From: Joel Fernandes <joel@joelfernandes.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Viktor Rosendahl <viktor.rosendahl@gmail.com>,
paulmck@kernel.org, Steven Rostedt <rostedt@goodmis.org>,
Ingo Molnar <mingo@redhat.com>,
linux-kernel@vger.kernel.org, linux-rt-users@vger.kernel.org
Subject: Re: [PATCH v5 1/4] ftrace: Implement fs notification for tracing_max_latency
Date: Wed, 4 Sep 2019 09:42:03 -0400 [thread overview]
Message-ID: <20190904134203.GA240514@google.com> (raw)
In-Reply-To: <20190904081919.GA2349@hirez.programming.kicks-ass.net>
On Wed, Sep 04, 2019 at 10:19:19AM +0200, Peter Zijlstra wrote:
> On Wed, Sep 04, 2019 at 12:00:39AM -0400, Joel Fernandes wrote:
> > [ Resending since I messed up my last email's headers! ]
> >
> > On Tue, Sep 03, 2019 at 03:25:59PM +0200, Viktor Rosendahl wrote:
> > > This patch implements the feature that the tracing_max_latency file,
> > > e.g. /sys/kernel/debug/tracing/tracing_max_latency will receive
> > > notifications through the fsnotify framework when a new latency is
> > > available.
> > >
> > > One particularly interesting use of this facility is when enabling
> > > threshold tracing, through /sys/kernel/debug/tracing/tracing_thresh,
> > > together with the preempt/irqsoff tracers. This makes it possible to
> > > implement a user space program that can, with equal probability,
> > > obtain traces of latencies that occur immediately after each other in
> > > spite of the fact that the preempt/irqsoff tracers operate in overwrite
> > > mode.
> >
> > Adding Paul since RCU faces similar situations, i.e. raising softirq risks
> > scheduler deadlock in rcu_read_unlock_special() -- but RCU's solution is to
> > avoid raising the softirq and instead use irq_work.
>
> Which is right.
Cool.
> > I was wondering, if we can rename __raise_softirq_irqoff() to
> > raise_softirq_irqoff_no_wake() and call that from places where there is risk
> > of scheduler related deadlocks. Then I think this can be used from Viktor's
> > code. Let us discuss - what would happen if the softirq is raised, but
> > ksoftirqd is not awakened for this latency notification path? Is this really
> > an issue considering the softirq will execute during the next interrupt exit?
>
> You'd get unbounded latency for processing the softirq and warnings on
> going idle with softirqs pending.
Thanks for sharing that.
> I really don't see why we should/want to be using softirq here.
Sure. makes sense.
thanks,
- Joel
next prev parent reply other threads:[~2019-09-04 13:42 UTC|newest]
Thread overview: 19+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-09-03 13:25 [PATCH v5 0/4] Some new features for the preempt/irqsoff tracers Viktor Rosendahl
2019-09-03 13:25 ` [PATCH v5 1/4] ftrace: Implement fs notification for tracing_max_latency Viktor Rosendahl
2019-09-04 4:00 ` Joel Fernandes
2019-09-04 8:19 ` Peter Zijlstra
2019-09-04 13:42 ` Joel Fernandes [this message]
2019-09-04 18:17 ` Viktor Rosendahl
2019-09-04 10:21 ` Paul E. McKenney
2019-09-04 8:20 ` Peter Zijlstra
2019-09-04 11:37 ` Steven Rostedt
2019-09-04 11:39 ` Steven Rostedt
2019-09-04 19:00 ` Viktor Rosendahl
2019-09-03 13:26 ` [PATCH v5 2/4] preemptirq_delay_test: Add the burst feature and a sysfs trigger Viktor Rosendahl
2019-09-04 11:42 ` Steven Rostedt
2019-09-04 19:10 ` Viktor Rosendahl
2019-09-05 16:52 ` Steven Rostedt
2019-09-05 18:27 ` Viktor Rosendahl
2019-09-03 13:26 ` [PATCH v5 3/4] Add the latency-collector to tools Viktor Rosendahl
2019-09-03 13:26 ` [PATCH v5 4/4] ftrace: Add an option for tracing console latencies Viktor Rosendahl
2019-09-04 5:02 ` kbuild test robot
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