On Sat, Aug 13, 2016 at 11:45 AM, Ingo Molnar <mi...@kernel.org> wrote:
>
> * Brian Gerst <brge...@gmail.com> wrote:
>
>> On Sat, Aug 13, 2016 at 1:16 PM, Linus Torvalds
>> <torva...@linux-foundation.org> wrote:
>> > On Sat, Aug 13, 2016 at 9:38 AM, Brian Gerst <brge...@gmail.com> wrote:
>> >> This patch set simplifies the switch_to() code, by moving the stack switch
>> >> code out of line into an asm stub before calling __switch_to(). This ends
>> >> up being more readable, and using the C calling convention instead of
>> >> clobbering all registers improves code generation. It also allows newly
>> >> forked processes to construct a special stack frame to seamlessly flow
>> >> to ret_from_fork, instead of using a test and branch, or an unbalanced
>> >> call/ret.
>> >
>> > Do you have performance numbers? Is it noticeable/measurable?
>>
>> How do I measure it? The perf documentation isn't easy to understand.
>
> Something like this:
>
> taskset 1 perf stat -a -e '{instructions,cycles}' --repeat 10 perf bench
> sched pipe
>
> ... will give a very good idea about the general impact of these changes on
> context switch overhead.
>
I will be quite surprised if you can measure any effect at all. I've
never seen context switches take fewer than ~2k cycles, and on my
laptop, they take 8k-9k cycles. The scheduler is really, really slow.
(Why doesn't that perf command show cycles per context switch?)
--Andy
