linux-kernel - Re: [PATCH] x86: only use ERMS for user copies for larger sizes

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Date:   Fri, 23 Nov 2018 23:09:24 -0700
From:   Jens Axboe <axboe@...nel.dk>
To:     Linus Torvalds <torvalds@...ux-foundation.org>, pabeni@...hat.com
Cc:     Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, bp@...en8.de,
        Peter Anvin <hpa@...or.com>,
        the arch/x86 maintainers <x86@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andrew Lutomirski <luto@...nel.org>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>, dvlasenk@...hat.com,
        brgerst@...il.com,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86: only use ERMS for user copies for larger sizes

On 11/21/18 11:16 AM, Linus Torvalds wrote:
> On Wed, Nov 21, 2018 at 9:27 AM Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
>>
>> It would be interesting to know exactly which copy it is that matters
>> so much...  *inlining* the erms case might show that nicely in
>> profiles.
> 
> Side note: the fact that Jens' patch (which I don't like in that form)
> allegedly shrunk the resulting kernel binary would seem to indicate
> that there's a *lot* of compile-time constant-sized memcpy calls that
> we are missing, and that fall back to copy_user_generic().

Other kind of side note... This also affects memset(), which does
rep stosb if we have ERMS if any size memset. I noticed this from
sg_init_table(), which does a memset of the table. For my kind of
testing, the entry size is small. The below, too, reduces memset()
overhead by 50% here for me.

diff --git a/arch/x86/lib/memset_64.S b/arch/x86/lib/memset_64.S
index 9bc861c71e75..bad0fdb9ddcd 100644
--- a/arch/x86/lib/memset_64.S
+++ b/arch/x86/lib/memset_64.S
@@ -60,6 +60,8 @@ EXPORT_SYMBOL(__memset)
  * rax   original destination
  */
 ENTRY(memset_erms)
+	cmpl $128,%edx
+	jb memset_orig
 	movq %rdi,%r9
 	movb %sil,%al
 	movq %rdx,%rcx

-- 
Jens Axboe