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

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Date:   Mon, 7 Dec 2020 18:41:38 +0530
From:   Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
To:     "Gautham R. Shenoy" <ego@...ux.vnet.ibm.com>
Cc:     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

* 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.

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?


-- 
Thanks and Regards
Srikar Dronamraju