Re: [patch 12/12] x86/cacheinfo.c: check for block interference CPUs

[Marcelo Tosatti]

On Wed, Feb 07, 2024 at 01:41:36PM +0100, Thomas Gleixner wrote:
> On Tue, Feb 06 2024 at 15:49, Marcelo Tosatti wrote:
> > @@ -396,6 +397,7 @@ static void amd_l3_disable_index(struct
> >  	 *  disable index in all 4 subcaches
> >  	 */
> >  	for (i = 0; i < 4; i++) {
> > +		int ret;
> >  		u32 reg = idx | (i << 20);
> >  
> >  		if (!nb->l3_cache.subcaches[i])
> > @@ -409,6 +411,7 @@ static void amd_l3_disable_index(struct
> >  		 * is not sufficient.
> >  		 */
> >  		ret = wbinvd_on_cpu(cpu);
> > +		WARN_ON(ret == -EPERM);
> 
> What? You create inconsistent state here.

That should not happen, since we checked for 

+   idx = block_interf_srcu_read_lock();
+
+   if (block_interf_cpu(cpu))
+           ret = -EPERM;

Earlier.

Thus the WARN_ON (hum, can change to BUG_ON...).

> > -	amd_l3_disable_index(nb, cpu, slot, index);
> > +	ret = 0;
> > +	idx = block_interf_srcu_read_lock();
> > +
> > +	if (block_interf_cpu(cpu))
> > +		ret = -EPERM;
> > +	else
> > +		amd_l3_disable_index(nb, cpu, slot, index);
> > +
> > +	block_interf_srcu_read_unlock(idx);
> 
> Again. This is a root only operation.
> 
> This whole patch series is just voodoo programming with zero
> justification for the mess it creates.
> 
> Thanks,
> 
>         tglx

Its not really voodoo programming: its simply returning errors to
userspace if a CPU is set in a particular cpumask.

Do you have a better idea? (i am in the process of converting more
users).
Can improve on the patchset quality.

Ok, the justification is as follows. Today, there is a reactive approach
to interruptions to isolated CPUs:

1) Run Linux+latency sensitive application on isolated CPU.

2) Wait for IPI or other interruption to happen on that isolated CPU,
which breaks the application.

3) Remove that interruption to the isolated CPU.

This is (for a class of IPIs), an active approach, where those IPIs are 
not permitted to interrupt the latency sensitive applications.

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