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    SubjectRe: [PATCH 1/2] workqueue: Move wq_update_unbound_numa() to the beginning of CPU_ONLINE
    Hello,

    So, the issue of the initial worker not having its affinity set
    correctly wasn't caused by the order of the operations. Reordering
    just made set_cpus_allowed tried one more time late enough so that it
    hides the race condition most of the time. The problem is that
    CPU_ONLINE callbacks are called while the cpu being onlined is online
    but not active and select_fallback_rq() only considers active cpus, so
    if a kthread gets scheduled in the meantime and it doesn't have any
    cpu which is active in its allowed mask, it's allowed mask gets reset
    to cpu_possible_mask.

    Would something like the following make sense?

    Thanks.
    ------ 8< ------
    Subject: [PATCH] sched: allow kthreads to fallback to online && !active cpus

    During CPU hotplug, CPU_ONLINE callbacks are run while the CPU is
    online but not active. A CPU_ONLINE callback may create or bind a
    kthread so that its cpus_allowed mask only allows the CPU which is
    being brought online. The kthread may start executing before the CPU
    is made active and can end up in select_fallback_rq().

    In such cases, the expected behavior is selecting the CPU which is
    coming online; however, because select_fallback_rq() only chooses from
    active CPUs, it determines that the task doesn't have any viable CPU
    in its allowed mask and ends up overriding it to cpu_possible_mask.

    CPU_ONLINE callbacks should be able to put kthreads on the CPU which
    is coming online. Update select_fallback_rq() so that it follows
    cpu_online() rather than cpu_active() for kthreads.

    Signed-off-by: Tejun Heo <tj@kernel.org>
    Reported-by: Gautham R Shenoy <ego@linux.vnet.ibm.com>
    ---
    kernel/sched/core.c | 4 +++-
    1 file changed, 3 insertions(+), 1 deletion(-)

    diff --git a/kernel/sched/core.c b/kernel/sched/core.c
    index 017d539..a12e3db 100644
    --- a/kernel/sched/core.c
    +++ b/kernel/sched/core.c
    @@ -1536,7 +1536,9 @@ static int select_fallback_rq(int cpu, struct task_struct *p)
    for (;;) {
    /* Any allowed, online CPU? */
    for_each_cpu(dest_cpu, tsk_cpus_allowed(p)) {
    - if (!cpu_active(dest_cpu))
    + if (!(p->flags & PF_KTHREAD) && !cpu_active(dest_cpu))
    + continue;
    + if (!cpu_online(dest_cpu))
    continue;
    goto out;
    }
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