Re: [tip:perfcounters/core] perf_counter: x86: Fix call-chainsupport to use NMI-safe methods

[Ingo Molnar]

* Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:

> On Mon, 15 Jun 2009, Ingo Molnar wrote:
> > 
> > See the numbers in the other mail: about 33 million pagefaults 
> > happen in a typical kernel build - that's ~400K/sec - and that 
> > is not a particularly really pagefault-heavy workload.
> 
> Did you do any function-level profiles?
> 
> Last I looked at it, the real cost of page faults were all in the 
> memory copies and page clearing, and while it would be nice to 
> speed up the kernel entry and exit, the few tens of cycles we 
> might be able to get from there really aren't all that important.

Yeah.

Here's the function level profiles of a typical kernel build on a 
Nehalem box:

 $ perf report --sort symbol

 #
 # (14317328 samples)
 #
 # Overhead  Symbol
 # ........  ......
 #
    44.05%  0x000000001a0b80
     5.09%  0x0000000001d298
     3.56%  0x0000000005742c
     2.48%  0x0000000014026d
     2.31%  0x00000000007b1a
     2.06%  0x00000000115ac9
     1.83%  [.] _int_malloc
     1.71%  0x00000000064680
     1.50%  [.] memset
     1.37%  0x00000000125d88
     1.28%  0x000000000b7642
     1.17%  [k] clear_page_c
     0.87%  [k] page_fault
     0.78%  [.] is_defined_config
     0.71%  [.] _int_free
     0.68%  [.] __GI_strlen
     0.66%  0x000000000699e8
     0.54%  [.] __GI_memcpy

Most is dominated by user-space symbols. (no proper ELF+debuginfo on 
this box so they are unnamed.) It also sows that page clearing and 
pagefault handling dominates the kernel overhead - but is dwarved by 
other overhead. Any page-fault-entry costs are a drop in the bucket.

In fact with call-chain graphs we can get a precise picture, as we 
can do a non-linear 'slice' set operation over the samples and 
filter out the ones that have the 'page_fault' pattern in one of 
their parent functions:

 $ perf report --sort symbol --parent page_fault

 #
 # (14317328 samples)
 #
 # Overhead  Symbol
 # ........  ......
 #
     1.12%  [k] clear_page_c
     0.87%  [k] page_fault
     0.43%  [k] get_page_from_freelist
     0.25%  [k] _spin_lock
     0.24%  [k] do_page_fault
     0.23%  [k] perf_swcounter_ctx_event
     0.16%  [k] perf_swcounter_event
     0.15%  [k] handle_mm_fault
     0.15%  [k] __alloc_pages_nodemask
     0.14%  [k] __rmqueue
     0.12%  [k] find_get_page
     0.11%  [k] copy_page_c
     0.11%  [k] find_vma
     0.10%  [k] _spin_lock_irqsave
     0.10%  [k] __wake_up_bit
     0.09%  [k] _spin_unlock_irqrestore
     0.09%  [k] do_anonymous_page
     0.09%  [k] __inc_zone_state

This "sub-profile" shows the true summary overhead that 'page_fault' 
and all its child functions have. Note that for example clear_page_c 
decreased from 1.17% to 1.12%:

     1.12%  [k] clear_page_c
     1.17%  [k] clear_page_c

because there's 0.05% of other callers to clear_page_c() that do not 
involve page_fault. Those are filtered out via --parent 
filtering/matching.

	Ingo
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