Re: [PATCH] sparc: perf: Add support M7 processor

[David Ahern <david.ahern@oracle.com>]

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On 4/22/15 5:25 PM, David Miller wrote:
> From: David Ahern <david.ahern@oracle.com>
> Date: Wed, 22 Apr 2015 17:19:23 -0600
>
>> Only thing left in my queue is optimized versions of the ffs / fls
>> families, but that patch is v9b specific, not M7.
>
> Something faster than the popc thing in arch/sparc/lib/ffs.S?

hmmm... i saw that, but wasn't clear 1) how it got inserted and 2) the 
overhead of a function call versus inline. Anyways, what I have is the 
same 3 instructions as an inline. But really the __ffs was just along 
for the ride; the focus was on __fls.

>
> Are you thinking of using "lzcnt"?  I wasn't impressed with the
> performance of that instruction last time I played around with it.

A comparison of what I hacked together is attached (columns too wide for 
inline). Data is from a T4-2. It shows lzcnt to be better for __fls, fls 
and fl64.


>
>> I'd like to put some attention on precise mode for perf counters; it
>> just has not bubbled to the top.
>
> That plus the backtrace deadlock thing we're discussing in another
> thread, that bug is irritating because your pagefault_disable() change
> should "just work".
>

oh, yes. forgot about that one. I spent too many hours trying to figure 
out why processes get killed with a sigbus. I added an option to perf 
tool to skip userspace chains until I can get back to it.



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- "slow" means version from asm-generic.
- Times are in nsec.
- 'bit' column shown to ensure correct answer between current and lzcnt
- average of 10 back-to-back calls


                 |        __fls        |         fls         |      fls64
            word |  lzcnt       slow   |  lzcnt       slow   |  lzcnt     slow
                 | bit   dt   bit   dt | bit   dt   bit   dt | bit   dt   bit   dt
               0 |   0   15     0   67 |   0   19     0   21 |   0   14     0   14
               1 |   0   13     0   50 |   1   32     1   61 |   1   20     1   51
        80000000 |  31   13    31   39 |  32   30    32   49 |  64   25    64   37
8000000000000000 |  63   13    63   34 |   0   17     0   16 |   0   12     0   14