From: Christophe Leroy <christophe.leroy@c-s.fr>
To: Segher Boessenkool <segher@kernel.crashing.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>,
Paul Mackerras <paulus@samba.org>,
Michael Ellerman <mpe@ellerman.id.au>,
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: Surprising code generated for vdso_read_begin()
Date: Fri, 10 Jan 2020 07:45:44 +0100 [thread overview]
Message-ID: <77a8bf25-6615-6c0a-56d4-eae7aa8a8f09@c-s.fr> (raw)
In-Reply-To: <20200109200733.GS3191@gate.crashing.org>
Le 09/01/2020 à 21:07, Segher Boessenkool a écrit :
> On Thu, Jan 09, 2020 at 05:52:34PM +0000, Christophe Leroy wrote:
>> Wondering why we get something so complicated/redundant for
>> vdso_read_begin() <include/vdso/helpers.h>
>>
>> static __always_inline u32 vdso_read_begin(const struct vdso_data *vd)
>> {
>> u32 seq;
>>
>> while ((seq = READ_ONCE(vd->seq)) & 1)
>> cpu_relax();
>>
>> smp_rmb();
>> return seq;
>> }
>>
>>
>> 6e0: 81 05 00 f0 lwz r8,240(r5)
>> 6e4: 71 09 00 01 andi. r9,r8,1
>> 6e8: 41 82 00 10 beq 6f8 <__c_kernel_clock_gettime+0x158>
>> 6ec: 81 05 00 f0 lwz r8,240(r5)
>> 6f0: 71 0a 00 01 andi. r10,r8,1
>> 6f4: 40 82 ff f8 bne 6ec <__c_kernel_clock_gettime+0x14c>
>> 6f8:
>>
>> r5 being vd pointer
>>
>> Why the first triplet, not only the second triplet ? Something wrong
>> with using READ_ONCE() for that ?
>
> It looks like the compiler did loop peeling. What GCC version is this?
> Please try current trunk (to become GCC 10), or at least GCC 9?
>
It is with GCC 5.5
https://mirrors.edge.kernel.org/pub/tools/crosstool/ doesn't have more
recent than 8.1
With 8.1, the problem doesn't show up.
Thanks
Christophe
WARNING: multiple messages have this Message-ID (diff)
From: Christophe Leroy <christophe.leroy@c-s.fr>
To: Segher Boessenkool <segher@kernel.crashing.org>
Cc: 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: Surprising code generated for vdso_read_begin()
Date: Fri, 10 Jan 2020 07:45:44 +0100 [thread overview]
Message-ID: <77a8bf25-6615-6c0a-56d4-eae7aa8a8f09@c-s.fr> (raw)
In-Reply-To: <20200109200733.GS3191@gate.crashing.org>
Le 09/01/2020 à 21:07, Segher Boessenkool a écrit :
> On Thu, Jan 09, 2020 at 05:52:34PM +0000, Christophe Leroy wrote:
>> Wondering why we get something so complicated/redundant for
>> vdso_read_begin() <include/vdso/helpers.h>
>>
>> static __always_inline u32 vdso_read_begin(const struct vdso_data *vd)
>> {
>> u32 seq;
>>
>> while ((seq = READ_ONCE(vd->seq)) & 1)
>> cpu_relax();
>>
>> smp_rmb();
>> return seq;
>> }
>>
>>
>> 6e0: 81 05 00 f0 lwz r8,240(r5)
>> 6e4: 71 09 00 01 andi. r9,r8,1
>> 6e8: 41 82 00 10 beq 6f8 <__c_kernel_clock_gettime+0x158>
>> 6ec: 81 05 00 f0 lwz r8,240(r5)
>> 6f0: 71 0a 00 01 andi. r10,r8,1
>> 6f4: 40 82 ff f8 bne 6ec <__c_kernel_clock_gettime+0x14c>
>> 6f8:
>>
>> r5 being vd pointer
>>
>> Why the first triplet, not only the second triplet ? Something wrong
>> with using READ_ONCE() for that ?
>
> It looks like the compiler did loop peeling. What GCC version is this?
> Please try current trunk (to become GCC 10), or at least GCC 9?
>
It is with GCC 5.5
https://mirrors.edge.kernel.org/pub/tools/crosstool/ doesn't have more
recent than 8.1
With 8.1, the problem doesn't show up.
