linux-kernel - Re: [PATCH v3 03/11] ARC: Allow irq threading

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Date:   Fri, 25 Aug 2017 13:45:06 -0700
From:   Vineet Gupta <Vineet.Gupta1@...opsys.com>
To:     Noam Camus <noamca@...lanox.com>,
        <linux-snps-arc@...ts.infradead.org>
CC:     <noamikong@...il.com>, <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH v3 03/11] ARC: Allow irq threading

+CC Peter, Tglx, Steven

On 06/15/2017 01:43 AM, Noam Camus wrote:
> From: Noam Camus <noamc@...hip.com>
> 
> Working with NPS400 we noticed that there is a possibility of L1
> interrupt nesting that may run out kernel stack.
> The scenario include serving invoke_softirqs() from irq_exit()
> and once local_irq_enable() called can hit another one before we
> managed to restore last one and pop some place from kernel stack.
> 
> Serving softirqs at dedicated kernel thread may mitigate this.
> We see that many architectures, including x86, behave like this.
> 
> Note 1: All interrupts which must be non threaded
> should be marked IRQF_NO_THREAD.
> Note 2: using kernel param "threadirqs" is needed to actually
> turn this on. This configuration is only a preperation.
> 
> Signed-off-by: Noam Camus <noamc@...hip.com>
> ---
>   arch/arc/Kconfig |    1 +
>   1 files changed, 1 insertions(+), 0 deletions(-)
> 
> diff --git a/arch/arc/Kconfig b/arch/arc/Kconfig
> index a545969..f464f97 100644
> --- a/arch/arc/Kconfig
> +++ b/arch/arc/Kconfig
> @@ -33,6 +33,7 @@ config ARC
>   	select HAVE_OPROFILE
>   	select HAVE_PERF_EVENTS
>   	select HANDLE_DOMAIN_IRQ
> +	select IRQ_FORCED_THREADING
>   	select IRQ_DOMAIN
>   	select MODULES_USE_ELF_RELA
>   	select NO_BOOTMEM> 

As Noam notes above and looking at work needed in other arches prior to switching 
to this: we need mark the low level ints as NO_THREAD.

ARC ipi, timer, perf interrupts happen to use request_percpu_irq() which doesn't 
take flags and doesn't seem to force NO_THREAD internally either. Does that mean 
we first need to convert all of these sites to __request_percpu_irq(.. NO_THREAD 
...) or is there a better way of doing this !

Thx,
-Vineet