linux-kernel - Re: [RFC][PATCH 0/4] tracing: Add new hwlat

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Date:	Thu, 23 Apr 2015 21:51:47 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Steven Rostedt <rostedt@...dmis.org>
cc:	linux-kernel@...r.kernel.org, linux-rt-users@...r.kernel.org,
	Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Carsten Emde <C.Emde@...dl.org>,
	Daniel Wagner <wagi@...om.org>, Jon Masters <jcm@...hat.com>,
	Clark Williams <williams@...hat.com>
Subject: Re: [RFC][PATCH 0/4] tracing: Add new hwlat_detector tracer

On Thu, 23 Apr 2015, Steven Rostedt wrote:
> This is the port of the hardware latency detector from the -rt patch
> to mainline. Instead of keeping it as a device that had its own debugfs
> filesystem infrastructure, it made more sense to make it into a tracer
> like irqsoff and wakeup latency tracers currently are.
> 
> With this patch set, a new tracer is enabled if CONFIG_HWLAT_TRACER is
> enabled. Inside the available_tracers file will be hwlat_detector.
> 
>  # cd /sys/kernel/debug/tracing
>  # echo hwlat_detector > current_tracer
> 
> will enable the hwlat_detector that will create per cpu kernel threads
> (which cpus is defined by the tracing/hwlat_detector/cpumask, default
>  is just CPU 0).
> 
> Like the other tracers (function, function_graph, preemptirqsoff,
> and mmiotracer), the hwlat_detector can add a significant performance
> penalty when enabled. As each of the threads created will go into a spin
> checking the trace_local_clock (sched_clock) for any gaps of time
> and will report them if they are greater than the threshold defined
> by tracing/tracing_thresh (usecs, default 10). The spin is performed with
> interrupts disabled and runs for "width" usecs in "window" usecs time. The

That's fine, but this still lacks a detection of NMI
disturbance. We've seen false positives reported over and over when
stuff like the NMI watchdog or perf was enabled while running this.

Aside of that isn't there a way to detect SMI crap with performance
counters on recent hardware?

Thanks,

	tglx
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