Re: [PATCH 3/3] context_tracking,x86: remove extraneous irq disable & enable from context tracking on syscall entry

[Rik van Riel]

On 05/01/2015 02:40 AM, Ingo Molnar wrote:

>> This patch builds on top of these patches by Paolo:
>> https://lkml.org/lkml/2015/4/28/188
>> https://lkml.org/lkml/2015/4/29/139
>>
>> Together with this patch I posted earlier this week, the syscall path
>> on a nohz_full cpu seems to be about 10% faster.
>> https://lkml.org/lkml/2015/4/24/394
>>
>> My test is a simple microbenchmark that calls getpriority() in a loop
>> 10 million times:
>>
>> 		run time	system time
>> vanilla		5.49s		2.08s
>> __acct patch	5.21s		1.92s
>> both patches	4.88s		1.71s
>
> Just curious, what are the numbers if you don't have context tracking 
> enabled, i.e. without nohz_full?
> 
> I.e. what's the baseline we are talking about?

It's an astounding difference. This is not a kernel without nohz_full,
just a CPU without nohz_full running the same kernel I tested with
yesterday:

 		run time	system time
vanilla		5.49s		2.08s
__acct patch	5.21s		1.92s
both patches	4.88s		1.71s
CPU w/o nohz	3.12s		1.63s    <-- your numbers, mostly

What is even more interesting is that the majority of the time
difference seems to come from _user_ time, which has gone down
from around 3.4 seconds in the vanilla kernel to around 1.5 seconds
on the CPU without nohz_full enabled...

At syscall entry time, the nohz_full context tracking code is very
straightforward. We check thread_info->flags & _TIF_WORK_SYSCALL_ENTRY,
and call syscall_trace_enter_phase1, which handles USER -> KERNEL
context transition.

Syscall exit time is a convoluted mess. Both do_notify_resume and
syscall_trace_leave call exit_user() on entry and enter_user()
on exit, leaving the time spent looping around between int_with_check
and syscall_return: in entry_64.S accounted as user time.

I sent an email about this last night, it may be useful to add a
third test & function call point to the syscall return code, where
we can call user_enter() just ONCE, and remove the other context
tracking calls from that loop.

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