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Date:	Thu, 07 Aug 2014 10:33:40 -0700
From:	David Daney <ddaney.cavm@...il.com>
To:	Ralf Baechle <ralf@...ux-mips.org>
CC:	Wei.Yang@...driver.com, a.p.zijlstra@...llo.nl, paulus@...ba.org,
	mingo@...hat.com, acme@...nel.org, linux-kernel@...r.kernel.org,
	linux-mips@...ux-mips.org
Subject: Re: [PATCH v1] MIPS: perf: Mark pmu interupt IRQF_NO_THREAD

On 08/07/2014 04:48 AM, Ralf Baechle wrote:
> On Tue, Aug 05, 2014 at 01:37:47PM +0800, Wei.Yang@...driver.com wrote:
>
>> From: Yang Wei <Wei.Yang@...driver.com>
>>
>> In RT kernel, I ran into the following calltrace, so PMU interrupts cannot
>> be threaded
>>
>> in_atomic(): 1, irqs_disabled(): 1, pid: 0, name: swapper/0
>> INFO: lockdep is turned off.
>> Call Trace:
>> [<ffffffff8088595c>] dump_stack+0x1c/0x50
>> [<ffffffff801a958c>] __might_sleep+0x13c/0x148
>> [<ffffffff80891c54>] rt_spin_lock+0x3c/0xb0
>> [<ffffffff801ad29c>] __wake_up+0x3c/0x80
>> [<ffffffff80243ba4>] perf_event_wakeup+0x8c/0xf8
>> [<ffffffff80243c50>] perf_pending_event+0x40/0x78
>> [<ffffffff8023d88c>] irq_work_run+0x74/0xc0
>> [<ffffffff80152640>] mipsxx_pmu_handle_shared_irq+0x110/0x228
>> [<ffffffff8015276c>] mipsxx_pmu_handle_irq+0x14/0x30
>> [<ffffffff801ffda4>] handle_irq_event_percpu+0xbc/0x470
>> [<ffffffff80204478>] handle_percpu_irq+0x98/0xc8
>> [<ffffffff801ff284>] generic_handle_irq+0x4c/0x68
>> [<ffffffff8089748c>] do_IRQ+0x2c/0x48
>> [<ffffffff80105864>] plat_irq_dispatch+0x64/0xd0
>
> Hm...  I don't see why based on this backtrace you concluce the
> handler needs to be marked IRQF_NO_THREAD.  However there's another
> reason to mark it IRQF_NO_THREAD.  IRQ threads may be rescheduled to
> other CPUs but this handler is fiddling with per-CPU resources.
>

Also by its nature, the profiling code needs synchronous access to the 
register state of the interrupted code.  If you are running on a 
different thread, then I don't think this would be available.

> See https://patchwork.linux-mips.org/patch/2818/ for a similar
> scenario a few years ago.
>
>    Ralf
>
>

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