linux-kernel - Re: [RFC 4/5] x86, perf: implements lwp-perf-integration (rc1)

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Date:	Mon, 19 Dec 2011 19:13:22 +0100
From:	Benjamin <bebl@...eta.org>
To:	Avi Kivity <avi@...hat.com>
CC:	Ingo Molnar <mingo@...e.hu>,
	Robert Richter <robert.richter@....com>,
	Hans Rosenfeld <hans.rosenfeld@....com>, hpa@...or.com,
	tglx@...utronix.de, suresh.b.siddha@...el.com, eranian@...gle.com,
	brgerst@...il.com, Andreas.Herrmann3@....com, x86@...nel.org,
	linux-kernel@...r.kernel.org,
	Benjamin Block <benjamin.block@....com>
Subject: Re: [RFC 4/5] x86, perf: implements lwp-perf-integration (rc1)

Am 19.12.2011 12:58, schrieb Avi Kivity:
>> I'd suggest the easy hack first, to get things going - we can
>> then help out with the proper solution.
> I think you're underestimating the complexity there.  LWP wasn't
> designed for this.
>

LWP is highly limited in its ability's to support more than one
"LWP-Instance" being active for a thread, IOW it is not possible.
You can't activate LWP from a threads context and simultaneously
activate lwp-system-wide-profiling in the way you suggested it,
Ingo. Either do the first xor do the last, because you only have
one xsave-area/msr/lwpcb that is read by the hardware and only one
LWP-Buffer that is written by the hw.

So, if one thread is running LWP, because he wants to
(selfmonitoring and stuff [like for what lwp was designed]) and a
su or u would activate this system-wide-monitoring, both would
frequently interfere with the each other. I don't think you want
this to be possible at all.

Frankly, it was already a pain to get LWP running from in-kernel,
like it is done now. I would expect a much higher pain, if you
would want to do this with a transparent buffer, that gets passed
around each scheduling (and this would permanently eliminate the
"lightweight" in "LWP").

best regards,
- Benjamin
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