linux-kernel - boot tracing

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Date:	Fri, 12 Jul 2013 10:47:12 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Robin Holt <holt@....com>, Robert Richter <rric@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>, Nate Zimmer <nzimmer@....com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Linux MM <linux-mm@...ck.org>, Rob Landley <rob@...dley.net>,
	Mike Travis <travis@....com>,
	Daniel J Blueman <daniel@...ascale-asia.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Greg KH <gregkh@...uxfoundation.org>,
	Yinghai Lu <yinghai@...nel.org>, Mel Gorman <mgorman@...e.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: boot tracing

On Fri, Jul 12, 2013 at 10:27:56AM +0200, Ingo Molnar wrote:
> Robert Richter and Boris Petkov are working on 'persistent events'
> support for perf, which will eventually allow boot time profiling -
> I'm not sure if the patches and the tooling support is ready enough
> yet for your purposes.

Nope, not yet but we're getting there.

> Robert, Boris, the following workflow would be pretty intuitive:
> 
>  - kernel developer sets boot flag: perf=boot,freq=1khz,size=16MB

What does perf=boot mean? I assume boot tracing.

If so, does it mean we want to enable *all* tracepoints and collect
whatever hits us?

What makes more sense to me is to hijack what the function tracer does -
i.e. simply collect all function calls.

>  - we'd get a single (cycles?) event running once the perf subsystem is up
>    and running, with a sampling frequency of 1 KHz, sending profiling
>    trace events to a sufficiently sized profiling buffer of 16 MB per
>    CPU.

Right, what would those trace events be?

>  - once the system reaches SYSTEM_RUNNING, profiling is stopped either
>    automatically - or the user stops it via a new tooling command.

Ok.

>  - the profiling buffer is extracted into a regular perf.data via a
>    special 'perf record' call or some other, new perf tooling 
>    solution/variant.
> 
>    [ Alternatively the kernel could attempt to construct a 'virtual'
>      perf.data from the persistent buffer, available via /sys/debug or
>      elsewhere in /sys - just like the kernel constructs a 'virtual' 
>      /proc/kcore, etc. That file could be copied or used directly. ]

Yeah, that.

>  - from that point on this workflow joins the regular profiling workflow: 
>    perf report, perf script et al can be used to analyze the resulting
>    boot profile.

Agreed.

-- 
Regards/Gruss,
    Boris.

Sent from a fat crate under my desk. Formatting is fine.
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