On 05/28/20 20:29, Peter Zijlstra wrote: > On Thu, May 28, 2020 at 05:51:31PM +0100, Qais Yousef wrote: > > > In my head, the simpler version of > > > > if (rt_task(p) && !uc->user_defined) > > // update_uclamp_min > > > > Is a single branch and write to cache, so should be fast. I'm failing to see > > how this could generate an overhead tbh, but will not argue about it :-) > > Mostly true; but you also had a load of that sysctl in there, which is > likely to be a miss, and those are expensive. Hmm yes there's no guarantee the sysctl global variable will be in LLC, though I thought that would be the likely case. > > Also; if we're going to have to optimize this, less logic is in there, > the less we need to take out. Esp. for stuff that 'never' changes, like > this. Agreed. > > > > It's more code, but it is all outside of the normal paths where we care > > > about performance. > > > > I am happy to take that direction if you think it's worth it. I'm thinking > > task_woken_rt() is good. But again, maybe I am missing something. > > Basic rule, if the state 'never' changes, don't touch fast paths. > > Such little things can be very difficult to measure, but at some point > they cause death-by-a-thousnd-cuts. Yeah we're bound to reach the critical mass at some point if too much bloat creeps up on the hot path. Thanks -- Qais Yousef > > > > Indeed, that one. The fact that regular distros cannot enable this > > > feature due to performance overhead is unfortunate. It means there is a > > > lot less potential for this stuff. > > > > I had a humble try to catch the overhead but wasn't successful. The observation > > wasn't missed by us too then. > > Right, I remember us doing benchmarks when we introduced all this and > clearly we missed something. I would be good if Mel can share which > benchmark hurt most so we can go have a look.
