linux-kernel - Re: [PATCH v2 1/4] ftrace: Implement fs notification for preempt/irqsoff tracers

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Date:   Sun, 5 May 2019 19:01:33 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Viktor Rosendahl <viktor.rosendahl@...il.com>
Cc:     Joel Fernandes <joel@...lfernandes.org>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/4] ftrace: Implement fs notification for
 preempt/irqsoff tracers

On Mon,  6 May 2019 00:39:15 +0200
Viktor Rosendahl <viktor.rosendahl@...il.com> wrote:

> Can you explain more precisely what you agree with?
> 
> The general idea of being able to trace bursts of latencies?

One thing I have an issue with the current approach is the use of the
trace file for this.

> 
> Or the slightly more specific idea of notifying user space when
> /sys/kernel/debug/tracing/trace has new data, and let user space do the rest?
> 
> > We do have a notification mechanism already in the form of trace_pipe. Can we
> > not improve that in some way to be notified of a new trace data? In theory,
> > the trace_pipe does fit into the description in the documentation: "Reads
> > from this file will block until new data is retrieved"
> >  
> 
> I am not quite sure what kind of solution you are after here. Could you be more
> specific?
> 
> I think that it would be weird if we used trace_pipe to send a message with
> the meaning "new data is available in the trace file". To me this would seem
> like (re)inventing a special purpose alternative to inotify.
> 
> Another option would be to allow the user to consume the the latency traces
> directly from trace_pipe, without using the trace file. This would make sense
> to me and would indeed be a much better solution. If somebody is able to make
> such an implementation I am all for it.
> 
> However, I do not know how to do this. I believe that it would lead towards a
> significant rewrite of the latency tracers, probably also how the ftrace
> buffering works.
> 

Hmm, what about adding a notifier to tracing_max_latency instead? And
do it not as a config option, but have it always enabled. It would send a
notification when it changes, and that only happens when there's a new
max latency. Would that work for you?

-- Steve