Message-ID: <20190812211211.GA197038@google.com>
Date:   Mon, 12 Aug 2019 14:12:11 -0700
From:   Brendan Higgins <brendanhiggins@...gle.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Petr Mladek <pmladek@...e.com>, Jeff Dike <jdike@...toit.com>,
        Kevin Hilman <khilman@...libre.com>,
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Subject: Re: [PATCH v9 04/18] kunit: test: add kunit_stream a std::stream
 like logger

On Fri, Aug 02, 2019 at 09:37:53AM +0200, John Ogness wrote:
> On 2019-08-01, Brendan Higgins <brendanhiggins@...gle.com> wrote:
> > On Fri, Jul 26, 2019 at 1:31 AM Petr Mladek <pmladek@...e.com> wrote:
> >> On Thu 2019-07-25 13:21:12, Brendan Higgins wrote:
> >>> On Wed, Jul 24, 2019 at 12:31 AM Petr Mladek <pmladek@...e.com> wrote:
> >>>> On Mon 2019-07-22 16:54:10, Stephen Boyd wrote:
> >>>>> Quoting Brendan Higgins (2019-07-22 15:30:49)
> >>>>>> On Mon, Jul 22, 2019 at 1:03 PM Stephen Boyd <sboyd@...nel.org> wrote:
> >>>>>>> What's the calling context of the assertions and expectations? I
> >>>>>>> still don't like the fact that string stream needs to allocate
> >>>>>>> buffers and throw them into a list somewhere because the calling
> >>>>>>> context matters there.
> >>>>>>
> >>>>>> The calling context is the same as before, which is anywhere.
> >>>>>
> >>>>> Ok. That's concerning then.
> >>>>>
> >>>>>>> I'd prefer we just wrote directly to the console/log via printk
> >>>>>>> instead. That way things are simple because we use the existing
> >>>>>>> buffering path of printk, but maybe there's some benefit to the
> >>>>>>> string stream that I don't see? Right now it looks like it
> >>>>>>> builds a string and then dumps it to printk so I'm sort of lost
> >>>>>>> what the benefit is over just writing directly with printk.
> >>>>>>
> >>>>>> It's just buffering it so the whole string gets printed
> >>>>>> uninterrupted.  If we were to print out piecemeal to printk,
> >>>>>> couldn't we have another call to printk come in causing it to
> >>>>>> garble the KUnit message we are in the middle of printing?
> >>>>>
> >>>>> Yes, printing piecemeal by calling printk many times could lead to
> >>>>> interleaving of messages if something else comes in such as an
> >>>>> interrupt printing something. Printk has some support to hold
> >>>>> "records" but I'm not sure how that would work here because
> >>>>> KERN_CONT talks about only being used early on in boot code. I
> >>>>> haven't looked at printk in detail though so maybe I'm all wrong
> >>>>> and KERN_CONT just works?
> >>>>
> >>>> KERN_CONT does not guarantee that the message will get printed
> >>>> together. The pieces get interleaved with messages printed in
> >>>> parallel.
> >>>>
> >>>> Note that KERN_CONT was originally really meant to be used only
> >>>> during boot. It was later used more widely and ended in the best
> >>>> effort category.
> >>>>
> >>>> There were several attempts to make it more reliable. But it was
> >>>> always either too complicated or error prone or both.
> >>>>
> >>>> You need to use your own buffering if you rely want perfect output.
> >>>> The question is if it is really worth the complexity. Also note
> >>>> that any buffering reduces the chance that the messages will reach
> >>>> the console.
> >>>
> >>> Seems like that settles it then. Thanks!
> >>>
> >>>> BTW: There is a work in progress on a lockless printk ring buffer.
> >>>> It will make printk() more secure regarding deadlocks. But it might
> >>>> make transparent handling of continuous lines even more tricky.
> >>>>
> >>>> I guess that local buffering, before calling printk(), will be
> >>>> even more important then. Well, it might really force us to create
> >>>> an API for it.
> >>>
> >>> Cool! Can you CC me on that discussion?
> >>
> >> Adding John Oggness into CC.
> >>
> >> John, please CC Brendan Higgins on the patchsets eventually switching
> >> printk() into the lockless buffer. The test framework is going to
> >> do its own buffering to keep the related messages together.
> >>
> >> The lockless ringbuffer might make handling of related (partial)
> >> lines worse or better. It might justify KUnit's extra buffering
> >> or it might allow to get rid of it.
> >
> > Thanks for CC'ing me on the printk ringbuffer thread. It looks like it
> > actually probably won't affect my needs for KUnit logging. The biggest
> > reason I need some sort of buffering system is to be able to compose
> > messages piece meal into a single message that will be printed out to
> > the user as a single message with no messages from other printk
> > callers printed out in the middle of mine.
> 
> printk has this same requirement for its CONT messages. You can read
> about how I propose to implement that here[0], using a separate prb
> ringbuffer for buffered storage until all the pieces are available.
> 
> It is not my goal that multiple subsystems start making use of the prb
> ringbuffer. However, its features can be attractive if you don't want to
> worry about multiple writers/readers or context (including NMI). Before

