linux-kernel - Re: [tip:perfcounters/core] perf_counter: x86: Fix call-chain support to use NMI-safe methods

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Date:	Mon, 15 Jun 2009 22:30:07 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>, mingo@...hat.com,
	hpa@...or.com, paulus@...ba.org, acme@...hat.com,
	linux-kernel@...r.kernel.org, a.p.zijlstra@...llo.nl,
	penberg@...helsinki.fi, vegard.nossum@...il.com, efault@....de,
	jeremy@...p.org, npiggin@...e.de, tglx@...utronix.de,
	linux-tip-commits@...r.kernel.org
Subject: Re: [tip:perfcounters/core] perf_counter: x86: Fix call-chain
	support to use NMI-safe methods


* Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> 
> 
> On Mon, 15 Jun 2009, Ingo Molnar wrote:
> > 
> > Well i guess it depends. For server apps it is true - syscalls are a 
> > lot more dominant, MMs are long-running so any startup cost gets 
> > amortized and pagefaults are avoided.
> > 
> > For something like a kernel build we have 7 times as many pagefaults 
> > as syscalls:
> 
> Ingo - calm down.
> 
> This is not about page faults.
> 
> This is purely about taps FROM KERNEL SPACE.
> 
> Yes, for the kernel build we have 7 times as many page faults as 
> system calls, BUT I BET 99.9% of them are from user mode!
> 
> The whole "open-code iret" only works for exceptions that happened 
> in kernel mode. That's a _vanishingly_ small number (outside of 
> device interrupts that happen during idle).

Ah, i misunderstood you, sorry. Yes, you are right.

Btw., we can have a precise number - we can sample all pagefaults 
via a soft-counter:

$ perf record -e page-faults -c 1 -f -m 512 -- make -j32

 [ perf record: Captured and wrote 783.697 MB perf.data (~34240256 samples) ]

#
# (33858154 samples)
#
# Overhead  Symbol
# ........  ......
#
    41.62%  [.] memset
    18.21%  0x007fff07bff634
     5.85%  [.] __GI_memcpy
     5.35%  [.] parse_config_file
     4.34%  [.] _int_malloc
     1.24%  0x0000000000dc90
     1.14%  [k] file_read_actor
     1.10%  [.] _dl_addr
     0.64%  [.] __GI_strlen
     0.47%  [.] _dl_load_cache_lookup
     0.43%  [.] _dl_cache_libcmp

(i dont have debuginfo installed for that gcc build, hence that raw 
entry.)

Kernel triggered pagefaults:

 $ perf report -s s | grep '\[k\]'

     1.14%  [k] file_read_actor
     0.15%  [k] __clear_user
     0.12%  [k] strncpy_from_user
     0.12%  [k] copy_user_generic_string
     0.04%  [k] __put_user_4
     0.04%  [k] pipe_read
     0.02%  [k] load_elf_binary
     0.00%  [k] do_notify_resume
     0.00%  [k] iov_iter_fault_in_readable

so 1.7% of the pagefaults are kernel-initiated - 98.3% via 
user-space. Your 99.9% figure was within 1.6% of the real number :-)

	Ingo
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