Re: [PATCH v6 5/9] x86/sysctl: Add sysctl for ITMT scheduling feature

[Thomas Gleixner]

On Wed, 26 Oct 2016, Peter Zijlstra wrote:
> On Wed, Oct 26, 2016 at 12:49:36PM +0200, Thomas Gleixner wrote:
> 
> > > +	/*
> > > +	 * ITMT capability automatically enables ITMT
> > > +	 * scheduling for small systems (single node).
> > > +	 */
> > > +	if (topology_num_packages() == 1)
> > > +		sysctl_sched_itmt_enabled = 1;
> > 
> > I really hate this. This is policy and the kernel should not impose
> > policy. Why would I like to have this enforced on my single socket XEON
> > server?
> 
> So this really wants to be enabled by default; otherwise nobody will use
> this, and it really does help single threaded workloads.

Fair enough. Then this wants to be documented.
 
> There were reservations on the multi-socket case of ITMT, maybe it would
> help to spell those out in great detail here. That is, have the comment
> explain the policy instead of simply stating what the code does (which
> is always bad comment policy, you can read the code just fine).

What is the objection for multi sockets? If it improves the behaviour then
why would this be a bad thing for multi sockets?

Thanks,

	tglx