[OE-core] [PATCH 1/1] db: disable the ARM assembler mutex code

Khem Raj raj.khem at gmail.com
Fri Jun 15 14:42:42 UTC 2018 

On Fri, Jun 15, 2018 at 3:41 AM Herve Jourdain <herve.jourdain at neuf.fr> wrote:
>
> Hi,
>
> Actually, I meant "works" in the sense of "does compile" - as opposed to armv8 where it does not compile, which is why we're having this discussion in the first place.
> So I was merely suggesting to not modify previous oe behavior for the db package for previous architectures, and just add the removal of 'swp' for armv8, where it matters most.
>
> If we want to look at it in details, based on the "ARM Architecture Reference Manual ARMv7-A and ARMv7-R edition":
> 1. SWP is the way to go before ARMv6.
> 2. SWP has been deprecated in ARMv6.
> 3. SWP has been deprecated AND made optional in ARMv7ve.
> 4. "The SWP and SWPB instructions rely on the properties of the system beyond the processor to ensure that no stores from other observers can occur between the load access and the store access, and this might not be implemented for all regions of memory on some system implementations. In all cases, SWP and SWPB do ensure that no stores from the processor that executed the SWP or SWPB instruction can occur between the load access and the store access of the SWP or SWPB."
>
> This latest part means that it may or may not work in SMP environments, it depends on how the system is architecture around the cores - most likely how the bus system is designed I believe. So it may actually be working fine if the system/bus designer has taken that into account.
>
> This said, I believe that from point #3 above, it might make sense to disable SWP for armv7ve as well, since being optional means that it might be compiled correctly, but still fail at runtime, depending on the choices of the SoC manufacturer.
> So my recommendation would be to add:
> MUTEX_armv7ve = ""
> MUTEX_armv8 = ""
>
> To disable 'swp' by default only for those 2 archs, while keeping things like they are for previous architectures.
>

the least intrusive fix it to fix what we need and let the defaults be
as it in this case.

