Subject: Re: [PATCH 1/2] workqueue: Move wq_update_unbound_numa() to the beginning of CPU_ONLINE

From: Gautham R Shenoy <[email-masked]>
Date: Tue, 21 Jun 2016 19:42:31 +0530

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 <[email-masked]>
> Reported-by: Gautham R Shenoy <[email-masked]>

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 <[email-masked]>

> ---
>  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;
>  		}
>