Thanks
Christophe
WARNING: multiple messages have this Message-ID (diff)
From: Christophe Leroy <christophe.leroy@c-s.fr>
To: Segher Boessenkool <segher@kernel.crashing.org>
Cc: 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: Surprising code generated for vdso_read_begin()
Date: Fri, 10 Jan 2020 07:45:44 +0100 [thread overview]
Message-ID: <77a8bf25-6615-6c0a-56d4-eae7aa8a8f09@c-s.fr> (raw)
In-Reply-To: <20200109200733.GS3191@gate.crashing.org>
Le 09/01/2020 à 21:07, Segher Boessenkool a écrit :
> On Thu, Jan 09, 2020 at 05:52:34PM +0000, Christophe Leroy wrote:
>> Wondering why we get something so complicated/redundant for
>> vdso_read_begin() <include/vdso/helpers.h>
>>
>> static __always_inline u32 vdso_read_begin(const struct vdso_data *vd)
>> {
>> u32 seq;
>>
>> while ((seq = READ_ONCE(vd->seq)) & 1)
>> cpu_relax();
>>
>> smp_rmb();
>> return seq;
>> }
>>
>>
>> 6e0: 81 05 00 f0 lwz r8,240(r5)
>> 6e4: 71 09 00 01 andi. r9,r8,1
>> 6e8: 41 82 00 10 beq 6f8 <__c_kernel_clock_gettime+0x158>
>> 6ec: 81 05 00 f0 lwz r8,240(r5)
>> 6f0: 71 0a 00 01 andi. r10,r8,1
>> 6f4: 40 82 ff f8 bne 6ec <__c_kernel_clock_gettime+0x14c>
>> 6f8:
>>
>> r5 being vd pointer
>>
>> Why the first triplet, not only the second triplet ? Something wrong
>> with using READ_ONCE() for that ?
>
> It looks like the compiler did loop peeling. What GCC version is this?
> Please try current trunk (to become GCC 10), or at least GCC 9?
>
It is with GCC 5.5
https://mirrors.edge.kernel.org/pub/tools/crosstool/ doesn't have more
recent than 8.1
With 8.1, the problem doesn't show up.
Thanks
Christophe
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next prev parent reply other threads:[~2020-01-10 6:45 UTC|newest]
Thread overview: 137+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-12-23 14:31 [RFC PATCH v2 00/10] powerpc/32: switch VDSO to C implementation Christophe Leroy
2019-12-23 14:31 ` Christophe Leroy
2019-12-23 14:31 ` Christophe Leroy
2019-12-23 14:31 ` [RFC PATCH v2 01/10] lib: vdso: ensure all arches have 32bit fallback Christophe Leroy
2019-12-23 14:31 ` Christophe Leroy
2019-12-23 14:31 ` Christophe Leroy
2019-12-24 2:07 ` Andy Lutomirski
2019-12-24 2:07 ` Andy Lutomirski
2019-12-24 2:07 ` Andy Lutomirski
2020-01-10 20:56 ` Thomas Gleixner
2020-01-10 20:56 ` Thomas Gleixner
2020-01-10 20:56 ` Thomas Gleixner
2020-01-10 21:02 ` Andy Lutomirski
2020-01-10 21:02 ` Andy Lutomirski
2020-01-10 21:02 ` Andy Lutomirski
2019-12-25 2:05 ` kbuild test robot
2019-12-25 6:01 ` kbuild test robot
2019-12-30 12:27 ` Arnd Bergmann
2019-12-30 12:27 ` Arnd Bergmann
2019-12-30 12:27 ` Arnd Bergmann
2020-01-02 11:29 ` Arnd Bergmann
2020-01-02 11:29 ` Arnd Bergmann
2020-01-02 11:29 ` Arnd Bergmann
2020-01-09 15:43 ` Christophe Leroy
2020-01-09 15:43 ` Christophe Leroy
2020-01-09 15:43 ` Christophe Leroy
2019-12-23 14:31 ` [RFC PATCH v2 02/10] lib: vdso: move call to fallback out of common code Christophe Leroy
2019-12-23 14:31 ` Christophe Leroy
2019-12-23 14:31 ` Christophe Leroy
2019-12-24 2:24 ` Andy Lutomirski
2019-12-24 2:24 ` Andy Lutomirski
2019-12-24 2:24 ` Andy Lutomirski
2019-12-24 11:41 ` christophe leroy
2019-12-24 11:41 ` christophe leroy
2019-12-24 11:41 ` christophe leroy
2019-12-24 12:09 ` Andy Lutomirski
2019-12-24 12:09 ` Andy Lutomirski
2019-12-24 12:09 ` Andy Lutomirski
2019-12-25 2:19 ` kbuild test robot
2019-12-23 14:31 ` [RFC PATCH v2 03/10] lib: vdso: Change __cvdso_clock_gettime/getres_common() to __cvdso_clock_gettime/getres() Christophe Leroy
2019-12-23 14:31 ` Christophe Leroy
2019-12-23 14:31 ` Christophe Leroy
2019-12-23 14:31 ` [RFC PATCH v2 04/10] lib: vdso: get pointer to vdso data from the arch Christophe Leroy
2019-12-23 14:31 ` Christophe Leroy
2019-12-23 14:31 ` Christophe Leroy
