Messages in this thread | | | Date | Fri, 01 May 2015 11:20:49 -0400 | From | Rik van Riel <> | Subject | Re: [PATCH 3/3] context_tracking,x86: remove extraneous irq disable & enable from context tracking on syscall entry |
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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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