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Date: Fri, 24 Jul 2020 12:40:38 +0530
From: Gautham R Shenoy <ego@linux.vnet.ibm.com>
To: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Subject: Re: [PATCH v2 05/10] powerpc/smp: Dont assume l2-cache to be
 superset of sibling
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Cc: Nathan Lynch <nathanl@linux.ibm.com>,
 Gautham R Shenoy <ego@linux.vnet.ibm.com>,
 Oliver OHalloran <oliveroh@au1.ibm.com>, Michael Neuling <mikey@linux.ibm.com>,
 Michael Ellerman <michaele@au1.ibm.com>, Peter Zijlstra <peterz@infradead.org>,
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 LKML <linux-kernel@vger.kernel.org>,
 Valentin Schneider <valentin.schneider@arm.com>,
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 linuxppc-dev <linuxppc-dev@lists.ozlabs.org>, Ingo Molnar <mingo@kernel.org>
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 <linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org>

On Wed, Jul 22, 2020 at 12:27:47PM +0530, Srikar Dronamraju wrote:
> * Gautham R Shenoy <ego@linux.vnet.ibm.com> [2020-07-22 11:51:14]:
> 
> > Hi Srikar,
> > 
> > > diff --git a/arch/powerpc/kernel/smp.c b/arch/powerpc/kernel/smp.c
> > > index 72f16dc0cb26..57468877499a 100644
> > > --- a/arch/powerpc/kernel/smp.c
> > > +++ b/arch/powerpc/kernel/smp.c
> > > @@ -1196,6 +1196,7 @@ static bool update_mask_by_l2(int cpu, struct cpumask *(*mask_fn)(int))
> > >  	if (!l2_cache)
> > >  		return false;
> > > 
> > > +	cpumask_set_cpu(cpu, mask_fn(cpu));
> > 
> > 
> > Ok, we need to do this because "cpu" is not yet set in the
> > cpu_online_mask. Prior to your patch the "cpu" was getting set in
> > cpu_l2_cache_map(cpu) as a side-effect of the code that is removed in
> > the patch.
> > 
> 
> Right.
> 
> > 
> > >  	for_each_cpu(i, cpu_online_mask) {
> > >  		/*
> > >  		 * when updating the marks the current CPU has not been marked
> > > @@ -1278,29 +1279,30 @@ static void add_cpu_to_masks(int cpu)
> > >  	 * add it to it's own thread sibling mask.
> > >  	 */
> > >  	cpumask_set_cpu(cpu, cpu_sibling_mask(cpu));
> > > +	cpumask_set_cpu(cpu, cpu_core_mask(cpu));
> 
> Note: Above, we are explicitly setting the cpu_core_mask.

You are right. I missed this.

> 
> > > 
> > >  	for (i = first_thread; i < first_thread + threads_per_core; i++)
> > >  		if (cpu_online(i))
> > >  			set_cpus_related(i, cpu, cpu_sibling_mask);
> > > 
> > >  	add_cpu_to_smallcore_masks(cpu);
> > > -	/*
> > > -	 * Copy the thread sibling mask into the cache sibling mask
> > > -	 * and mark any CPUs that share an L2 with this CPU.
> > > -	 */
> > > -	for_each_cpu(i, cpu_sibling_mask(cpu))
> > > -		set_cpus_related(cpu, i, cpu_l2_cache_mask);
> > >  	update_mask_by_l2(cpu, cpu_l2_cache_mask);
> > > 
> > > -	/*
> > > -	 * Copy the cache sibling mask into core sibling mask and mark
> > > -	 * any CPUs on the same chip as this CPU.
> > > -	 */
> > > -	for_each_cpu(i, cpu_l2_cache_mask(cpu))
> > > -		set_cpus_related(cpu, i, cpu_core_mask);
> > > +	if (pkg_id == -1) {
> > 
> > I suppose this "if" condition is an optimization, since if pkg_id != -1,
> > we anyway set these CPUs in the cpu_core_mask below.
> > 
> > However...
> 
> This is not just an optimization.
> The hunk removed would only work if cpu_l2_cache_mask is bigger than
> cpu_sibling_mask. (this was the previous assumption that we want to break)
> If the cpu_sibling_mask is bigger than cpu_l2_cache_mask and pkg_id is -1,
> then setting only cpu_l2_cache_mask in cpu_core_mask will result in a broken 
> topology.
> 
> > 
> > > +		struct cpumask *(*mask)(int) = cpu_sibling_mask;
> > > +
> > > +		/*
> > > +		 * Copy the sibling mask into core sibling mask and
> > > +		 * mark any CPUs on the same chip as this CPU.
> > > +		 */
> > > +		if (shared_caches)
> > > +			mask = cpu_l2_cache_mask;
> > > +
> > > +		for_each_cpu(i, mask(cpu))
> > > +			set_cpus_related(cpu, i, cpu_core_mask);
> > > 
> > > -	if (pkg_id == -1)
> > >  		return;
> > > +	}
> > 
> > 
> > ... since "cpu" is not yet set in the cpu_online_mask, do we not miss setting
> > "cpu" in the cpu_core_mask(cpu) in the for-loop below ?
> > 
> > 
> 
> As noted above, we are setting before. So we don't missing the cpu and hence
> have not different from before.


Fair enough.

> 
> > --
> > Thanks and Regards
> > gautham.
> 
> -- 
> Thanks and Regards
> Srikar Dronamraju