From: Segher Boessenkool <segher@kernel.crashing.org>
To: Christophe Leroy <christophe.leroy@c-s.fr>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>,
Paul Mackerras <paulus@samba.org>,
Michael Ellerman <mpe@ellerman.id.au>,
nathanl@linux.ibm.com, arnd@arndb.de, tglx@linutronix.de,
vincenzo.frascino@arm.com, luto@kernel.org, x86@kernel.org,
linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org,
linux-arm-kernel@lists.infradead.org, linux-mips@vger.kernel.org
Subject: Re: [RFC PATCH v4 00/11] powerpc: switch VDSO to C implementation.
Date: Mon, 20 Jan 2020 11:27:32 -0600 [thread overview]
Message-ID: <20200120172732.GC3191@gate.crashing.org> (raw)
In-Reply-To: <4b0e5941-c37e-3c85-3809-45f33ce35657@c-s.fr>
On Mon, Jan 20, 2020 at 06:08:23PM +0100, Christophe Leroy wrote:
> Not easy I think.
>
> First we have the unavoidable ASM entry function that can't be dropped
> because of the CR[SO] bit the set on error or clear on no error and that
> can't be done in C.
Yup.
> In our ASM VDSO, fixed shifts are used, while in generic C VDSO, shifts
> are generic and read from the VDSO data.
Does that cost more than just a few cycles?
> And there is still some funny code generated by GCC (8.1), like:
>
> 620: 7d 29 3c 30 srw r9,r9,r7
> 624: 21 87 00 20 subfic r12,r7,32
> 628: 7d 07 3c 31 srw. r7,r8,r7
> 62c: 7d 08 60 30 slw r8,r8,r12
> 630: 7d 0b 4b 78 or r11,r8,r9
(This can be done cheaper for fixed shifts, you can use rlwimi then).
> 634: 39 40 00 00 li r10,0
> 638: 40 82 00 84 bne 6bc <__c_kernel_clock_gettime+0x114>
> 63c: 81 23 00 24 lwz r9,36(r3)
> 640: 81 05 00 00 lwz r8,0(r5)
> ...
> 6bc: 7d 69 5b 78 mr r9,r11
> 6c0: 7c ea 3b 78 mr r10,r7
> 6c4: 7d 2b 4b 78 mr r11,r9
> 6c8: 4b ff ff 74 b 63c <__c_kernel_clock_gettime+0x94>
>
> This branch to 6bc is totally useless:
> - copying r11 into r9 is pointless as r9 is overwritten in 63c
> - copying back r9 into r11 is pointless as r11 has not been modified
> inbetween.
Yeah, huh, how did that happen.
> - loading r10 with 0 then overwritting r10 with r7 when r7 is not 0 is
> pointless as well, could have directly put the result of srw. in r10.
This may be harder to make the compiler do.
But the r9/r11 thing suggests you are preventing optimisation somewhere,
maybe with some asm? Do you have some small testcase I can compile?
Segher
WARNING: multiple messages have this Message-ID (diff)
From: Segher Boessenkool <segher@kernel.crashing.org>
To: Christophe Leroy <christophe.leroy@c-s.fr>
Cc: nathanl@linux.ibm.com, arnd@arndb.de, x86@kernel.org,
linux-kernel@vger.kernel.org, linux-mips@vger.kernel.org,
Paul Mackerras <paulus@samba.org>,
luto@kernel.org, tglx@linutronix.de, vincenzo.frascino@arm.com,
linuxppc-dev@lists.ozlabs.org,
linux-arm-kernel@lists.infradead.org
Subject: Re: [RFC PATCH v4 00/11] powerpc: switch VDSO to C implementation.
Date: Mon, 20 Jan 2020 11:27:32 -0600 [thread overview]
Message-ID: <20200120172732.GC3191@gate.crashing.org> (raw)
In-Reply-To: <4b0e5941-c37e-3c85-3809-45f33ce35657@c-s.fr>
On Mon, Jan 20, 2020 at 06:08:23PM +0100, Christophe Leroy wrote:
> Not easy I think.
