From: Segher Boessenkool <segher@kernel.crashing.org>
To: Arnd Bergmann <arnd@arndb.de>
Cc: Christophe Leroy <christophe.leroy@c-s.fr>,
Benjamin Herrenschmidt <benh@kernel.crashing.org>,
Paul Mackerras <paulus@samba.org>,
Michael Ellerman <mpe@ellerman.id.au>,
Thomas Gleixner <tglx@linutronix.de>,
Vincenzo Frascino <vincenzo.frascino@arm.com>,
Andy Lutomirski <luto@kernel.org>,
the arch/x86 maintainers <x86@kernel.org>,
linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
Linux ARM <linux-arm-kernel@lists.infradead.org>,
"open list:BROADCOM NVRAM DRIVER" <linux-mips@vger.kernel.org>
Subject: Re: Surprising code generated for vdso_read_begin()
Date: Wed, 19 Feb 2020 07:08:08 -0600 [thread overview]
Message-ID: <20200219130808.GU22482@gate.crashing.org> (raw)
In-Reply-To: <CAK8P3a0SfCwP04CJPThCuDmngGhwtejN8Px_UQpSwi=s_ww=bw@mail.gmail.com>
On Wed, Feb 19, 2020 at 10:52:16AM +0100, Arnd Bergmann wrote:
> On Wed, Feb 19, 2020 at 9:45 AM Christophe Leroy
> <christophe.leroy@c-s.fr> wrote:
> > Le 16/02/2020 à 19:10, Arnd Bergmann a écrit :
> > > On Sat, Jan 11, 2020 at 12:33 PM Segher Boessenkool
> > > <segher@kernel.crashing.org> wrote:
> > >>
> > >> On Fri, Jan 10, 2020 at 07:45:44AM +0100, Christophe Leroy wrote:
> > >>> Le 09/01/2020 à 21:07, Segher Boessenkool a écrit :
> > >>>> 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
> > >>
> > >> Arnd, can you update the tools? We are at 8.3 and 9.2 now :-) Or is
> > >> this hard and/or painful to do?
> > >
> > > To follow up on this older thread, I have now uploaded 6.5, 7.5, 8.3 and 9.2
> > > binaries, as well as a recent 10.0 snapshot.
> > >
> >
> > Thanks Arnd,
> >
> > I have built the VDSO with 9.2, I get less performant result than with
> > 8.2 (same performance as with 5.5).
> >
> > After a quick look, I see:
> > - Irrelevant NOPs to align loops and stuff, allthough -mpcu=860 should
> > avoid that.
> > - A stack frame is set for saving r31 in __c_kernel_clock_gettime. GCC
> > 8.1 don't need that, all VDSO functions are frameless with 8.1
>
> If you think it should be fixed in gcc, maybe try to reproduce it in
> https://godbolt.org/
(Feel free to skip this step; and don't put links to godbolt (or anything
else external) in our bugzilla, please; such links go stale before you
know it.)
> and open a gcc bug against that.
Yes please :-)
> Also, please try the gcc-10 snapshot, which has the highest chance
> of getting fixes if it shows the same issue (or worse).
If it is a regression, chances are it will be backported. (But not to
9.3, which is due in just a few weeks, just like 8.4). If it is just a
side effect of some other change, it will probably *not* be undone, not
on trunk (GCC 10) either. It depends.
But sure, always test trunk if you can.
Segher
prev parent reply other threads:[~2020-02-19 13:08 UTC|newest]
Thread overview: 45+ 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 ` [RFC PATCH v2 01/10] lib: vdso: ensure all arches have 32bit fallback Christophe Leroy
2019-12-24 2:07 ` Andy Lutomirski
2020-01-10 20:56 ` Thomas Gleixner
2020-01-10 21:02 ` Andy Lutomirski
2019-12-30 12:27 ` Arnd Bergmann
2020-01-02 11:29 ` Arnd Bergmann
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
2019-12-24 2:24 ` Andy Lutomirski
2019-12-24 11:41 ` christophe leroy
2019-12-24 12:09 ` Andy Lutomirski
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 ` [RFC PATCH v2 04/10] lib: vdso: get pointer to vdso data from the arch Christophe Leroy
2019-12-24 2:27 ` Andy Lutomirski
2019-12-24 11:53 ` christophe leroy
2019-12-24 12:15 ` Andy Lutomirski
2019-12-24 12:41 ` Andy Lutomirski
2019-12-24 14:46 ` Segher Boessenkool
2019-12-23 14:31 ` [RFC PATCH v2 05/10] lib: vdso: inline do_hres() Christophe Leroy
2019-12-24 2:29 ` Andy Lutomirski
2019-12-30 12:07 ` Arnd Bergmann
2020-01-10 21:07 ` Thomas Gleixner
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 ` [RFC PATCH v2 07/10] lib: vdso: don't use READ_ONCE() in __c_kernel_time() Christophe Leroy
2019-12-24 1:58 ` Andy Lutomirski
2019-12-24 11:12 ` christophe leroy
2019-12-24 12:04 ` Andy Lutomirski
2020-01-10 21:12 ` Thomas Gleixner
2020-01-11 8:05 ` Christophe Leroy
2020-01-11 11:07 ` Thomas Gleixner
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
2019-12-24 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 ` [RFC PATCH v2 10/10] powerpc/32: Switch VDSO to C implementation Christophe Leroy
2020-01-09 17:52 ` Surprising code generated for vdso_read_begin() Christophe Leroy
2020-01-09 20:07 ` Segher Boessenkool
2020-01-10 6:45 ` Christophe Leroy
2020-01-11 11:33 ` Segher Boessenkool
2020-02-16 18:10 ` Arnd Bergmann
2020-02-19 8:45 ` Christophe Leroy
2020-02-19 9:52 ` Arnd Bergmann
2020-02-19 13:08 ` Segher Boessenkool [this message]
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