Message-ID: <551EE4BE.9090507@zytor.com>
Date: Fri, 03 Apr 2015 12:06:38 -0700
From: "H. Peter Anvin" <hpa@zytor.com>
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.5.0
MIME-Version: 1.0
To: Linus Torvalds <torvalds@linux-foundation.org>
CC: Denys Vlasenko <dvlasenk@redhat.com>, Ingo Molnar <mingo@kernel.org>,
        Steven Rostedt <rostedt@goodmis.org>, Borislav Petkov <bp@alien8.de>,
        Andy Lutomirski <luto@amacapital.net>, Oleg Nesterov <oleg@redhat.com>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Alexei Starovoitov <ast@plumgrid.com>, Will Drewry <wad@chromium.org>,
        Kees Cook <keescook@chromium.org>,
        the arch/x86 maintainers <x86@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] x86/asm/entry/64: pack interrupt dispatch table tighter
References: <1428059634-11782-1-git-send-email-dvlasenk@redhat.com>	<20150403140336.GA16422@gmail.com>	<551EC5CD.3050901@redhat.com>	<CA+55aFxHSCUmsWZR4ey_NfZep-h5oyEoUL15GamQt9v17znyvQ@mail.gmail.com>	<551EDD7E.3050505@zytor.com> <CA+55aFwQV5Qb1V++wJQXDfv92mGfuwCidO9Y2rRP1ezZG9v4jA@mail.gmail.com>
In-Reply-To: <CA+55aFwQV5Qb1V++wJQXDfv92mGfuwCidO9Y2rRP1ezZG9v4jA@mail.gmail.com>
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 982
Lines: 26

On 04/03/2015 11:37 AM, Linus Torvalds wrote:
> On Fri, Apr 3, 2015 at 11:35 AM, H. Peter Anvin <hpa@zytor.com> wrote:
>>
>> For the record, I actually measured the impact of the jump-to-jump when
>> I wrote it.  It has a small, *but measurable*, positive impact.
> 
> What did you compare against, and how did you measure that? I don't
> see how it could *possibly* be faster than just a simple aligned "push
> + jmp".
> 

I wish I remembered the exact details; it took a fair bit of gathering
numbers as the spread was quite a bit wider than the delta, but in the
end there were two distribution peaks clearly offset.

I seem to remember it involving a loop running RDTSC continuously and
another RDTSC in the interrupt path.

	-hpa


--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/