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

[Qais Yousef]

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.