Re: [PATCH V2 1/2] trace,x86: Add external_interrupts to the irq_vectors class

From: Daniel Bristot de Oliveira <bristot@redhat.com>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Ingo Molnar <mingo@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	Andy Lutomirski <luto@kernel.org>,
	Clark Williams <williams@redhat.com>,
	x86@kernel.org
Subject: Re: [PATCH V2 1/2] trace,x86: Add external_interrupts to the irq_vectors class
Date: Mon, 8 Apr 2019 11:48:50 +0200	[thread overview]
Message-ID: <7912cfa4-cd20-c764-2a36-32d3853066be@redhat.com> (raw)
In-Reply-To: <alpine.DEB.2.21.1904052314410.1802@nanos.tec.linutronix.de>

On 4/5/19 11:15 PM, Thomas Gleixner wrote:
> On Mon, 1 Apr 2019, Daniel Bristot de Oliveira wrote:
> 
>> Currently, the irq_vectors is showing the entry and exit events for
>> the interrupts of the architecture, but not for external interrupts.
> 
> Those are covered by the irq tracepoints. Is there a really good reason why
> we need both?

The irq_handler_* tracepoints might give the "imprecise" idea that more than one
interrupts were raised when we have shared handlers. For instance:

------------ %< ----------------
f-892   [000] d.h.   790.617251: external_interrupt_entry: vector=37
f-892   [000] d.h.   790.617257: irq_handler_entry: irq=11 name=uhci_hcd:usb3
f-892   [000] d.h.   790.617343: irq_handler_exit: irq=11 ret=handled
f-892   [000] d.h.   790.617343: irq_handler_entry: irq=11 name=uhci_hcd:usb4
f-892   [000] d.h.   790.617349: irq_handler_exit: irq=11 ret=unhandled
f-892   [000] d.h.   790.617350: irq_handler_entry: irq=11 name=qxl
f-892   [000] d.h.   790.617360: irq_handler_exit: irq=11 ret=handled
f-892   [000] d.h.   790.617387: external_interrupt_exit: vector=37
------------ >% ----------------

In this case, a single interrupt occurrence (vector 37) caused two handlers to
handle their interrupt.

From a latency analysis perspective, the external_interrupt_* tracepoints turn
clearer that a single interrupt interfered in the thread execution happened, not
two separated executions of the same vector.

The outer-most tracepoints also help to have a more precise accounting of the
interference:

Using the sum of the irq_handler_* tracepoints we have:
(790.617343−790.617257)+(790.617349−790.617343)+(790.617360−790.617350) = 102 us

While using the irq_vector based one:
 790.617387-790.617251 = 136 us

So, the proposed tracepoints help clarify the logical sequence of the interrupt
handling, while increasing the precision of the measurements.

Thoughts?

-- Daniel
> Thanks,
> 
> 	tglx
> 