linux-kernel - Re: [PATCH 4/4] ARCv2: entry: Reduce perf intr return path

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Message-ID: <64e093da-1778-f1ea-9eae-a1c2ce29556d@synopsys.com>
Date:   Tue, 21 Nov 2017 15:26:32 -0800
From:   Vineet Gupta <Vineet.Gupta1@...opsys.com>
To:     Vineet Gupta <Vineet.Gupta1@...opsys.com>,
        Peter Zijlstra <peterz@...radead.org>
CC:     <linux-snps-arc@...ts.infradead.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/4] ARCv2: entry: Reduce perf intr return path

On 11/17/2017 03:42 PM, Vineet Gupta wrote:

>>>> What do you (on ARC) do about irq_work ?
>>>
> 
>> So the reason I'm asking is that some architectures that don't have NMIs
>> call irq_work_run() at the very end of their perf-interrupt handler (ARM
>> does this for instance).
> 
> But on ARC, we don't call irq_work_run() in perf intr return path and that seem to 
> imply it is broken - as in latency to service a perf induced preemption.

[snip...]

>>> Although I'm sure it is, can you please explain how irq_work is relevant in
>>> the context of this patch.
>>
>> Since the perf interrupt (in general) cannot call a whole lot of things
>> for it needs to assume running from NMI context, it needs to defer
>> things to a more regular context. It does this with irq_work.

So given my understanding of this topic, ARC (or any non NMI based perf intr 
system) is potentially broken without irq_work_run() ?

I can follow up with a patch for ARC, or does this need to addressed for others 
too - say irq_exit_perf() or some such ?