From: Brendan Higgins <brendanhiggins@google.com>
To: John Ogness <john.ogness@linutronix.de>
Cc: Petr Mladek <pmladek@suse.com>, Jeff Dike <jdike@addtoit.com>,
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Subject: Re: [PATCH v9 04/18] kunit: test: add kunit_stream a std::stream like logger
Date: Mon, 12 Aug 2019 14:12:11 -0700 [thread overview]
Message-ID: <20190812211211.GA197038@google.com> (raw)
In-Reply-To: <871ry4yq3y.fsf@linutronix.de>
On Fri, Aug 02, 2019 at 09:37:53AM +0200, John Ogness wrote:
> On 2019-08-01, Brendan Higgins <brendanhiggins@google.com> wrote:
> > On Fri, Jul 26, 2019 at 1:31 AM Petr Mladek <pmladek@suse.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@suse.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@kernel.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!
next prev parent reply other threads:[~2019-08-12 21:12 UTC|newest]
Thread overview: 57+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-07-12 8:17 [PATCH v9 00/18] kunit: introduce KUnit, the Linux kernel unit testing framework Brendan Higgins
2019-07-12 8:17 ` [PATCH v9 01/18] kunit: test: add KUnit test runner core Brendan Higgins
2019-07-15 20:10 ` Stephen Boyd
2019-07-15 21:25 ` Brendan Higgins
2019-07-12 8:17 ` [PATCH v9 02/18] kunit: test: add test resource management API Brendan Higgins
2019-07-15 20:24 ` Stephen Boyd
2019-07-15 20:30 ` Brendan Higgins
2019-07-15 20:51 ` Stephen Boyd
2019-07-12 8:17 ` [PATCH v9 03/18] kunit: test: add string_stream a std::stream like string builder Brendan Higgins
2019-07-15 20:43 ` Stephen Boyd
2019-07-15 21:11 ` Brendan Higgins
2019-07-15 22:04 ` Stephen Boyd
2019-07-15 22:11 ` Brendan Higgins
2019-07-15 22:43 ` Brendan Higgins
2019-07-16 15:33 ` Stephen Boyd
2019-07-16 18:55 ` Brendan Higgins
2019-07-12 8:17 ` [PATCH v9 04/18] kunit: test: add kunit_stream a std::stream like logger Brendan Higgins
2019-07-15 22:15 ` Stephen Boyd
2019-07-16 7:57 ` Brendan Higgins
2019-07-16 8:37 ` Brendan Higgins
2019-07-16 15:30 ` Stephen Boyd
2019-07-16 17:51 ` Brendan Higgins
2019-07-16 17:50 ` Stephen Boyd
2019-07-16 18:52 ` Brendan Higgins
2019-07-18 17:50 ` Stephen Boyd
2019-07-18 19:22 ` Brendan Higgins
2019-07-19 0:08 ` Brendan Higgins
2019-07-22 18:10 ` Brendan Higgins
2019-07-22 20:03 ` Stephen Boyd
2019-07-22 22:30 ` Brendan Higgins
2019-07-22 23:54 ` Stephen Boyd
2019-07-23 0:32 ` Brendan Higgins
2019-07-24 7:31 ` Petr Mladek
2019-07-25 20:21 ` Brendan Higgins
2019-07-26 8:31 ` Petr Mladek
2019-08-01 18:55 ` Brendan Higgins
2019-08-01 18:59 ` Brendan Higgins
2019-08-01 21:14 ` Stephen Boyd
2019-08-01 21:43 ` Brendan Higgins
2019-08-12 20:41 ` Brendan Higgins
2019-08-02 7:37 ` John Ogness
2019-08-12 21:12 ` Brendan Higgins [this message]
2019-07-12 8:17 ` [PATCH v9 05/18] kunit: test: add the concept of expectations Brendan Higgins
2019-07-12 8:17 ` [PATCH v9 06/18] kbuild: enable building KUnit Brendan Higgins
2019-07-15 20:49 ` Stephen Boyd
2019-07-12 8:17 ` [PATCH v9 07/18] kunit: test: add initial tests Brendan Higgins
2019-07-12 8:17 ` [PATCH v9 08/18] objtool: add kunit_try_catch_throw to the noreturn list Brendan Higgins
2019-07-12 8:17 ` [PATCH v9 09/18] kunit: test: add support for test abort Brendan Higgins
2019-07-12 8:17 ` [PATCH v9 10/18] kunit: test: add tests for kunit " Brendan Higgins
2019-07-12 8:17 ` [PATCH v9 11/18] kunit: test: add the concept of assertions Brendan Higgins
2019-07-12 8:17 ` [PATCH v9 12/18] kunit: test: add tests for KUnit managed resources Brendan Higgins
2019-07-12 8:17 ` [PATCH v9 13/18] kunit: tool: add Python wrappers for running KUnit tests Brendan Higgins
2019-07-12 8:17 ` [PATCH v9 14/18] kunit: defconfig: add defconfigs for building " Brendan Higgins
2019-07-12 8:17 ` [PATCH v9 15/18] Documentation: kunit: add documentation for KUnit Brendan Higgins
2019-07-12 8:17 ` [PATCH v9 16/18] MAINTAINERS: add entry for KUnit the unit testing framework Brendan Higgins
2019-07-12 8:17 ` [PATCH v9 17/18] kernel/sysctl-test: Add null pointer test for sysctl.c:proc_dointvec() Brendan Higgins
2019-07-12 8:17 ` [PATCH v9 18/18] MAINTAINERS: add proc sysctl KUnit test to PROC SYSCTL section Brendan Higgins
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