Date:   Tue, 25 Jun 2019 15:14:45 -0700
From:   Brendan Higgins <brendanhiggins@...gle.com>
To:     Luis Chamberlain <mcgrof@...nel.org>
Cc:     Stephen Boyd <sboyd@...nel.org>,
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Subject: Re: [PATCH v5 01/18] kunit: test: add KUnit test runner core

On Tue, Jun 25, 2019 at 2:44 PM Luis Chamberlain <mcgrof@...nel.org> wrote:
>
> On Tue, Jun 25, 2019 at 01:28:25PM -0700, Brendan Higgins wrote:
> > On Wed, Jun 19, 2019 at 5:15 PM Stephen Boyd <sboyd@...nel.org> wrote:
> > >
> > > Quoting Brendan Higgins (2019-06-17 01:25:56)
> > > > diff --git a/kunit/test.c b/kunit/test.c
> > > > new file mode 100644
> > > > index 0000000000000..d05d254f1521f
> > > > --- /dev/null
> > > > +++ b/kunit/test.c
> > > > @@ -0,0 +1,210 @@
> > > > +// SPDX-License-Identifier: GPL-2.0
> > > > +/*
> > > > + * Base unit test (KUnit) API.
> > > > + *
> > > > + * Copyright (C) 2019, Google LLC.
> > > > + * Author: Brendan Higgins <brendanhiggins@...gle.com>
> > > > + */
> > > > +
> > > > +#include <linux/sched/debug.h>
> > > > +#include <kunit/test.h>
> > > > +
> > > > +static bool kunit_get_success(struct kunit *test)
> > > > +{
> > > > +       unsigned long flags;
> > > > +       bool success;
> > > > +
> > > > +       spin_lock_irqsave(&test->lock, flags);
> > > > +       success = test->success;
> > > > +       spin_unlock_irqrestore(&test->lock, flags);
> > >
> > > I still don't understand the locking scheme in this code. Is the
> > > intention to make getter and setter APIs that are "safe" by adding in a
> > > spinlock that is held around getting and setting various members in the
> > > kunit structure?
> >
> > Yes, your understanding is correct. It is possible for a user to write
> > a test such that certain elements may be updated in different threads;
> > this would most likely happen in the case where someone wants to make
> > an assertion or an expectation in a thread created by a piece of code
> > under test. Although this should generally be avoided, it is possible,
> > and there are occasionally good reasons to do so, so it is
> > functionality that we should support.
> >
> > Do you think I should add a comment to this effect?
> >
> > > In what situation is there more than one thread reading or writing the
> > > kunit struct? Isn't it only a single process that is going to be
> >
> > As I said above, it is possible that the code under test may spawn a
> > new thread that may make an expectation or an assertion. It is not a
> > super common use case, but it is possible.
>
> I wonder if it is worth to have then different types of tests based on
> locking requirements. One with no locking, since it seems you imply
> most tests would fall under this category, then locking, and another
> with IRQ context.
>
> If no locking is done at all for all tests which do not require locking,
> is there any gains at run time? I'm sure it might be minimum but
> curious.

Yeah, I don't think it is worth it.

I don't think we need to be squeezing every ounce of performance out
of unit tests, since they are inherently a cost and are not intended
to be run in a production deployed kernel as part of normal production
usage.

> > > operating on this structure? And why do we need to disable irqs? Are we
> > > expecting to be modifying the unit tests from irq contexts?
> >
> > There are instances where someone may want to test a driver which has
> > an interrupt handler in it. I actually have (not the greatest) example
> > here. Now in these cases, I expect someone to use a mock irqchip or
> > some other fake mechanism to trigger the interrupt handler and not
> > actual hardware; technically speaking in this case, it is not going to
> > be accessed from a "real" irq context; however, the code under test
> > should think that it is in an irq context; given that, I figured it is
> > best to just treat it as a real irq context. Does that make sense?
>
> Since its a new architecture and since you seem to imply most tests
> don't require locking or even IRQs disabled, I think its worth to
> consider the impact of adding such extreme locking requirements for
> an initial ramp up.

Fair enough, I can see the point of not wanting to use irq disabled
until we get someone complaining about it, but I think making it
thread safe is reasonable. It means there is one less thing to confuse
a KUnit user and the only penalty paid is some very minor performance.

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