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

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Date:	Tue, 20 Dec 2011 09:56:14 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Benjamin <bebl@...eta.org>
Cc:	Avi Kivity <avi@...hat.com>,
	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)

* Benjamin <bebl@...eta.org> wrote:

> 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.

That's OK, we can deal with various PMU constraints just fine.

> 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,

We have other PMU resources that are exclusive in that sense.

> 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.

That's similar to PEBS (which we already support), there's only 
one Debug Store per CPU, obviously.

> 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.

THe LWPCB is designed to allow multiple events, and the LWP 
ring-buffer is shared between these events.

If the kernel properly manages the lwpcb then no such 
'interference' happens during normal use - both outside and 
self-installed events can be activated at once, up to the event 
limit - similar to how we handle regular PMU events.

[ This is why the threshold IRQ support i requested is key: it 
  is needed for the flow of events and for the kernel 
  event-demultiplexer to work transparently. ]

> 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").

There's no heavyweight 'passing around' of a buffer needed at 
context switch time. The buffer context has to be flipped - part 
of the job of context switching.

So no, i don't think any of your objections have any merit.

Thanks,

	Ingo
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