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Date: Wed, 18 Mar 2015 22:32:42 +0100
From: Denys Vlasenko <dvlasenk@redhat.com>
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To: Andy Lutomirski <luto@amacapital.net>
CC: Linus Torvalds <torvalds@linux-foundation.org>,
        Steven Rostedt <rostedt@goodmis.org>, Ingo Molnar <mingo@kernel.org>,
        Borislav Petkov <bp@alien8.de>, "H. Peter Anvin" <hpa@zytor.com>,
        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>, X86 ML <x86@kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 2/3] x86: entry_64.S: use PUSH insns to build pt_regs
 on stack
References: <1426708056-2785-1-git-send-email-dvlasenk@redhat.com> <1426708056-2785-2-git-send-email-dvlasenk@redhat.com> <CALCETrVhsr1-OcUiWeuU3ATfbYGpYOc=3P96vAwtk=_G2AxsFg@mail.gmail.com> <5509EA52.7080904@redhat.com> <CALCETrUKjsbZGUUHNG95V98g0mfv=70x-+EzPT+YBEf-ZUxkWQ@mail.gmail.com>
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On 03/18/2015 10:22 PM, Andy Lutomirski wrote:
> On Wed, Mar 18, 2015 at 2:12 PM, Denys Vlasenko <dvlasenk@redhat.com> wrote:
>> On 03/18/2015 10:01 PM, Andy Lutomirski wrote:
>>> On Wed, Mar 18, 2015 at 12:47 PM, Denys Vlasenko <dvlasenk@redhat.com> wrote:
>>>> We lose a number of large insns there:
>>>>
>>>>     text    data     bss     dec     hex filename
>>>>     9863       0       0    9863    2687 entry_64_before.o
>>>>     9671       0       0    9671    25c7 entry_64.o
>>>>
>>>> What's more important, we convert two "MOVQ $imm,off(%rsp)" to "PUSH $imm"
>>>> (the ones which fill pt_regs->cs,ss).
>>>>
>>>> Before this patch, placing them on fast path was slowing it down by two cycles:
>>>> this form of MOV is very large, 12 bytes, and this probably reduces decode bandwidth
>>>> to one insn per cycle when it meets them.
>>>> Therefore they were living in FIXUP_TOP_OF_STACK instead (away from hot path).
>>>
>>> Does that mean that this has zero performance impact, or is it
>>> actually a speedup?
>>
>>
>> No, it's not a speedup because those big bad instructions weren't
>> on hot path to begin with.
>>
>> We want them to be there.
>>
>> Inserting them in a form of MOVs into hot path (say, in order
>> to eliminate FIXUP_TOP_OF_STACK) *would be* a slowdown.
>>
>> But we switch to PUSH method, and then inserting them _as PUSHes_
>> seems to be a wash.
>>
> 
> Sorry, what I meant was: what was the performance impact of this patch
> on fast-path syscalls?

I measured the next patch (which added one additional push)
and it was a wash compared to timings before both patches.
See comment there.

I did not measure this patch in isolation this time around,
on the previous iteration of this patch it was a single-cycle speedup.