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. 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. > > 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. > > 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.
