linux-kernel - Re: [patch 08/13] ACPI/processor: Replace racy task affinity logic.

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Date:   Thu, 13 Apr 2017 14:52:24 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Sebastian Siewior <bigeasy@...utronix.de>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        "David S. Miller" <davem@...emloft.net>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Len Brown <lenb@...nel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>, Tejun Heo <tj@...nel.org>,
        Tony Luck <tony.luck@...el.com>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        linux-acpi@...r.kernel.org
Subject: Re: [patch 08/13] ACPI/processor: Replace racy task affinity logic.

On Thu, Apr 13, 2017 at 02:01:42PM +0200, Thomas Gleixner wrote:
> On Thu, 13 Apr 2017, Peter Zijlstra wrote:
> > 
> > That makes my machine sad...
> > [    9.786610]  work_on_cpu+0x82/0x90
> > [    9.790404]  ? __usermodehelper_disable+0x110/0x110
> > [    9.795846]  ? __acpi_processor_get_throttling+0x20/0x20
> > [    9.801773]  acpi_processor_set_throttling+0x199/0x220
> > [    9.807506]  ? trace_hardirqs_on_caller+0xfb/0x1d0
> > [    9.812851]  acpi_processor_get_throttling_ptc+0xec/0x180
> > [    9.818876]  __acpi_processor_get_throttling+0xf/0x20
> > [    9.824511]  work_for_cpu_fn+0x14/0x20
> > [    9.828692]  process_one_work+0x261/0x670
> > [    9.833165]  worker_thread+0x21b/0x3f0
> > [    9.837348]  kthread+0x108/0x140
> > [    9.840947]  ? process_one_work+0x670/0x670
> > [    9.845611]  ? kthread_create_on_node+0x40/0x40
> > [    9.850667]  ret_from_fork+0x31/0x40
> 
> Yuck. So the call chain is:
> 
> acpi_processor_get_throttling()
>   work_on_cpu(acpi_processor_get_throttling)
> 
> That work does:
> 
> __acpi_processor_get_throttling()
>     acpi_processor_get_throttling_ptc()
>       acpi_processor_set_throttling()
>         work_on_cpu(__acpi_processor_set_throttling)
> 
> Why the heck calls a get_throttling() function set_throttling()? I'm mildly
> surprised.
> 
> Does the delta patch below cure the problem?

Yes, aside from a compile warn.

But that's disgusting... then again, you merely reintroduced this hack.