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From: Dmitry Vyukov <dvyukov@google.com>
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>
Subject: Re: for_each_cpu() is buggy for UP kernel?
Date: Mon, 14 May 2018 09:28:19 +0200	[thread overview]
Message-ID: <CACT4Y+aUV5XoynuhCnNTUNveQQGBhD8iHeYSMXbp7U6SVBAckw@mail.gmail.com> (raw)
In-Reply-To: <CA+55aFx5_qcUHBQ50RfOnH+pS=jqA_Wg2YYCcm=fTXRFJjs3rg@mail.gmail.com>

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.

  reply	other threads:[~2018-05-14  7:28 UTC|newest]

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-05-09  6:24 Dexuan Cui
2018-05-09 23:20 ` Andrew Morton
2018-05-13 13:35   ` Thomas Gleixner
2018-05-13 18:21 ` Linus Torvalds
2018-05-14  7:28   ` Dmitry Vyukov [this message]
2018-05-15  3:02   ` Dexuan Cui
2018-05-15 17:21     ` Linus Torvalds
2018-05-15 20:10       ` Dexuan Cui
2018-05-15 19:52 ` [PATCH] tick/broadcast: Use for_each_cpu() specially on UP kernels Dexuan Cui
2018-05-15 20:48   ` [tip:timers/urgent] " tip-bot for Dexuan Cui

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