Re: [patch 1/2] x86_64 page fault NMI-safe

[Ingo Molnar <mingo@elte.hu>]

* Peter Zijlstra <peterz@infradead.org> wrote:

> > What I am proposing does not even involve a copy: when we want to take a 
> > snapshot, we just have to force a sub-buffer switch on the ring buffer. 
> > The "returns" happening at the beginning of the next (empty) sub-buffer 
> > would clearly fail to discard records (expecting non-existing entry 
> > records). We would then have to save a small record saying that a function 
> > return occurred. The current stack frame at the end of the next sub-buffer 
> > could be deduced from the complete collection of stack frame samples.
> 
> And suppose the stack-trace was all of 16 entries (not uncommon for a kernel 
> stack), then you waste a whole page for 128 bytes (assuming your sub-buffer 
> is page sized). I'll take the memcopy, thank you.

To throw some hard numbers into the discussion, i found two random callgraph 
perf.data's on my boxes (both created prior the start of this discussion) and 
here is the distribution of their call-chain length:

aldebaran:~> perf report -D | grep 'chain: nr:' | cut -d: -f3- | sort -n | uniq -c
      2 4
     21 6
     23 8
     13 9
     20 10
     29 11
     21 12
     25 13
     54 14
    112 15
     72 16
     77 17
     35 18
     38 19
     48 20
     29 21
     10 22
     97 23
      3 24
      1 25
      2 26
      2 28
      2 29
      1 30
      2 31

So the peak/average here is around 15 entries.

The other one:

phoenix:~> perf report -D | grep 'chain: nr:' | cut -d: -f3- | sort -n | uniq -c
      1 2
     70 3
    222 4
    112 5
    116 6
    329 7
    241 8
    163 9
    203 10
    287 11
    159 12
      4 13
      6 14
     22 15
      2 16
     11 17
      5 18

Here the average is even lower - around 8 entries.

Thanks,

	Ingo