> > > Other approaches under consideration include making CONFIG_PREEMPT_COUNT > > > unconditional and thus allowing call_rcu() and kvfree_rcu() to determine > > > whether direct calls to the allocator are safe (some guy named Linus > > > doesn't like this one), > > > > I assume that the primary argument is the overhead, right? Do you happen > > to have any reference? > > Jon Corbet wrote a very nice article summarizing the current situation: > https://lwn.net/Articles/831678/. Thomas's measurements show no visible > system-level performance impact. I will let Uladzislau present his more > microbenchmarky performance work. > I have done some analysis of the !PREEMPT kernel with and without PREEMPT_COUNT configuration. The aim is to show a performance impact if the PREEMPT_COUNT is unconditionally enabled. As for the test i used the refscale kernel module, that does: <snip> static void ref_rcu_read_section(const int nloops) { int i; for (i = nloops; i >= 0; i--) { rcu_read_lock(); rcu_read_unlock(); } } <snip> How to run the microbenchmark: <snip> urezki@pc638:~$ sudo modprobe refscale <snip> The below is an average duration per loop (nanoseconds): !PREEMPT_COUNT PREEMPT_COUNT Runs Time(ns) Runc Time(ns) 1 109.640 1 99.915 2 102.303 2 111.106 3 90.520 3 98.713 4 106.347 4 111.239 5 108.374 5 111.797 6 108.012 6 111.558 7 103.989 7 113.122 8 106.194 8 111.515 9 107.330 9 107.559 10 105.877 10 105.965 11 104.860 11 104.835 12 104.299 12 106.342 13 104.794 13 106.664 14 104.916 14 104.914 15 105.485 15 104.280 16 104.610 16 105.642 17 104.981 17 105.646 18 103.089 18 106.370 19 105.251 19 105.284 20 104.133 20 105.973 21 105.589 21 105.271 22 104.154 22 106.063 23 104.963 23 106.248 24 102.431 24 105.568 25 102.610 25 105.556 26 103.474 26 105.655 27 100.194 27 102.887 28 102.340 28 104.347 29 102.075 29 102.389 30 102.808 30 103.123 The difference is ~1.8% in average. The maximum value is 109.640 vs 113.122 The minimum value is 90.520 vs 98.713. Tested on: processor : 63 vendor_id : AuthenticAMD cpu family : 6 model : 6 model name : QEMU Virtual CPU version 2.5+ cpu MHz : 3700.204 I also can do more detailed testing using "perf" tool. -- Vlad Rezki
