Re: [PATCH 1/2] sched/uclamp: Add a new sysctl to control RT default boost value

[Peter Zijlstra]

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.