Re: [PATCH v5 1/4] ftrace: Implement fs notification for tracing_max_latency

[Joel Fernandes <joel@joelfernandes.org>]

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