On 01/29/2016 10:36 PM, Frederic Weisbecker wrote: > On Sat, Jan 30, 2016 at 12:10:18AM +0100, Peter Zijlstra wrote: >> On Fri, Jan 29, 2016 at 05:22:59PM -0500, r...@redhat.com wrote: >>> From: Rik van Riel <r...@redhat.com> >>> >>> When running a microbenchmark calling an invalid syscall number >>> in a loop, on a nohz_full CPU, we spend a full 9% of our CPU >>> time in __acct_update_integrals. >>> >>> This function converts cputime_t to jiffies, to a timeval, only to >>> convert the timeval back to microseconds before discarding it. >>> >>> This patch leaves __acct_update_integrals functionally equivalent, >>> but speeds things up by about 11%, with 10 million calls to an >>> invalid syscall number dropping from 3.7 to 3.3 seconds. >> >> WTH is this taskstat crap anyway? Who uses it and can't we kill it? > > I have no idea what it's used for, it seems to be related to taskstats > over netlink. I'm not even sure if it's actually used. There don't seem > to be a runtime offcase and I bet distros enable it. So that stuff does > some work every millisecond on millions of machines while it probably > has very few users. > > SGI introduced it in 2006 and it seems that their last contribution there is > in 2008. The rest is kernel maintainance and fixes. > > If there are still users of it, then at least we should disable it on runtime > by > default.
With all the non-power-of-2 divides removed, __acct_update_integrals disappears from the profile, even running many times per millisecond. At that point, native_sched_clock takes over the top of the profile, and the only way I can think of getting rid of that one is making sure it is not called twice for every syscall, irq, and kvm guest entry/exit. -- All rights reversed