>
> First we have the unavoidable ASM entry function that can't be dropped
> because of the CR[SO] bit the set on error or clear on no error and that
> can't be done in C.
Yup.
> In our ASM VDSO, fixed shifts are used, while in generic C VDSO, shifts
> are generic and read from the VDSO data.
Does that cost more than just a few cycles?
> And there is still some funny code generated by GCC (8.1), like:
>
> 620: 7d 29 3c 30 srw r9,r9,r7
> 624: 21 87 00 20 subfic r12,r7,32
> 628: 7d 07 3c 31 srw. r7,r8,r7
> 62c: 7d 08 60 30 slw r8,r8,r12
> 630: 7d 0b 4b 78 or r11,r8,r9
(This can be done cheaper for fixed shifts, you can use rlwimi then).
> 634: 39 40 00 00 li r10,0
> 638: 40 82 00 84 bne 6bc <__c_kernel_clock_gettime+0x114>
> 63c: 81 23 00 24 lwz r9,36(r3)
> 640: 81 05 00 00 lwz r8,0(r5)
> ...
> 6bc: 7d 69 5b 78 mr r9,r11
> 6c0: 7c ea 3b 78 mr r10,r7
> 6c4: 7d 2b 4b 78 mr r11,r9
> 6c8: 4b ff ff 74 b 63c <__c_kernel_clock_gettime+0x94>
>
> This branch to 6bc is totally useless:
> - copying r11 into r9 is pointless as r9 is overwritten in 63c
> - copying back r9 into r11 is pointless as r11 has not been modified
> inbetween.
Yeah, huh, how did that happen.
> - loading r10 with 0 then overwritting r10 with r7 when r7 is not 0 is
> pointless as well, could have directly put the result of srw. in r10.
This may be harder to make the compiler do.
But the r9/r11 thing suggests you are preventing optimisation somewhere,
maybe with some asm? Do you have some small testcase I can compile?
Segher
WARNING: multiple messages have this Message-ID (diff)
From: Segher Boessenkool <segher@kernel.crashing.org>
To: Christophe Leroy <christophe.leroy@c-s.fr>
Cc: nathanl@linux.ibm.com, arnd@arndb.de,
Benjamin Herrenschmidt <benh@kernel.crashing.org>,
x86@kernel.org, linux-kernel@vger.kernel.org,
linux-mips@vger.kernel.org, Paul Mackerras <paulus@samba.org>,
luto@kernel.org, Michael Ellerman <mpe@ellerman.id.au>,
tglx@linutronix.de, vincenzo.frascino@arm.com,
linuxppc-dev@lists.ozlabs.org,
linux-arm-kernel@lists.infradead.org
Subject: Re: [RFC PATCH v4 00/11] powerpc: switch VDSO to C implementation.
Date: Mon, 20 Jan 2020 11:27:32 -0600 [thread overview]
Message-ID: <20200120172732.GC3191@gate.crashing.org> (raw)
In-Reply-To: <4b0e5941-c37e-3c85-3809-45f33ce35657@c-s.fr>
On Mon, Jan 20, 2020 at 06:08:23PM +0100, Christophe Leroy wrote:
> Not easy I think.
>
> First we have the unavoidable ASM entry function that can't be dropped
> because of the CR[SO] bit the set on error or clear on no error and that
> can't be done in C.
Yup.
> In our ASM VDSO, fixed shifts are used, while in generic C VDSO, shifts
> are generic and read from the VDSO data.
Does that cost more than just a few cycles?
> And there is still some funny code generated by GCC (8.1), like:
>
> 620: 7d 29 3c 30 srw r9,r9,r7
> 624: 21 87 00 20 subfic r12,r7,32
> 628: 7d 07 3c 31 srw. r7,r8,r7
> 62c: 7d 08 60 30 slw r8,r8,r12
> 630: 7d 0b 4b 78 or r11,r8,r9
(This can be done cheaper for fixed shifts, you can use rlwimi then).
