linux-kernel - Re: [PATCH v2 0/2] arm64: Only call into preempt_schedule() if need

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Message-ID: <20181206150850.GI13538@hirez.programming.kicks-ass.net>
Date:   Thu, 6 Dec 2018 16:08:50 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Will Deacon <will.deacon@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        ard.biesheuvel@...aro.org, catalin.marinas@....com, rml@...h9.net,
        tglx@...utronix.de, schwidefsky@...ibm.com
Subject: Re: [PATCH v2 0/2] arm64: Only call into preempt_schedule() if
 need_resched()

On Fri, Nov 30, 2018 at 05:34:29PM +0000, Will Deacon wrote:
> Hi all,
> 
> This is version two of the patches I originally posted here:
> 
>   http://lkml.kernel.org/r/1543347902-21170-1-git-send-email-will.deacon@arm.com
> 
> The only change since v1 is that  __preempt_count_dec_and_test() now
> reloads the need_resched flag if it initially saw that it was set. This
> resolves the issue spotted by Peter, where an IRQ coming in during the
> decrement can cause a reschedule to be missed.

Yes, I think this one will work, so:

Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org>

However, this leaves me wondering if the sequence is actually much
better than what you had?

I suppose there's a win due to cache locality -- you only have to load a
single line -- but I'm thinking that on pure instruction count, you're
not actually winning much.