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Date: Wed, 25 Feb 2009 18:13:25 -0500
From: Mathieu Desnoyers <compudj@...stal.dyndns.org>
To: Frederic Weisbecker <fweisbec@...il.com>
Cc: Steven Rostedt <rostedt@...dmis.org>,
Jason Baron <jbaron@...hat.com>,
Masami Hiramatsu <mhiramat@...hat.com>,
KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
Peter Zijlstra <peterz@...radead.org>,
"Frank Ch. Eigler" <fche@...hat.com>, mingo@...e.hu,
linux-kernel@...r.kernel.org, acme@...stprotocols.net,
Dominique Toupin <dominique.toupin@...csson.com>
Subject: Re: [PATCH] new irq tracer
* Frederic Weisbecker (fweisbec@...il.com) wrote:
> On Wed, Feb 25, 2009 at 05:12:10PM -0500, Mathieu Desnoyers wrote:
> > * Steven Rostedt (rostedt@...dmis.org) wrote:
> > >
> > >
> > > On Wed, 25 Feb 2009, Mathieu Desnoyers wrote:
> > >
> > > > * Jason Baron (jbaron@...hat.com) wrote:
> > > > > On Wed, Feb 25, 2009 at 11:48:28AM -0500, Masami Hiramatsu wrote:
> > > > > > KOSAKI Motohiro wrote:
> > > > > > >> /**
> > > > > > >> * handle_IRQ_event - irq action chain handler
> > > > > > >> * @irq: the interrupt number
> > > > > > >> @@ -354,7 +358,9 @@ irqreturn_t handle_IRQ_event(unsigned int irq, struct irqaction *action)
> > > > > > >> local_irq_enable_in_hardirq();
> > > > > > >>
> > > > > > >> do {
> > > > > > >> + trace_irq_entry(irq);
> > > > > > >> ret = action->handler(irq, action->dev_id);
> > > > > > >> + trace_irq_exit(irq, ret);
> > > > > > >> if (ret == IRQ_HANDLED)
> > > > > > >> status |= action->flags;
> > > > > > >> retval |= ret;
> > > > > > >
> > > > > > > Nobdy want unnecessary redundant tracepoint.
> > > > > > > Please discuss with mathieu, and merge his tracepoint.
> > > > > >
> > > > > > Hmm, from the viewpoint of trouble shooting, the place of LTTng's tracepoint
> > > > > > is enough. However, from the same viewpoint, it should pass irq-number
> > > > > > to irq-exit event too, because we may lost some previous events by buffer-overflow
> > > > > > etc.
> > > > > >
> > > > > > trace_irq_entry(irq, NULL);
> > > > > > ret = _handle_IRQ_event(irq, action);
> > > > > > trace_irq_exit(irq, ret);
> > > > > > ^^^^
> > > > > >
> > > > >
> > > > > the lttng tracepoints wrap the calls to _handle_IRQ_event in 3
> > > > > different places. So the above suggested irq tracepoint provides the
> > > > > same information with 4 less tracepoints in the code. So I believe its
> > > > > simpler - plus we can understand which action handlers are handling the
> > > > > interrupt.
> > > > >
> > > >
> > > > The main thing I dislike about only tracing action->handler() calls is
> > > > that you are not tracing an IRQ per se, but rather the invocation of a
> > > > given handler within the interrupt. For instance, it would be difficult
> > > > to calculate the maximum interrupt latency for a given interrupt line,
> > > > because you don't have the "real" irq entry/exit events, just the
> > > > individual handler() calls.
> > >
> > > Then use the function_graph tracer.
> > >
> >
> > Sadly, the function tracer cannot be enabled on production systems.
> > Therefore it's not a usable solution in the context where I need this.
> >
> > Mathieu
>
>
> Why?
> If this is about the interrupt latency caused by stop_machine, I still don't understand.
> This latency happens before the tracing (function filter which use patching), not during
> tracing.
>
Given this scenario :
A telecommunication system runs, but the client notices something wrong.
