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[66.29.188.166]) by smtp.gmail.com with ESMTPSA id y12sm8710141pfk.70.2018.11.23.22.09.26 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Fri, 23 Nov 2018 22:09:28 -0800 (PST) Subject: Re: [PATCH] x86: only use ERMS for user copies for larger sizes To: Linus Torvalds , pabeni@redhat.com Cc: Ingo Molnar , Thomas Gleixner , Ingo Molnar , bp@alien8.de, Peter Anvin , the arch/x86 maintainers , Andrew Morton , Andrew Lutomirski , Peter Zijlstra , dvlasenk@redhat.com, brgerst@gmail.com, Linux List Kernel Mailing References: <02bfc577-32a5-66be-64bf-d476b7d447d2@kernel.dk> <20181121063609.GA109082@gmail.com> <48e27a3a-2bb2-ff41-3512-8aeb3fd59e57@kernel.dk> <1c22125bb5d22c2dcd686d0d3b390f115894f746.camel@redhat.com> From: Jens Axboe Message-ID: <3f739385-5bc5-6061-cf49-00c92ded3166@kernel.dk> Date: Fri, 23 Nov 2018 23:09:24 -0700 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.2.1 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 11/21/18 11:16 AM, Linus Torvalds wrote: > On Wed, Nov 21, 2018 at 9:27 AM Linus Torvalds > 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