[ovs-dev] [PATCH v1] configure: Allow opt-in to CPU ISA opts at compile time

Flavio Leitner fbl at sysclose.org
Thu Nov 25 17:53:39 UTC 2021


On Mon, Sep 13, 2021 at 02:36:41PM +0000, Van Haaren, Harry wrote:
> > -----Original Message-----
> > From: Eelco Chaudron <echaudro at redhat.com>
> > Sent: Friday, September 10, 2021 3:41 PM
> > To: Van Haaren, Harry <harry.van.haaren at intel.com>; i.maximets at ovn.org;
> > Stokes, Ian <ian.stokes at intel.com>; fbl at sysclose.org
> > Cc: Amber, Kumar <kumar.amber at intel.com>; ovs-dev at openvswitch.org;
> > ktraynor at redhat.com
> > Subject: Re: [PATCH v1] configure: Allow opt-in to CPU ISA opts at compile time
> > 
> > 
> > 
> > On 8 Sep 2021, at 17:28, Van Haaren, Harry wrote:
> > 
> > >> -----Original Message-----
> > >> From: Eelco Chaudron <echaudro at redhat.com>
> > >> Sent: Wednesday, September 8, 2021 9:16 AM
> > >> To: Amber, Kumar <kumar.amber at intel.com>
> > >> Cc: ovs-dev at openvswitch.org; ktraynor at redhat.com; i.maximets at ovn.org;
> > >> Stokes, Ian <ian.stokes at intel.com>; fbl at sysclose.org; Van Haaren, Harry
> > >> <harry.van.haaren at intel.com>
> > >> Subject: Re: [PATCH v1] configure: Allow opt-in to CPU ISA opts at compile
> > time
> > >>
> > >> Not a real review of the patch, but just some comment/questions glancing
> > over
> > >> the patch.
> > >
> > > Sure, thanks for input.
> > >
> > >> On 3 Sep 2021, at 15:53, Kumar Amber wrote:
> > >>
> > >>> This commit allows "opt-in" to CPU ISA optimized implementations of
> > >>> OVS SW datapath components at compile time. This can be useful in some
> > >>> deployments where the CPU ISA optimized implementation is to be chosen
> > >>> by default.
> > >
> > > <snip some contents>
> > >
> > >>> +Enabling all AVX512 options
> > >>> +---------------------------
> > >>> +
> > >>> +A user can enable all the three DPIF, Miniflow Extract and DPLCS optimized
> > >>> +AVX512 options at build time, if the CPU supports the required AVX512 ISA
> > >>> +by using the following command ::
> > >>> +
> > >>> +    ./configure --enable-cpu-isa
> > >>
> > >> If we have different ISA architectures, i.e., i86 vs ARM, we are ok with a single
> > >> option. Have you thought about AMD adding its own AMDXXX instructions in
> > >> addition to AVX512? How would this configuration option work? Maybe an
> > >> optional option to prioritize one over the other.
> > >
> > > The ISA enabling efforts have been generic so far, any reference to specific ISA
> > (e.g. AVX512)
> > > has been solely in the implementation choice - never in a general component.
> > Intention here
> > > is to stay in line with that - and "enable CPU ISA" seemed a logical string to
> > achieve that to me..
> > >
> > > It is of course possible to provide multiple configure command lines, but I was
> > hoping to avoid
> > > creating too many compile time flags. Typically I think projects attempt to avoid
> > due to expanding
> > > testing & validation. A single flag would limit overhead to the minimum...
> > >
> > > Typically ISA sets have a "good - better - best" type relationship - which could
> > lead to a general
> > > acceptance of what ISA is best. We have runtime functions to switch
> > implementation - so today
> > > the code already enables a log of runtime/dynamic updating of
> > implementation. If there's a
> > > need to expose that at compile time too, then that's easy to add - but comes
> > with a burden in
> > > testing & validation...
> > 
> > The main reason to mention this is the inconsistent behavior across
> > builds/releases. With this flag being as general as it is, if someone decides to add
> > AVX1024, it now gets selected as the default isa function (assuming the target
> > was already supporting this). This is a change in behavior that happens without
> > any configure option change. The difference to any other general change is that
> > this is not a global change, but something that changes based on your target.
> 
> Note that the default setting for this option is suggested as "off", meaning this is an entirely
> *opt in* strategy, to allow people to deploy OVS and automatically benefit from CPU ISA.
> 
> To be more specific, this feature is a request from folks who intend to deploy with CPU ISA
> enabled by default - it suited their CI/CD/QA tooling to have this enabled by default compile
> time switch to ease validation as the CPU ISA will get picked up automatically when available.
> 
> Note that it is not a "change in behaviour", because functionally its identical.
> (The fuzzing, autovalidation & unit tests are there to ensure it is functionally identical).
> It correct that there is a change in the default *implementation* of the functionality,
> which I think you meant (just clarifying the "change in behaviour" as not being "functional behaviour",
> only "implementation of behaviour")

It seems to me this is coming to address the distro release process
requirement of not going back in the process to change the RPM
package.

To give others some background: An overview of release process is
like: we build a RPM package, then send it to QA, and if it gets
approved, then ship it to users/customers.

Therefore, there is no way to go back to rebuild/change the RPM to
enable/disable something, then ship to users/customers.

That was a concern initially with AVX512 implementation.

However, the implementation selection can be done at runtime, and
it doesn't require changing the RPM. So, from the distro release
process point of view, there would be no need for this build option.

See below.

> > Anyone else has an opinion on this? I think an alternative to this, is a proper
> > configuration option, which will survive a restart.
> >
> > Thinking about it a bit more during my afternoon walk, I think we should minimize
> > (not allow) any compile-time default behavior variations. It should all be based on
> > the configuration, which if required to survive a reboot, be added to the ovsdb.

That sounds like a better solution to me. OVS stores persistent settings
in the ovsdb, and that's what we look for when we need to support OVS.

Does that address the original requirements of this patch?

> > >> Even if we decide a single option is right, this one to me looks more about
> > >> compile-time flags than enable ISA-specific code. Maybe something more in
> > line
> > >> with --prioritize-cpu-isa-functions? Or some other catchy name.
> > >
> > > Sure - that string is fine for me too.

Yeah, the concern is more like how we could expand and express that
in the future in case there is a new optimization available. Because
--enable-cpu-isa seems way too generic and we won't be able to
change that without breaking stuff.

Also that, it may open doors to change other defaults, and that
makes life harder to support OVS because we will need to find out
how OVS was built instead of just checking the ovsdb.

Thanks,
-- 
fbl


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