From: Alexey Dobriyan <adobriyan@gmail.com>
To: Ingo Molnar <mingo@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Thomas Gleixner <tglx@linutronix.de>,
Peter Zijlstra <a.p.zijlstra@chello.nl>,
Andrew Morton <akpm@linux-foundation.org>,
Andy Lutomirski <luto@kernel.org>, Borislav Petkov <bp@alien8.de>,
Josh Poimboeuf <jpoimboe@redhat.com>,
"H. Peter Anvin" <hpa@zytor.com>,
Denys Vlasenko <dvlasenk@redhat.com>
Subject: Re: x86/asm: __clear_user() micro-optimization (was: "Re: [GIT PULL] x86/asm changes for v4.18")
Date: Tue, 5 Jun 2018 20:22:43 +0300 [thread overview]
Message-ID: <20180605172243.GA2059@avx2> (raw)
In-Reply-To: <20180605150514.GA31065@gmail.com>
On Tue, Jun 05, 2018 at 05:05:14PM +0200, Ingo Molnar wrote:
>
> * Linus Torvalds <torvalds@linux-foundation.org> wrote:
>
> > On Mon, Jun 4, 2018 at 5:21 AM Ingo Molnar <mingo@kernel.org> wrote:
> > >
> > > - __clear_user() micro-optimization (Alexey Dobriyan)
> >
> > Was this actually tested?
>
> I'm not sure - Alexey?
>
> > I think one reason people avoided the constant was that on some
> > microarchitecture it ended up being a separate uop just for the
> > constant generation, because it wouldn't fit in a single uop.
> Ok, fair point and agreed - if Alexey sends some measurements to back the change
> I'll keep this, otherwise queue up a revert.
Tested? :^) I had P4 maybe ~15(?) years ago.
godbolt.org earliest compiler is 4.1.2 and it generates "movb [r32], imm8"
with "-m32 -O2 -march=pentium4" for simple memset-style loop
if it counts for something.
Actually I think __clear_user should be rewritten in C with assembly.
It's biggest user is probably ELF loader and those partial page .bss
clears should be noticeable.
next prev parent reply other threads:[~2018-06-05 17:22 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-06-04 12:21 [GIT PULL] x86/asm changes for v4.18 Ingo Molnar
2018-06-05 1:58 ` Linus Torvalds
2018-06-05 15:05 ` x86/asm: __clear_user() micro-optimization (was: "Re: [GIT PULL] x86/asm changes for v4.18") Ingo Molnar
2018-06-05 15:47 ` Linus Torvalds
2018-06-05 17:22 ` Alexey Dobriyan [this message]
2018-06-05 17:32 ` Linus Torvalds
2018-06-05 22:41 ` Alexey Dobriyan
2018-06-05 23:01 ` Linus Torvalds
2018-06-05 23:04 ` Linus Torvalds
2018-06-05 23:20 ` Alexey Dobriyan
2018-06-05 23:27 ` Linus Torvalds
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