[PATCH] perf tooling: Add 'perf bench syscall' benchmark

[Ingo Molnar]

* riel@xxxxxxxxxx <riel@xxxxxxxxxx> wrote:

> (v3: address comments raised by Frederic)
> 
> Running with nohz_full introduces a fair amount of overhead.
> Specifically, various things that are usually done from the
> timer interrupt are now done at syscall, irq, and guest
> entry and exit times.
> 
> However, some of the code that is called every single time
> has only ever worked at jiffy resolution. The code in
> __acct_update_integrals was also doing some unnecessary
> calculations.
> 
> Getting rid of the unnecessary calculations, without
> changing any of the functionality in __acct_update_integrals
> gets us about an 11% win.
> 
> Not calling the time statistics updating code more than
> once per jiffy, like is done on housekeeping CPUs and on
> all the CPUs of a non-nohz_full system, shaves off a
> further 30%.
> 
> I tested this series with a microbenchmark calling
> an invalid syscall number ten million times in a row,
> on a nohz_full cpu.
> 
>     Run times for the microbenchmark:
>     
> 4.4				3.8 seconds
> 4.5-rc1				3.7 seconds
> 4.5-rc1 + first patch		3.3 seconds
> 4.5-rc1 + first 3 patches	3.1 seconds
> 4.5-rc1 + all patches		2.3 seconds

Another suggestion (beyond fixing the 32-bit build ;-), could you please stick 
your syscall microbenchmark into 'perf bench', so that we have a standardized way 
of checking such numbers?

In fact I'd suggest we introduce an entirely new sub-tool for system call 
performance measurement - and this might be the first functionality of it.

I've attached a quick patch that is basically a copy of 'perf bench numa' and 
which measures getppid() performance (simple syscall where the result is not 
cached by glibc).

I kept the process, threading and memory allocation bits of numa.c, just in case 
we need them to measure more complex syscalls. Maybe we could keep the threading 
bits and remove the memory allocation parameters, to simplify the benchmark?

Anyway, this could be a good base to start off on.

Thanks,

	Ingo

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