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

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Message-ID: <4EEF1C3B.3010307@redhat.com>
Date:	Mon, 19 Dec 2011 13:12:59 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Robert Richter <robert.richter@....com>,
	Benjamin Block <bebl@...eta.org>,
	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)

On 12/19/2011 12:54 PM, Ingo Molnar wrote:
> * Robert Richter <robert.richter@....com> wrote:
>
> > On 19.12.11 00:43:10, Ingo Molnar wrote:
> >
> > > So the question becomes, how well is it integrated: can perf 
> > > 'record -a + perf report', or 'perf top' use LWP, to do 
> > > system-wide precise [user-space] profiling and such?
> > 
> > There is only self-monitoring of a process possible, no kernel 
> > and system-wide profiling. This is because we can not allocate 
> > memory regions in the kernel for a thread other than the 
> > current. This would require a complete rework of mm code.
>
> Hm, i don't think a rework is needed: check the 
> vmalloc_to_page() code in kernel/events/ring_buffer.c. Right now 
> CONFIG_PERF_USE_VMALLOC is an ARM, MIPS, SH and Sparc specific 
> feature, on x86 it turns on if CONFIG_DEBUG_PERF_USE_VMALLOC=y.
>
> That should be good enough for prototyping the kernel/user 
> shared buffering approach.
>

LWP wants user memory, vmalloc is insufficient.  You need do_mmap() with
a different mm.

You could let a workqueue call use_mm() and then do_mmap().  Even then
it is subject to disruption by the monitored thread (and may disrupt the
monitored thread by playing with its address space).  This is for thread
monitoring only, I don't think system-wide monitoring is possible with LWP.

-- 
error compiling committee.c: too many arguments to function

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