They call their service provider. The provider enables tracing
_remotely_ on the _production system_ while it's _active in the field_.
Bam, those few milliseconds interrupt latencies become unacceptable.
Hopefully this scenario makes the use-case clearer. The problem is not
that interrupt latencies would occur while tracing is on, but rather
that it would happen on a running production system when switching
tracing on. This is what is totally unacceptable for this use-case.
For more details about such requirements, I'm CCing Dominique Toupin
from Ericsson who I'm sure would be happy to give more details about
this if needed.
Mathieu
>
> > > # echo smp_apic_timer_interrupt > /debug/tracing/set_ftrace_filter
> > > # echo __irqentry_text_start >> /debug/tracing/set_ftrace_filter
> > > # echo function_graph > /debug/tracing/current_tracer
> > > # cat /debug/tracing/trace
> > >
> > > # tracer: function_graph
> > > #
> > > # CPU DURATION FUNCTION CALLS
> > > # | | | | | | |
> > > ------------------------------------------
> > > 1) kblockd-4012 => <idle>-0
> > > ------------------------------------------
> > >
> > > 1) ==========> |
> > > 1) + 35.783 us | smp_apic_timer_interrupt();
> > > ------------------------------------------
> > > 2) kstop/2-4013 => <idle>-0
> > > ------------------------------------------
> > >
> > > 2) ==========> |
> > > 2) + 13.819 us | smp_apic_timer_interrupt();
> > > ------------------------------------------
> > > 3) kstop/3-4014 => <idle>-0
> > > ------------------------------------------
> > >
> > > 3) ==========> |
> > > 3) + 24.919 us | __irqentry_text_start();
> > > ------------------------------------------
> > > 0) kstop/0-4011 => <idle>-0
> > > ------------------------------------------
> > >
> > > 0) ==========> |
> > > 0) + 24.444 us | smp_apic_timer_interrupt();
> > > ------------------------------------------
> > > 1) <idle>-0 => bash-3899
> > > ------------------------------------------
> > >
> > > 1) ==========> |
> > > 1) + 13.642 us | smp_apic_timer_interrupt();
> > > 2) ==========> |
> > > 2) + 15.014 us | smp_apic_timer_interrupt();
> > > 3) ==========> |
> > > 3) + 31.004 us | smp_apic_timer_interrupt();
> > > ------------------------------------------
> > > 0) <idle>-0 => sshd-3892
> > > ------------------------------------------
> > >
> > > 0) ==========> |
> > > 0) + 15.655 us | __irqentry_text_start();
> > > 0) ==========> |
> > > 0) + 37.722 us | __irqentry_text_start();
> > > ------------------------------------------
> > > 0) sshd-3892 => <idle>-0
> > > ------------------------------------------
> > >
> > > 0) ==========> |
> > > 0) + 26.139 us | __irqentry_text_start();
> > > 3) ==========> |
> > > 3) + 27.240 us | __irqentry_text_start();
> > > 3) ==========> |
> > > 3) + 21.987 us | __irqentry_text_start();
> > >
> > >
> > > And there you have your latencies ;-)
> > >
> > > With Jasons added trace points, we could add the to the event tracer
> > > and those would show up as comments here.
> > >
> > > -- Steve
> > >
> > >
> > >
> > > >
> > > > But I agree that knowing which handler is called is important.
> > > >
> > > > How about this compromise :
> > > >
> > > > trace_irq_entry(irq, action)
> > > > _handle_IRQ_event()
> > > > for each action {
> > > > trace_irq_handler(action, ret);
> > > > ret = action->handler(irq, action->dev_id);
> > > > ...
> > > > }
> > > > trace_irq_exit(action_ret);
> > > >
> > > > Would that give you the information you need ?
> > > >
> > > > Here trace_irq_handler would be passed the _current_ action invoked and
> > > > the _previous_ action return value. Note that we should initialize
> > > > irqreturn_t ret to some initial value if we do this. That should keep
> > > > the tracing overhead minimal.
> > > >
> > >
> >
> > --
> > Mathieu Desnoyers
> > OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
>
--
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
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