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Date:   Fri, 27 Jul 2018 11:55:24 -0700
From:   Joel Fernandes <joel@joelfernandes.org>
To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     linux-kernel@vger.kernel.org, kernel-team@android.com,
        Boqun Feng <boqun.feng@gmail.com>,
        Byungchul Park <byungchul.park@lge.com>,
        Erick Reyes <erickreyes@google.com>,
        Ingo Molnar <mingo@redhat.com>,
        Julia Cartwright <julia@ni.com>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Namhyung Kim <namhyung@kernel.org>,
        Paul McKenney <paulmck@linux.vnet.ibm.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Glexiner <tglx@linutronix.de>,
        Todd Kjos <tkjos@google.com>,
        Tom Zanussi <tom.zanussi@linux.intel.com>,
        Will Deacon <will.deacon@arm.com>
Subject: Re: [PATCH v11 3/3] tracing: Centralize preemptirq tracepoints and
 unify their usage
Message-ID: <20180727185524.GB83926@joelaf.mtv.corp.google.com>
References: <20180726235044.10195-1-joel@joelfernandes.org>
 <20180726235044.10195-4-joel@joelfernandes.org>
 <20180727122825.281a3a67@gandalf.local.home>
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On Fri, Jul 27, 2018 at 12:28:25PM -0400, Steven Rostedt wrote:
> On Thu, 26 Jul 2018 16:50:44 -0700
> Joel Fernandes <joel@joelfernandes.org> wrote:
> 
> > From: "Joel Fernandes (Google)" <joel@joelfernandes.org>
> > 
> > This patch detaches the preemptirq tracepoints from the tracers and
> > keeps it separate.
> > 
> > Advantages:
> > * Lockdep and irqsoff event can now run in parallel since they no longer
> > have their own calls.
> > 
> > * This unifies the usecase of adding hooks to an irqsoff and irqson
> > event, and a preemptoff and preempton event.
> >   3 users of the events exist:
> >   - Lockdep
> >   - irqsoff and preemptoff tracers
> >   - irqs and preempt trace events
> > 
> > The unification cleans up several ifdefs and makes the code in preempt
> > tracer and irqsoff tracers simpler. It gets rid of all the horrific
> > ifdeferry around PROVE_LOCKING and makes configuration of the different
> > users of the tracepoints more easy and understandable. It also gets rid
> > of the time_* function calls from the lockdep hooks used to call into
> > the preemptirq tracer which is not needed anymore. The negative delta in
> > lines of code in this patch is quite large too.
> > 
> > In the patch we introduce a new CONFIG option PREEMPTIRQ_TRACEPOINTS
> > as a single point for registering probes onto the tracepoints. With
> > this,
> > the web of config options for preempt/irq toggle tracepoints and its
> > users becomes:
> > 
> >  PREEMPT_TRACER   PREEMPTIRQ_EVENTS  IRQSOFF_TRACER PROVE_LOCKING
> >        |                 |     \         |           |
> >        \    (selects)    /      \        \ (selects) /
> >       TRACE_PREEMPT_TOGGLE       ----> TRACE_IRQFLAGS
> >                       \                  /
> >                        \ (depends on)   /
> >                      PREEMPTIRQ_TRACEPOINTS
> > 
> > One note, I have to check for lockdep recursion in the code that calls
> > the trace events API and bail out if we're in lockdep recursion
> > protection to prevent something like the following case: a spin_lock is
> > taken. Then lockdep_acquired is called.  That does a raw_local_irq_save
> > and then sets lockdep_recursion, and then calls __lockdep_acquired. In
> > this function, a call to get_lock_stats happens which calls
> > preempt_disable, which calls trace IRQS off somewhere which enters my
> > tracepoint code and sets the tracing_irq_cpu flag to prevent recursion.
> > This flag is then never cleared causing lockdep paths to never be
> > entered and thus causing splats and other bad things.
> > 
> > Other than the performance tests mentioned in the previous patch, I also
> > ran the locking API test suite. I verified that all tests cases are
> > passing.
> > 
> > I also injected issues by not registering lockdep probes onto the
> > tracepoints and I see failures to confirm that the probes are indeed
> > working.
> > 
> > This series + lockdep probes not registered (just to inject errors):
> > [    0.000000]      hard-irqs-on + irq-safe-A/21:  ok  |  ok  |  ok  |
> > [    0.000000]      soft-irqs-on + irq-safe-A/21:  ok  |  ok  |  ok  |
> > [    0.000000]        sirq-safe-A => hirqs-on/12:FAILED|FAILED|  ok  |
> > [    0.000000]        sirq-safe-A => hirqs-on/21:FAILED|FAILED|  ok  |
> > [    0.000000]          hard-safe-A + irqs-on/12:FAILED|FAILED|  ok  |
> > [    0.000000]          soft-safe-A + irqs-on/12:FAILED|FAILED|  ok  |
> > [    0.000000]          hard-safe-A + irqs-on/21:FAILED|FAILED|  ok  |
> > [    0.000000]          soft-safe-A + irqs-on/21:FAILED|FAILED|  ok  |
> > [    0.000000]     hard-safe-A + unsafe-B #1/123:  ok  |  ok  |  ok  |
> > [    0.000000]     soft-safe-A + unsafe-B #1/123:  ok  |  ok  |  ok  |
> > 
> > With this series + lockdep probes registered, all locking tests pass:
> > 
> > [    0.000000]      hard-irqs-on + irq-safe-A/21:  ok  |  ok  |  ok  |
> > [    0.000000]      soft-irqs-on + irq-safe-A/21:  ok  |  ok  |  ok  |
> > [    0.000000]        sirq-safe-A => hirqs-on/12:  ok  |  ok  |  ok  |
> > [    0.000000]        sirq-safe-A => hirqs-on/21:  ok  |  ok  |  ok  |
> > [    0.000000]          hard-safe-A + irqs-on/12:  ok  |  ok  |  ok  |
> > [    0.000000]          soft-safe-A + irqs-on/12:  ok  |  ok  |  ok  |
> > [    0.000000]          hard-safe-A + irqs-on/21:  ok  |  ok  |  ok  |
> > [    0.000000]          soft-safe-A + irqs-on/21:  ok  |  ok  |  ok  |
> > [    0.000000]     hard-safe-A + unsafe-B #1/123:  ok  |  ok  |  ok  |
> > [    0.000000]     soft-safe-A + unsafe-B #1/123:  ok  |  ok  |  ok  |
> > 
> > Reviewed-by: Namhyung Kim <namhyung@kernel.org>
> > Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> > ---
> >  include/linux/ftrace.h            |  11 +-
> >  include/linux/irqflags.h          |  11 +-
> >  include/linux/lockdep.h           |   8 +-
> >  include/linux/preempt.h           |   2 +-
> >  include/trace/events/preemptirq.h |  23 +--
> >  init/main.c                       |   5 +-
> >  kernel/locking/lockdep.c          |  35 ++---
> 
> Did Peter ever give an Acked-by for this patch?

Peter didn't give an ack yet, but he had some simple comments last time
around lockdep recursrion and all those were addressed.

Peter, could you Ack this patch if you're Ok with the lockdep and/or other
parts of it?

thanks!

- Joel