> 634: 39 40 00 00 li r10,0
> 638: 40 82 00 84 bne 6bc <__c_kernel_clock_gettime+0x114>
> 63c: 81 23 00 24 lwz r9,36(r3)
> 640: 81 05 00 00 lwz r8,0(r5)
> ...
> 6bc: 7d 69 5b 78 mr r9,r11
> 6c0: 7c ea 3b 78 mr r10,r7
> 6c4: 7d 2b 4b 78 mr r11,r9
> 6c8: 4b ff ff 74 b 63c <__c_kernel_clock_gettime+0x94>
>
> This branch to 6bc is totally useless:
> - copying r11 into r9 is pointless as r9 is overwritten in 63c
> - copying back r9 into r11 is pointless as r11 has not been modified
> inbetween.
Yeah, huh, how did that happen.
> - loading r10 with 0 then overwritting r10 with r7 when r7 is not 0 is
> pointless as well, could have directly put the result of srw. in r10.
This may be harder to make the compiler do.
But the r9/r11 thing suggests you are preventing optimisation somewhere,
maybe with some asm? Do you have some small testcase I can compile?
Segher
_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel
next prev parent reply other threads:[~2020-01-20 17:28 UTC|newest]
Thread overview: 78+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-01-16 17:58 [RFC PATCH v4 00/11] powerpc: switch VDSO to C implementation Christophe Leroy
2020-01-16 17:58 ` Christophe Leroy
2020-01-16 17:58 ` Christophe Leroy
2020-01-16 17:58 ` [RFC PATCH v4 01/11] powerpc/64: Don't provide time functions in compat VDSO32 Christophe Leroy
2020-01-16 17:58 ` Christophe Leroy
2020-01-16 17:58 ` Christophe Leroy
2020-01-16 17:58 ` [RFC PATCH v4 02/11] powerpc/vdso: Switch VDSO to generic C implementation Christophe Leroy
2020-01-16 17:58 ` Christophe Leroy
2020-01-16 17:58 ` Christophe Leroy
2020-01-16 17:58 ` [RFC PATCH v4 03/11] lib: vdso: only read hrtimer_res when needed in __cvdso_clock_getres() Christophe Leroy
2020-01-16 17:58 ` Christophe Leroy
2020-01-16 17:58 ` Christophe Leroy
2020-01-16 17:58 ` [RFC PATCH v4 04/11] powerpc/vdso: simplify __get_datapage() Christophe Leroy
2020-01-16 17:58 ` Christophe Leroy
2020-01-16 17:58 ` Christophe Leroy
2020-01-16 17:58 ` [RFC PATCH v4 05/11] lib: vdso: allow arches to provide vdso data pointer Christophe Leroy
2020-01-16 17:58 ` Christophe Leroy
2020-01-16 17:58 ` Christophe Leroy
2020-01-16 17:58 ` [RFC PATCH v4 06/11] powerpc/vdso: provide inline alternative to __get_datapage() Christophe Leroy
2020-01-16 17:58 ` Christophe Leroy
2020-01-16 17:58 ` Christophe Leroy
2020-01-16 17:58 ` [RFC PATCH v4 07/11] powerpc/vdso: provide vdso data pointer from the ASM caller Christophe Leroy
2020-01-16 17:58 ` Christophe Leroy
2020-01-16 17:58 ` Christophe Leroy
2020-01-16 17:58 ` [RFC PATCH v4 08/11] lib: vdso: allow fixed clock mode Christophe Leroy
2020-01-16 17:58 ` Christophe Leroy
2020-01-16 17:58 ` Christophe Leroy
2020-01-16 20:13 ` Thomas Gleixner
2020-01-16 20:13 ` Thomas Gleixner
