Re: [PATCH 2/2] workqueue:Fix affinity of an unbound worker of a node with 1 online CPU

[Abdul Haleem <abdhalee@linux.vnet.ibm.com>]

Hi Gautham,

Thanks a lot for the fix.

With your patches applied, 4.7.0-rc2 builds fine on ppc64le bare metal.
Boot was successful with No call traces.

Thanks for all your support !

Regard's
Abdul

On Tuesday 07 June 2016 08:44 PM, Gautham R. Shenoy wrote:

> With commit e9d867a67fd03ccc ("sched: Allow per-cpu kernel threads to
> run on online && !active"), __set_cpus_allowed_ptr() expects that only
> strict per-cpu kernel threads can have affinity to an online CPU which
> is not yet active.
>
> This assumption is currently broken in the CPU_ONLINE notification
> handler for the workqueues where restore_unbound_workers_cpumask()
> calls set_cpus_allowed_ptr() when the first cpu in the unbound
> worker's pool->attr->cpumask comes online. Since
> set_cpus_allowed_ptr() is called with pool->attr->cpumask in which
> only one CPU is online which is not yet active, we get the following
> WARN_ON during an CPU online operation.
>
> ------------[ cut here ]------------
> WARNING: CPU: 40 PID: 248 at kernel/sched/core.c:1166
> __set_cpus_allowed_ptr+0x228/0x2e0
> Modules linked in:
> CPU: 40 PID: 248 Comm: cpuhp/40 Not tainted 4.6.0-autotest+ #4
> <..snip..>
> Call Trace:
> [c000000f273ff920] [c00000000010493c] __set_cpus_allowed_ptr+0x2cc/0x2e0 (unreliable)
> [c000000f273ffac0] [c0000000000ed4b0] workqueue_cpu_up_callback+0x2c0/0x470
> [c000000f273ffb70] [c0000000000f5c58] notifier_call_chain+0x98/0x100
> [c000000f273ffbc0] [c0000000000c5ed0] __cpu_notify+0x70/0xe0
> [c000000f273ffc00] [c0000000000c6028] notify_online+0x38/0x50
> [c000000f273ffc30] [c0000000000c5214] cpuhp_invoke_callback+0x84/0x250
> [c000000f273ffc90] [c0000000000c562c] cpuhp_up_callbacks+0x5c/0x120
> [c000000f273ffce0] [c0000000000c64d4] cpuhp_thread_fun+0x184/0x1c0
> [c000000f273ffd20] [c0000000000fa050] smpboot_thread_fn+0x290/0x2a0
> [c000000f273ffd80] [c0000000000f45b0] kthread+0x110/0x130
> [c000000f273ffe30] [c000000000009570] ret_from_kernel_thread+0x5c/0x6c
> ---[ end trace 00f1456578b2a3b2 ]---
>
> This patch sets the affinity of the worker to
> a) the only online CPU in the cpumask of the worker pool when it comes
>     online.
> b) the cpumask of the worker pool when the second CPU in the pool's
>     cpumask comes online.
>
> Reported-by: Abdul Haleem <abdhalee@linux.vnet.ibm.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Tejun Heo <htejun@gmail.com>
> Cc: Michael Ellerman <mpe@ellerman.id.au>
> Signed-off-by: Gautham R. Shenoy <ego@linux.vnet.ibm.com>
> ---