linux-kernel - Re: NMI for ARC

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Message-ID: <3b5cbc7c-ed5c-af95-8dc9-25fd92277e98@synopsys.com>
Date:   Wed, 28 Sep 2016 10:58:06 -0700
From:   Vineet Gupta <Vineet.Gupta1@...opsys.com>
To:     Peter Zijlstra <peterz@...radead.org>
CC:     arcml <linux-snps-arc@...ts.infradead.org>,
        Alexey Brodkin <Alexey.Brodkin@...opsys.com>,
        lkml <linux-kernel@...r.kernel.org>,
        "Andy Lutomirski" <luto@...capital.net>
Subject: Re: NMI for ARC

On 09/28/2016 12:16 AM, Peter Zijlstra wrote:
>> 1. Is it OK in general to short-circuit preemption off irq checks for NMI style
>> > interrupts. 
> Yes. If the NMI returns to kernel space you must not attempt preemption
> for reasons you found :-), if the NMI returns to userspace you should do
> the normal return to user bits, I think.

Just to add, eliding preemption check in return path might not be sufficient as we
could still have timer intr leading to scheduler_tick() - whcih is what i found as
well ;-) So scheduler nevertheless needs to be told to not reschedule in this code
path.

So eliding preempt-off-irq just becomes an optimization for NMI code paths IMHO.

Now I was a stupid fool, fudging preemption counts in low level code, to achieve
this, whereas we have nice generic nmi_{enter,exit}() and in_nmi() helpers which
can be used.

Thx,
-Vineet