2020-01-16 20:13 ` Thomas Gleixner
2020-01-16 20:19 ` Andy Lutomirski
2020-01-16 20:19 ` Andy Lutomirski
2020-01-16 20:19 ` Andy Lutomirski
2020-01-16 21:07 ` Thomas Gleixner
2020-01-16 21:07 ` Thomas Gleixner
2020-01-16 21:07 ` Thomas Gleixner
2020-01-16 17:58 ` [RFC PATCH v4 09/11] powerpc/vdso: override __arch_vdso_capable() Christophe Leroy
2020-01-16 17:58 ` Christophe Leroy
2020-01-16 17:58 ` Christophe Leroy
2020-01-16 17:58 ` [RFC PATCH v4 10/11] lib: vdso: Allow arches to override the ns shift operation Christophe Leroy
2020-01-16 17:58 ` Christophe Leroy
2020-01-16 17:58 ` Christophe Leroy
2020-01-16 19:47 ` Andy Lutomirski
2020-01-16 19:47 ` Andy Lutomirski
2020-01-16 19:47 ` Andy Lutomirski
2020-01-16 19:57 ` Thomas Gleixner
2020-01-16 19:57 ` Thomas Gleixner
2020-01-16 19:57 ` Thomas Gleixner
2020-01-16 20:20 ` Andy Lutomirski
2020-01-16 20:20 ` Andy Lutomirski
2020-01-16 20:20 ` Andy Lutomirski
2020-01-29 7:14 ` Thomas Gleixner
2020-01-29 7:14 ` Thomas Gleixner
2020-01-29 7:14 ` Thomas Gleixner
2020-01-29 7:26 ` Christophe Leroy
2020-01-29 7:26 ` Christophe Leroy
2020-01-29 7:26 ` Christophe Leroy
2020-01-16 17:58 ` [RFC PATCH v4 11/11] powerpc/32: provide vdso_shift_ns() Christophe Leroy
2020-01-16 17:58 ` Christophe Leroy
2020-01-16 17:58 ` Christophe Leroy
2020-01-17 8:58 ` [RFC PATCH v4 00/11] powerpc: switch VDSO to C implementation Segher Boessenkool
2020-01-17 8:58 ` Segher Boessenkool
2020-01-17 8:58 ` Segher Boessenkool
2020-01-17 9:26 ` Christophe Leroy
2020-01-17 9:26 ` Christophe Leroy
2020-01-17 9:26 ` Christophe Leroy
2020-01-20 14:56 ` Christophe Leroy
2020-01-20 14:56 ` Christophe Leroy
2020-01-20 14:56 ` Christophe Leroy
2020-01-20 15:19 ` Segher Boessenkool
2020-01-20 15:19 ` Segher Boessenkool
2020-01-20 15:19 ` Segher Boessenkool
2020-01-20 17:08 ` Christophe Leroy
2020-01-20 17:08 ` Christophe Leroy
2020-01-20 17:08 ` Christophe Leroy
2020-01-20 17:27 ` Segher Boessenkool [this message]
2020-01-20 17:27 ` Segher Boessenkool
2020-01-20 17:27 ` Segher Boessenkool
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20200120172732.GC3191@gate.crashing.org \
--to=segher@kernel.crashing.org \
--cc=arnd@arndb.de \
--cc=benh@kernel.crashing.org \
--cc=christophe.leroy@c-s.fr \
--cc=linux-arm-kernel@lists.infradead.org \
--cc=linux-kernel@vger.kernel.org \
--cc=linux-mips@vger.kernel.org \
--cc=linuxppc-dev@lists.ozlabs.org \
--cc=luto@kernel.org \
--cc=mpe@ellerman.id.au \
--cc=nathanl@linux.ibm.com \
--cc=paulus@samba.org \
--cc=tglx@linutronix.de \
--cc=vincenzo.frascino@arm.com \
--cc=x86@kernel.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is an external index of several public inboxes,
see mirroring instructions on how to clone and mirror
all data and code used by this external index.