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Date: Fri, 27 Jan 2012 13:09:01 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Stephane Eranian <eranian@...gle.com>
Cc: linux-kernel@...r.kernel.org, mingo@...e.hu, acme@...radead.org,
robert.richter@....com, ming.m.lin@...el.com, andi@...stfloor.org,
asharma@...com, ravitillo@....gov, vweaver1@...s.utk.edu
Subject: Re: [PATCH 00/13] perf_events: add support for sampling taken
branches (v3)
Arnaldo,
On Mon, 2012-01-09 at 17:49 +0100, Stephane Eranian wrote:
> I would like to thank Roberto Vitillo @ LBL for his work on the perf
> tool for this.
>
> Enough talking, let's take a simple example. Our trivial test program
> goes like this:
>
> void f2(void)
> {}
> void f3(void)
> {}
> void f1(unsigned long n)
> {
> if (n & 1UL)
> f2();
> else
> f3();
> }
> int main(void)
> {
> unsigned long i;
>
> for (i=0; i < N; i++)
> f1(i);
> return 0;
> }
>
> $ perf record -b any branchy
> $ perf report -b
> # Events: 23K cycles
> #
> # Overhead Source Symbol Target Symbol
> # ........ ................ ................
>
> 18.13% [.] f1 [.] main
> 18.10% [.] main [.] main
> 18.01% [.] main [.] f1
> 15.69% [.] f1 [.] f1
> 9.11% [.] f3 [.] f1
> 6.78% [.] f1 [.] f3
> 6.74% [.] f1 [.] f2
> 6.71% [.] f2 [.] f1
>
> Of the total number of branches captured, 18.13% were from f1() -> main().
>
> Let's make this clearer by filtering the user call branches only:
>
> $ perf record -b any_call -e cycles:u branchy
> $ perf report -b
> # Events: 19K cycles
> #
> # Overhead Source Symbol Target Symbol
> # ........ ......................... .........................
> #
> 52.50% [.] main [.] f1
> 23.99% [.] f1 [.] f3
> 23.48% [.] f1 [.] f2
> 0.03% [.] _IO_default_xsputn [.] _IO_new_file_overflow
> 0.01% [k] _start [k] __libc_start_main
>
> Now it is more obvious. %52 of all the captured branches where calls from main() -> f1().
> The rest is split 50/50 between f1() -> f2() and f1() -> f3() which is expected given
> that f1() dispatches based on odd vs. even values of n which is constantly increasing.
>
>
> Here is a kernel example, where we want to sample indirect calls:
> $ perf record -a -C 1 -b ind_call -e r1c4:k sleep 10
> $ perf report -b
> #
> # Overhead Source Symbol Target Symbol
> # ........ .......................... ..........................
> #
> 36.36% [k] __delay [k] delay_tsc
> 9.09% [k] ktime_get [k] read_tsc
> 9.09% [k] getnstimeofday [k] read_tsc
> 9.09% [k] notifier_call_chain [k] tick_notify
> 4.55% [k] cpuidle_idle_call [k] intel_idle
> 4.55% [k] cpuidle_idle_call [k] menu_reflect
> 2.27% [k] handle_irq [k] handle_edge_irq
> 2.27% [k] ack_apic_edge [k] native_apic_mem_write
> 2.27% [k] hpet_interrupt_handler [k] hrtimer_interrupt
> 2.27% [k] __run_hrtimer [k] watchdog_timer_fn
> 2.27% [k] enqueue_task [k] enqueue_task_rt
> 2.27% [k] try_to_wake_up [k] select_task_rq_rt
> 2.27% [k] do_timer [k] read_tsc
>
> Due to HW limitations, branch filtering may be approximate on
> Core, Atom processors. It is more accurate on Nehalem, Westmere
> and best on Sandy Bridge.
Can I have you ACK on this userspace stuff (patches 11-13)?
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