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From:   Dmitry Vyukov <dvyukov@google.com>
Date:   Mon, 14 May 2018 09:28:19 +0200
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Subject: Re: for_each_cpu() is buggy for UP kernel?
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Dexuan Cui <decui@microsoft.com>, Ingo Molnar <mingo@kernel.org>,
        Alexey Dobriyan <adobriyan@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Rakib Mullick <rakib.mullick@gmail.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Kostya Serebryany <kcc@google.com>
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On Sun, May 13, 2018 at 8:21 PM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
> On Tue, May 8, 2018 at 11:24 PM Dexuan Cui <decui@microsoft.com> wrote:
>
>> Should we fix the for_each_cpu() in include/linux/cpumask.h for UP?
>
> As Thomas points out, this has come up before.
>
> One of the issues is historical - we tried very hard to make the SMP code
> not cause code generation problems for UP, and part of that was just that
> all these loops were literally designed to entirely go away under UP. It
> still *looks* syntactically like a loop, but an optimizing compiler will
> see that there's nothing there, and "for_each_cpu(...) x" essentially just
> turns into "x" on UP.  An empty mask simply generally doesn't make sense,
> since opn UP you also don't have any masking of CPU ops, so the mask is
> ignored, and that helps the code generation immensely.
>
> If you have to load and test the mask, you immediately lose out badly in
> code generation.
>
> So honestly, I'd really prefer to keep our current behavior. Perhaps with a
> debug option that actually tests (on SMP - because that's what every
> developer is actually _using_ these days) that the mask isn't empty. But
> I'm not sure that would find this case, since presumably on SMP it might
> never be empty.

This looks like the problem automated testing traditionally and
effectively solves. If UP is an important config, there must be
automated pre/post commit checks for this.