> Cheers,
> Herve
>
> -----Original Message-----
> From: Andre McCurdy [mailto:armccurdy at gmail.com]
> Sent: vendredi 15 juin 2018 09:39
> To: Herve Jourdain <herve.jourdain at neuf.fr>
> Cc: Khem Raj <raj.khem at gmail.com>; Ovidiu Panait <ovidiu.panait at windriver.com>; Patches and discussions about the oe-core layer <openembedded-core at lists.openembedded.org>
> Subject: Re: [OE-core] [PATCH 1/1] db: disable the ARM assembler mutex code
>
> On Fri, Jun 15, 2018 at 12:10 AM, Herve Jourdain <herve.jourdain at neuf.fr> wrote:
> > Hi,
> >
> > So the issue is whether we want to change the behaviour of previous architectures, or if we try to fix the issue only for the architectures that don't work.
> > Until now, the db recipe was enabling the 'swp' optimization, and that behavior could be disabled on .bbappend if needed.
> > While that works fine until armv7ve, it does not work for armv8, which has removed support for those instructions.
>
> I don't know if "works fine until armv7ve" is correct. Although the swp instruction exists for armv7, according to the link I posted yesterday, it is not guaranteed to work.
>
>   https://community.arm.com/processors/b/blog/posts/locks-swps-and-two-smoking-barriers
>
> Or do you have other evidence to suggest that swp is safe to use for armv7?
>
> > Therefore, there is a need to fix it for armv8 - and armv8 only - whereas it can be safely used on previous architectures.
> > If we remove the use for all ARM architectures, that might create some regression/issues.
> > If we just remove the use of 'swp' only for armv8, we ensure it doesn't break anything that's running on previous ARM architectures.
> >
> > Cheers,
> > Herve
> >
> > -----Original Message-----
> > From: Andre McCurdy [mailto:armccurdy at gmail.com]
> > Sent: vendredi 15 juin 2018 00:03
> > To: Khem Raj <raj.khem at gmail.com>
> > Cc: Herve Jourdain <herve.jourdain at neuf.fr>; Ovidiu Panait
> > <ovidiu.panait at windriver.com>; Patches and discussions about the
> > oe-core layer <openembedded-core at lists.openembedded.org>
> > Subject: Re: [OE-core] [PATCH 1/1] db: disable the ARM assembler mutex
> > code
> >
> > On Thu, Jun 14, 2018 at 2:48 PM, Khem Raj <raj.khem at gmail.com> wrote:
> >> On Thu, Jun 14, 2018 at 1:01 PM Andre McCurdy <armccurdy at gmail.com> wrote:
> >>> On Thu, Jun 14, 2018 at 12:24 PM, Khem Raj <raj.khem at gmail.com> wrote:
> >>> > On Thu, Jun 14, 2018 at 12:12 PM Andre McCurdy
> >>> > <armccurdy at gmail.com>
> >>> > wrote:
> >>> >>
> >>> >> On Thu, Jun 14, 2018 at 9:40 AM, Khem Raj <raj.khem at gmail.com> wrote:
> >>> >> > On 6/14/18 5:10 AM, Herve Jourdain wrote:
> >>> >> >> Hi,
> >>> >> >>
> >>> >> >> I believe I solved that same problem by just adding, in the
> >>> >> >> case of
> >>> >> >> armv8
> >>> >> >> (which I believe may be the new architecture you're referring to):
> >>> >> >> MUTEX_armv8 = ""
> >>> >> >> This way, it allows previous versions to work just like they
> >>> >> >> did before, without having to disable ARM assembler mutex code
> >>> >> >> for architectures that support it correctly - up to armv7ve I
> >>> >> >> believe.
> >>> >> >> Of course, we might need to also have a good definition for
> >>> >> >> armv8, which is the object of another thread.
> >>> >> >
> >>> >> > right thats a better approach.
> >>> >>
> >>> >> SWP is not guaranteed to work on SMP systems... and even if it
> >>> >> does, performance is likely to be worse than the pthreads version
> >>> >> (which can take advantage of the newer instructions).
> >>> >>
> >>> >>
> >>> >> https://community.arm.com/processors/b/blog/posts/locks-swps-and-
> >>> >> t
> >>> >> wo-smoking-barriers
> >>> >>
> >>> >> In general, use of hand optimised assembler mutex implementations
> >>> >> in user space isn't something to be encouraged - use pthreads (or
> >>> >> maybe a gcc intrinsic) instead.
> >>> >>
> >>> >
> >>> > question is about disabling it on old arm machines, do we have
> >>> > data where we know that other sync methods without swp works
> >>> > better on
> >>> > armv5 and lower ?
> >>>
> >>> On armv5 and below a hand optimised implementation using SWP is
> >>> likely to be faster than pthreads. No one is suggesting otherwise.
> >>>
> >>> On SMP (highly likely nowadays for armv7 and above), SWP simply
> >>> might not work (aside from the fact that if it does work, it's
