linux-kernel - Re: [PATCH v5 5/9] x86/sysctl: Add sysctl for ITMT scheduling feature

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Message-ID: <alpine.DEB.2.20.1610061307030.22338@nanos>
Date:   Thu, 6 Oct 2016 13:13:08 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Tim Chen <tim.c.chen@...ux.intel.com>
cc:     Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        rjw@...ysocki.net, mingo@...hat.com, bp@...e.de, x86@...nel.org,
        linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-acpi@...r.kernel.org, peterz@...radead.org, jolsa@...hat.com
Subject: Re: [PATCH v5 5/9] x86/sysctl: Add sysctl for ITMT scheduling
 feature

On Wed, 5 Oct 2016, Tim Chen wrote:
> On Wed, 2016-10-05 at 16:35 +0200, Thomas Gleixner wrote:
> > > +	if (itmt_supported) {
> > > +		itmt_sysctl_header =
> > > +			register_sysctl_table(itmt_root_table);
> > > +		if (!itmt_sysctl_header) {
> > > +			mutex_unlock(&itmt_update_mutex);
> > > +			return;
> > So you now have a state of capable which cannot be enabled. Whats the
> > point?
> 
> For multi-socket system where ITMT is not enabled by default, the operator
> can still decide to enable it via sysctl.

With a sysctl which failed to be installed. Good luck with that.
 
> > > +		}
> > > +		/*
> > > +		 * ITMT capability automatically enables ITMT
> > > +		 * scheduling for small systems (single node).
> > > +		 */
> > > +		if (topology_num_packages() == 1)
> > > +			sysctl_sched_itmt_enabled = 1;
> > > +	} else {
> > > +		if (itmt_sysctl_header)
> > > +			unregister_sysctl_table(itmt_sysctl_header);
> > > +	}
> > > +
> > > +	if (sysctl_sched_itmt_enabled) {
> > > +		/* disable sched_itmt if we are no longer ITMT capable */
> > > +		if (!itmt_supported)
> > 
> > How do you get here if itmt is not supported? 
> 
> If the OS decides to turn off ITMT for any reason, (i.e. invoke 
> sched_set_itmt_support(false) after it has turned on itmt_support
> before), this is the logic to do it.  We don't turn off ITMT support
> after it has been turned on today, in the future the OS may.

Then please make this two functions (set/clear) so one can actually follow
the logic. The above is just too convoluted.

Thanks,

	tglx