Hi Tejun,
On Thu, Jun 16, 2016 at 03:35:04PM -0400, Tejun Heo wrote:
> 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 <t...@kernel.org>
> Reported-by: Gautham R Shenoy <e...@linux.vnet.ibm.com>
Hi Tejun,
This patch fixes the issue on POWER. I am able to see the worker
threads of the unbound workqueues of the newly onlined node with this.
Tested-by: Gautham R. Shenoy <e...@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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