> >>> likely to be slower than pthreads). It's not really a question of
> >>> performance there, so the proposal to only disable SWP for armv8
> >>> doesn't seem like a safe solution.
> >>
> >> Suggestion is not to just do it for armv8 but To keep it there where
> >> its beneficial
> >
> > You can always argue that micro optimisations are beneficial. The question is whether they make a big enough difference in some real world use case to be worth the maintenance effort.
> >
> >>> Using pthreads unconditionally is safe and easy. Unless you can
> >>> prove that hand optimised SWP is really a big win for armv5 (is
> >>> anyone really running a performance critical database on an armv5
> >>> system?) why not keep the recipe simple and use pthreads everywhere?
> >>>
> >>> >> I think the original patch is good.
> >>> >>
> >>> >> >> Cheers,
> >>> >> >> Herve
> >>> >> >>
> >>> >> >> -----Original Message-----
> >>> >> >> From: openembedded-core-bounces at lists.openembedded.org
> >>> >> >> [mailto:openembedded-core-bounces at lists.openembedded.org] On
> >>> >> >> Behalf Of Ovidiu Panait
> >>> >> >> Sent: jeudi 14 juin 2018 13:55
> >>> >> >> To: openembedded-core at lists.openembedded.org
> >>> >> >> Subject: [OE-core] [PATCH 1/1] db: disable the ARM assembler
> >>> >> >> mutex code
> >>> >> >>
> >>> >> >> The swpb in macro MUTEX_SET will cause "undefined instruction"
> >>> >> >> error on the new arm arches which don't support this assembly
> >>> >> >> instruction any more. If use ldrex/strex to replace swpb, the
> >>> >> >> old arm arches don't support them. So to avoid this issue,
> >>> >> >> just disable the ARM assembler mutex code, and use the default
> >>> >> >> pthreads mutex.
> >>> >> >>
> >>> >> >> Signed-off-by: Li Zhou <li.zhou at windriver.com>
> >>> >> >> Signed-off-by: Catalin Enache <catalin.enache at windriver.com>
> >>> >> >> Signed-off-by: Ovidiu Panait <ovidiu.panait at windriver.com>
> >>> >> >> ---
> >>> >> >>  meta/recipes-support/db/db_5.3.28.bb | 13 +------------
> >>> >> >>  1 file changed, 1 insertion(+), 12 deletions(-)
> >>> >> >>
> >>> >> >> diff --git a/meta/recipes-support/db/db_5.3.28.bb
> >>> >> >> b/meta/recipes-support/db/db_5.3.28.bb
> >>> >> >> index 093ee44909..15b4155a29 100644
> >>> >> >> --- a/meta/recipes-support/db/db_5.3.28.bb
> >>> >> >> +++ b/meta/recipes-support/db/db_5.3.28.bb
> >>> >> >> @@ -59,18 +59,7 @@ FILES_SOLIBSDEV = "${libdir}/libdb.so
> >>> >> >> ${libdir}/libdb_cxx.so"
> >>> >> >>  # All the --disable-* options replace --enable-smallbuild,
> >>> >> >> which breaks a bunch of stuff (eg. postfix)  DB5_CONFIG ?=
> >>> >> >> "--enable-o_direct --disable-cryptography --disable-queue
> >>> >> >> --disable-replication --disable-verify --disable-compat185
> >>> >> >> --disable-sql"
> >>> >> >>
> >>> >> >> -EXTRA_OECONF = "${DB5_CONFIG} --enable-shared --enable-cxx
> >>> >> >> --with-sysroot"
> >>> >> >> -
> >>> >> >> -# Override the MUTEX setting here, the POSIX library is -#
> >>> >> >> the default - "POSIX/pthreads/library".
> >>> >> >> -# Don't ignore the nice SWP instruction on the ARM:
> >>> >> >> -# These enable the ARM assembler mutex code, this won't -#
> >>> >> >> work with thumb compilation...
> >>> >> >> -ARM_MUTEX = "--with-mutex=ARM/gcc-assembly"
> >>> >> >> -MUTEX = ""
> >>> >> >> -MUTEX_arm = "${ARM_MUTEX}"
> >>> >> >> -MUTEX_armeb = "${ARM_MUTEX}"
> >>> >> >> -EXTRA_OECONF += "${MUTEX} STRIP=true"
> >>> >> >> +EXTRA_OECONF = "${DB5_CONFIG} --enable-shared --enable-cxx
> >>> >> >> --with-sysroot
> >>> >> >> STRIP=true"
> >>> >> >>  EXTRA_OEMAKE += "LIBTOOL='./${HOST_SYS}-libtool'"
> >>> >> >>
> >>> >> >>  EXTRA_AUTORECONF += "--exclude=autoheader  -I
> >>> >> >> ${S}/dist/aclocal -I${S}/dist/aclocal_java"
> >>> >> >> --
> >>> >> >> 2.17.1
> >>> >> >>
> >>> >> >> --
> >>> >> >> _______________________________________________
> >>> >> >> Openembedded-core mailing list
> >>> >> >> Openembedded-core at lists.openembedded.org
> >>> >> >> http://lists.openembedded.org/mailman/listinfo/openembedded-co
> >>> >> >> r
> >>> >> >> e
> >>> >> >>
> >>> >> >
> >>> >> >
> >>> >> >
> >>> >> > --
> >>> >> > _______________________________________________
> >>> >> > Openembedded-core mailing list
> >>> >> > Openembedded-core at lists.openembedded.org
> >>> >> > http://lists.openembedded.org/mailman/listinfo/openembedded-cor
> >>> >> > e
> >>> >> >
> >
>

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