to suggest its the node topology being wrecked.
    >
    > which with your code-base would be
    > cpu_node_mask()->sched_domain_node_span()..
    >
    > Did you specify any node topology on the qemu command line? If not,
    > it
    > should all reduce to cpumask_of_node(0).
    >
    > identify_secondary_cpu()->identify_cpu()->numa_add_cpu() should set
    > that
    > bit. which is well before the CPU_ONLINE->cpuset_update_active_cpus()
    > sched domain rebuild.
    >
    >
    > Most puzzling. Can you dig a little deeper as to why these masks
    > might
    > be wrong? Also, can you reproduce on actual hardware? The reason I
    probably not

    > never
    > use kvm or other virt for debugging is that I always end up spending
    > time chasing virt bugs, and I hate virt..

    qemu started with -smp 4,sockets=4,cores=1,threads=1

    I can answer it right away. It happens when one of cpus failed
    to come online on time and boot cpu decided to abandon attempt and exit
    do_boot_cpu with error. While exiting with error it clears some masks:

    if (boot_error) {
    /* Try to put things back the way they were before ... */
    numa_remove_cpu(cpu); /* was set by numa_add_cpu */

    /* was set by do_boot_cpu() */
    cpumask_clear_cpu(cpu, cpu_callout_mask);

    /* was set by cpu_init() */
    cpumask_clear_cpu(cpu, cpu_initialized_mask);

    set_cpu_present(cpu, false);
    per_cpu(x86_cpu_to_apicid, cpu) = BAD_APICID;
    }

    However cpu that has been blissfully forgotten, continues to boot and sets
    cpu_active_mask. So causing all sorts of havoc in places that use this mask.
    Then when next cpu is being onlined/offlined it eventually calls
    CPU_[ONLINE|DOWN]->cpuset_update_active_cpus()
    result is OOPS.

    That is why I've posted https://lkml.org/lkml/2012/5/9/125
    that prevent all of this from happening by 'gracefully' aborting boot
    process on failed cpu with necessary cleanup.

    Checked Ingo's git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git:master
    races, that patches 1, 2, 4 are fixing, are still there.
    If it is preferable for this patches to go into -tip, then I'll rebase them
    on top of it and repost. ???

    Please review them if you think it has a sense to fix races in mainline,
    before cpu-hotplug overhaul of this part is complete.


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