That sounds like it might be useful down the road, but not to replace
the string_stream.

> writing "yet another ringbuffer" [1] it might be worth the effort to at
> least see if one of the existing implementations can work (or be
> extended to work) for you.

In regards to the conversation here about string_stream/kunit_stream, I
think Petr already answered that question. As I said previously:
> [I]t appears that to get the
> semantics that I need, I would have to put my entire message in a
> single data block and would consequently need to know the size of my
> message a priori, which is problematic. Consequently, it seems as
> though I will probably need to compose my entire message using my own
> buffering system.

I could potentially use my own set of prbs for that buffering; however,
I think this use case is probably closer to seq_buf than your prb.
Really, I just want some kind of string builder, not a message queue.

The place where I think your prb is relevant here is once I have
composed the message and I just want to print it, having a way to print
it without worrying about context is nice, but I think that is a
separate discussion from the main topic here which was just figuring out
the right way to compose that message in the first place.

Cheers

> John Ogness
> 
> [0] https://lkml.kernel.org/r/87imt2bl0k.fsf@linutronix.de
> [1] https://lwn.net/Articles/789603/
> 
> > The prb does look interesting; however, it appears that to get the
> > semantics that I need, I would have to put my entire message in a
> > single data block and would consequently need to know the size of my
> > message a priori, which is problematic. Consequently, it seems as
> > though I will probably need to compose my entire message using my own
> > buffering system.
> >
> >>>> Note that stroring the messages into the printk log is basically
> >>>> safe in any context. It uses temporary per-CPU buffers for
> >>>> recursive messages and in NMI. The only problem is panic() when
> >>>> some CPU gets stuck with the lock taken. This will get solved by
> >>>> the lockless ringbuffer. Also the temporary buffers will not be
> >>>> necessary any longer.
> >>>
> >>> Sure, I think Stephen's concern is all the supporting code that is
> >>> involved. Not printk specifically. It just means a lot more of KUnit
> >>> has to be IRQ safe.
> >>
> >> I see.
> >>
> >> BTW: I wonder if KUnit could reuse the existing seq_buf
> >> implementation for buffering messages.
> >>
> >> I am sorry if it has already been proposed and rejected for some
> >> reason. I might have missed it. Feel free to just point me to
> >> same older mail.
> >
> > Yeah, we discussed it briefly here:
> >
> > https://lkml.org/lkml/2019/5/17/497
> >
> > Looks like I forgot to include my reasoning in the commit text, sorry
> > about that.
> >
> >>>> Much bigger problems are with consoles. There are many of them. It
> >>>> means a lot of code and more locks involved, including scheduler
> >>>> locks. Note that console lock is a semaphore.
> >>>
> >>> That shouldn't affect us though, right? As long as we continue to
> >>> use the printk interface?
> >>
> >> I guess that it should not affect KUnit.
> >>
> >> The only problem might be if the testing framework calls printk()
> >> inside scheduler or console code. And only when the tested code
> >> uses the same locks that will be used by the called printk().
> >
> > Yeah, well printk will not be our only problem in those instances.
> >
> >> To be honest I do not fully understand KUnit design. I am not
> >> completely sure how the tested code is isolated from the running
> >> system. Namely, I do not know if the tested code shares
> >> the same locks with the system running the test.
> >
> > No worries, I don't expect printk to be the hang up in those cases. It
> > sounds like KUnit has a long way to evolve before printk is going to
> > be a limitation.
> >
> > Thanks!

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