linux-kernel - Re: [PATCH 3/3] powerpc/cacheinfo: Print correct cache-sibling map/list for L2 cache

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Message-ID: <20201208175647.GC14206@in.ibm.com>
Date:   Tue, 8 Dec 2020 23:26:47 +0530
From:   Gautham R Shenoy <ego@...ux.vnet.ibm.com>
To:     Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Cc:     "Gautham R. Shenoy" <ego@...ux.vnet.ibm.com>,
        Anton Blanchard <anton@...abs.org>,
        Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Michael Neuling <mikey@...ling.org>,
        Nicholas Piggin <npiggin@...il.com>,
        Nathan Lynch <nathanl@...ux.ibm.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Valentin Schneider <valentin.schneider@....com>,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] powerpc/cacheinfo: Print correct cache-sibling
 map/list for L2 cache

On Mon, Dec 07, 2020 at 06:41:38PM +0530, Srikar Dronamraju wrote:
> * Gautham R. Shenoy <ego@...ux.vnet.ibm.com> [2020-12-04 10:18:47]:
> 
> > From: "Gautham R. Shenoy" <ego@...ux.vnet.ibm.com>
> > 
> > 
> > Signed-off-by: Gautham R. Shenoy <ego@...ux.vnet.ibm.com>
> > ---
> > 
> > +extern bool thread_group_shares_l2;
> >  /*
> >   * On big-core systems, each core has two groups of CPUs each of which
> >   * has its own L1-cache. The thread-siblings which share l1-cache with
> >   * @cpu can be obtained via cpu_smallcore_mask().
> > + *
> > + * On some big-core systems, the L2 cache is shared only between some
> > + * groups of siblings. This is already parsed and encoded in
> > + * cpu_l2_cache_mask().
> >   */
> >  static const struct cpumask *get_big_core_shared_cpu_map(int cpu, struct cache *cache)
> >  {
> >  	if (cache->level == 1)
> >  		return cpu_smallcore_mask(cpu);
> > +	if (cache->level == 2 && thread_group_shares_l2)
> > +		return cpu_l2_cache_mask(cpu);
> > 
> >  	return &cache->shared_cpu_map;
> 
> As pointed with lkp@...el.org, we need to do this only with #CONFIG_SMP,
> even for cache->level = 1 too.

Yes, I have fixed that in the next version.

> 
> I agree that we are displaying shared_cpu_map correctly. Should we have also
> update /clear shared_cpu_map in the first place. For example:- If for a P9
> core with CPUs 0-7, the cache->shared_cpu_map for L1 would have 0-7 but
> would display 0,2,4,6.
> 
> The drawback of this is even if cpus 0,2,4,6 are released L1 cache will not
> be released. Is this as expected?

cacheinfo populates the cache->shared_cpu_map on the basis of which
CPUs share the common device-tree node for a particular cache.  There
is one l1-cache object in the device-tree for a CPU node corresponding
to a big-core. That the L1 is further split between the threads of the
core is shown using ibm,thread-groups.

The ideal thing would be to add a "group_leader" field to "struct
cache" so that we can create separate cache objects , one per thread
group. I will take a stab at this in the v2.

Thanks for the review comments.



> 
> 
> -- 
> Thanks and Regards
> Srikar Dronamraju