linux-kernel - Re: [RFC Patch 0/4] Enhance perf-events to profile memory accesses using hw-breakpoints

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Date:	Thu, 29 Oct 2009 09:19:17 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	"K.Prasad" <prasad@...ux.vnet.ibm.com>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Alan Stern <stern@...land.harvard.edu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Paul Mackerras <paulus@...ba.org>
Subject: Re: [RFC Patch 0/4] Enhance perf-events to profile memory accesses
	using hw-breakpoints - ver II


* K.Prasad <prasad@...ux.vnet.ibm.com> wrote:

> Hi All,
> 	Please find version II of the patchset that enables perf-events to
> place hw-breakpoints over kernel symbols (along with requisite enhancements to
> the hw-breakpoint layer).
> 
> Changelog version II
> ---------------------
> Version I: http://lkml.org/lkml/2009/10/26/461
> 
> - Fixed parsing issues that disallowed other perf events to be invoked
> - Fixed user-space breakpoint usage which was broken due to patch 2/4
> - Introduced an instance of perf_sample_data for use by do_perf_sw_event()
> 
> An edited log of 'perf stat' and 'perf record' output is shown below for your
> reference.
> 
> Kindly let me know your suggestions/feedback about the same.
> 
> Thanks,
> K.Prasad
> 
> Screen logs
> ------------
> # perf stat -v -i -e breakpoint-readwrite:pid_max -e breakpoint-write:jiffies make kernel/futex.o
>   CHK     include/linux/version.h
>   CHK     include/linux/utsrelease.h
>   SYMLINK include/asm -> include/asm-x86
>   CALL    scripts/checksyscalls.sh
>   CC      kernel/futex.o
> breakpoint-readwrite: 68 298512531 298512531
> breakpoint-write: 235 298512531 298512531
> 
>  Performance counter stats for 'make kernel/futex.o':
> 
>              68  breakpoint-readwrite     #      0.000 M/sec
>             235  breakpoint-write         #      0.000 M/sec
> 
>    14.571235288  seconds time elapsed
> 
> #
> #
> # perf record -v -i -e breakpoint-readwrite:jiffies top
> 
> [Ran 'top' for about 10 seconds]

btw., you probably want to add the -a/--all option as well when you test 
via top, to do system-wide profiling. With this command you profile top 
itself (and its child tasks).

> 
> # perf report -i perf.data
> # Samples: 2022950155
> #
> # Overhead  Command  Shared Object  Symbol
> # ........  .......  .............  ......
> #
>     99.99%      top  [kernel]       [k] scheduler_tick
>      0.01%     perf  [kernel]       [k] scheduler_tick
>      0.00%      top  [kernel]       [k] set_track
>      0.00%      top  [kernel]       [k] run_timer_softirq
>      0.00%     perf  [kernel]       [k] set_track
>      0.00%      top  [kernel]       [k] __call_rcu
>      0.00%      top  [kernel]       [k] calc_global_load
>      0.00%      top  [kernel]       [k] do_timer
>      0.00%      top  [kernel]       [k] __rcu_process_callbacks
> #
> # (For a higher level overview, try: perf report --sort comm,dso)
> #

That output looks pretty awesome! This way we can map out how frequently 
global variables are used in the kernel - in stock distro kernels too. 
Previously we could only measure it indirectly (by looking at 
high-overhead functions and assembly level annotations), or by running 
very costly instrumentation like Valgrind.

I like it how you extended --event with the breakpoint-readwrite:jiffies 
method as well.

A few additional shortcuts/aliases would be nice, such as:

   perf record -v -i -e readwrite:jiffies top

as breakpoint-readwrite is pretty log users arent really interested in 
the mechanism (hardware-breakpoints), they are more interested that it's 
memory read-write profiling done at a given address.

Maybe even 'rw' would be a useful alias as well. There are alias tables 
for events which you can use for this. You can define them via:

  { CHBP(WRITE),               "memory-write",     "write",     "w"  },
  { CHBP(RW),                  "memory-readwrite", "readwrite", "rw" },

Anyway, this looks very good already - Frederic, if you like these 
patches too feel free to send it to me in your next hw-breakpoints pull 
request.

Thanks,

	Ingo
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