2019-12-24 2:27 ` Andy Lutomirski
2019-12-24 2:27 ` Andy Lutomirski
2019-12-24 2:27 ` Andy Lutomirski
2019-12-24 11:53 ` christophe leroy
2019-12-24 11:53 ` christophe leroy
2019-12-24 11:53 ` christophe leroy
2019-12-24 12:15 ` Andy Lutomirski
2019-12-24 12:15 ` Andy Lutomirski
2019-12-24 12:15 ` Andy Lutomirski
2019-12-24 12:41 ` Andy Lutomirski
2019-12-24 12:41 ` Andy Lutomirski
2019-12-24 12:41 ` Andy Lutomirski
2019-12-24 14:46 ` Segher Boessenkool
2019-12-24 14:46 ` Segher Boessenkool
2019-12-24 14:46 ` Segher Boessenkool
2019-12-23 14:31 ` [RFC PATCH v2 05/10] lib: vdso: inline do_hres() Christophe Leroy
2019-12-23 14:31 ` Christophe Leroy
2019-12-23 14:31 ` Christophe Leroy
2019-12-24 2:29 ` Andy Lutomirski
2019-12-24 2:29 ` Andy Lutomirski
2019-12-24 2:29 ` Andy Lutomirski
2019-12-30 12:07 ` Arnd Bergmann
2019-12-30 12:07 ` Arnd Bergmann
2019-12-30 12:07 ` Arnd Bergmann
2020-01-10 21:07 ` Thomas Gleixner
2020-01-10 21:07 ` Thomas Gleixner
2020-01-10 21:07 ` Thomas Gleixner
2020-01-11 9:06 ` Christophe Leroy
2020-01-11 9:06 ` Christophe Leroy
2020-01-11 9:06 ` Christophe Leroy
2019-12-23 14:31 ` [RFC PATCH v2 06/10] lib: vdso: make do_coarse() return 0 Christophe Leroy
2019-12-23 14:31 ` Christophe Leroy
2019-12-23 14:31 ` Christophe Leroy
2019-12-23 14:31 ` [RFC PATCH v2 07/10] lib: vdso: don't use READ_ONCE() in __c_kernel_time() Christophe Leroy
2019-12-23 14:31 ` Christophe Leroy
2019-12-23 14:31 ` Christophe Leroy
2019-12-24 1:58 ` Andy Lutomirski
2019-12-24 1:58 ` Andy Lutomirski
2019-12-24 1:58 ` Andy Lutomirski
2019-12-24 11:12 ` christophe leroy
2019-12-24 11:12 ` christophe leroy
2019-12-24 11:12 ` christophe leroy
2019-12-24 12:04 ` Andy Lutomirski
2019-12-24 12:04 ` Andy Lutomirski
2019-12-24 12:04 ` Andy Lutomirski
2020-01-10 21:12 ` Thomas Gleixner
2020-01-10 21:12 ` Thomas Gleixner
2020-01-10 21:12 ` Thomas Gleixner
2020-01-11 8:05 ` Christophe Leroy
2020-01-11 8:05 ` Christophe Leroy
2020-01-11 8:05 ` Christophe Leroy
2020-01-11 11:07 ` Thomas Gleixner
2020-01-11 11:07 ` Thomas Gleixner
2020-01-11 11:07 ` Thomas Gleixner
2020-01-13 6:52 ` Christophe Leroy
2020-01-13 6:52 ` Christophe Leroy
2020-01-13 6:52 ` Christophe Leroy
2019-12-23 14:31 ` [RFC PATCH v2 08/10] lib: vdso: Avoid duplication in __cvdso_clock_getres() Christophe Leroy
2019-12-23 14:31 ` Christophe Leroy
2019-12-23 14:31 ` Christophe Leroy
2019-12-24 1:59 ` Andy Lutomirski
2019-12-24 1:59 ` Andy Lutomirski
2019-12-24 1:59 ` Andy Lutomirski
2019-12-23 14:31 ` [RFC PATCH v2 09/10] powerpc/vdso32: inline __get_datapage() Christophe Leroy
2019-12-23 14:31 ` Christophe Leroy
2019-12-23 14:31 ` Christophe Leroy
2019-12-23 14:31 ` [RFC PATCH v2 10/10] powerpc/32: Switch VDSO to C implementation Christophe Leroy
2019-12-23 14:31 ` Christophe Leroy
2019-12-23 14:31 ` Christophe Leroy
2019-12-25 6:54 ` kbuild test robot
2020-01-09 17:52 ` Surprising code generated for vdso_read_begin() Christophe Leroy
2020-01-09 20:07 ` Segher Boessenkool
2020-01-09 20:07 ` Segher Boessenkool
2020-01-09 20:07 ` Segher Boessenkool
2020-01-10 6:45 ` Christophe Leroy [this message]
2020-01-10 6:45 ` Christophe Leroy
2020-01-10 6:45 ` Christophe Leroy
2020-01-11 11:33 ` Segher Boessenkool
2020-01-11 11:33 ` Segher Boessenkool
2020-01-11 11:33 ` Segher Boessenkool
2020-02-16 18:10 ` Arnd Bergmann
2020-02-16 18:10 ` Arnd Bergmann
2020-02-16 18:10 ` Arnd Bergmann
2020-02-19 8:45 ` Christophe Leroy
2020-02-19 8:45 ` Christophe Leroy
2020-02-19 8:45 ` Christophe Leroy
2020-02-19 9:52 ` Arnd Bergmann
2020-02-19 9:52 ` Arnd Bergmann
2020-02-19 9:52 ` Arnd Bergmann
2020-02-19 13:08 ` Segher Boessenkool
2020-02-19 13:08 ` Segher Boessenkool
2020-02-19 13:08 ` Segher Boessenkool
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