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From:   Brendan Higgins <brendanhiggins@google.com>
Date:   Wed, 6 Jul 2022 15:43:09 -0400
Message-ID: <CAFd5g44dp05DaEot23_a2QdOGfmg=eehtoe24=6yo_UKiGNukA@mail.gmail.com>
Subject: Re: [PATCH 1/2] kunit: tool: Add x86_64-smp architecture for SMP testing
To:     Daniel Latypov <dlatypov@google.com>
Cc:     David Gow <davidgow@google.com>, Marco Elver <elver@google.com>,
        Shuah Khan <skhan@linuxfoundation.org>,
        Dmitry Vyukov <dvyukov@google.com>,
        KUnit Development <kunit-dev@googlegroups.com>,
        kasan-dev <kasan-dev@googlegroups.com>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Thu, May 19, 2022 at 1:11 PM Daniel Latypov <dlatypov@google.com> wrote:
>
> On Thu, May 19, 2022 at 6:15 AM David Gow <davidgow@google.com> wrote:
> >
> > I tend to agree that having both would be nice: I think there are
> > enough useful "machine configs" that trying to maintain, e.g, a 1:1
> > mapping with kernel architectures is going to leave a bunch of things
> > on the table, particularly as we add more tests for, e.g., drivers and
> > specific CPU models.
>
> I agree that we don't necessarily need to maintain a 1:1 mapping.
> But I feel like we should have a pretty convincing reason for doing
> so, e.g. support for a CPU that requires we add in a bunch of
> kconfigs.

Agreed. That being said, if we have a good convention for archs that
are not in arch/, then it should be OK. The biggest thing is that all
archs passed into ARCH=, if supported, should have a default with the
same value for kunittool; as long as that is the case, I don't think
anyone will get confused.

> This particular one feels simple enough to me.
> Given we already have to put specific instructions in the
> kcsan/.kunitconfig, I don't know if there's much of a difference in
> cost between these two commands
>
> $ ./tools/testing/kunit/kunit.py run --kunitconfig=kernel/kcsan
> --arch=x86_64-smp
> $ ./tools/testing/kunit/kunit.py run --kunitconfig=kernel/kcsan
> --arch=x86_64 --kconfig_add CONFIG_SMP=y --qemu_args "-smp 8"

Also agree.

> I've generally learned to prefer more explicit commands like the
> second, even if they're quite a bit longer.

I agree, but I think I learned this from you :-)

> But I have the following biases
> * I use FZF heavily, so I don't re-type long commands much

Same.

> * I'm the person who proposed --kconfig_add and --qemu_args, so of
> course I'd think the longer form is easy to understand.
> so I'm not in a position to object to this change.

Yeah, I think I am a bit biased on this too, but I don't terribly care
one way or the other.

> Changing topics:
> Users can overwrite the '-smp 8' here via --qemu_args [1], so I'm much
> less worried about hard-coding any specific value in this file
> anymore.
> And given that, I think a more "natural" value for this file would be "-smp 2".
> I think anything that needs more than that should explicitly should --qemu_args.
>
> Thoughts?

If we have time, we could bring this topic up at LPC?

> [1] tested with --qemu_args='-smp 4' --qemu_args='-smp 8'
> and I see the following in the test.log
>  smpboot: Allowing 8 CPUs, 0 hotplug CPUs
> so QEMU respects the last value passed in, as expected.
>
> >
> > The problem, of course, is that the --kconfig_add flags don't allow us
> > to override anything explicitly stated in either the kunitconfig or
> > qemu_config (and I imagine there could be problems with --qemu_config,
> > too).
>
> This patch would fix that.
> https://lore.kernel.org/linux-kselftest/20220519164512.3180360-1-dlatypov@google.com
>
> It introduces an overwriting priority of
> * --kconfig_add
> * kunitconfig / --kunitconfig
> * qemu_config