linux-kernel - Re: [PATCH v2 1/3] cacheinfo: Check sib_leaf in cache_leaves_are

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Message-ID: <20230412-viewpoint-refutable-a31f3657093c@wendy>
Date:   Wed, 12 Apr 2023 12:27:20 +0100
From:   Conor Dooley <conor.dooley@...rochip.com>
To:     Pierre Gondois <pierre.gondois@....com>
CC:     <linux-kernel@...r.kernel.org>, Radu Rendec <rrendec@...hat.com>,
        Alexandre Ghiti <alexghiti@...osinc.com>,
        Will Deacon <will@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Sudeep Holla <sudeep.holla@....com>,
        Palmer Dabbelt <palmer@...osinc.com>,
        Gavin Shan <gshan@...hat.com>,
        Jeremy Linton <jeremy.linton@....com>
Subject: Re: [PATCH v2 1/3] cacheinfo: Check sib_leaf in
 cache_leaves_are_shared()

On Wed, Apr 12, 2023 at 09:18:04AM +0200, Pierre Gondois wrote:
> If 'this_leaf' is a L2 cache (or higher) and 'sib_leaf' is a L1 cache,
> the caches are detected as shared. Indeed, cache_leaves_are_shared()
> only checks the cache level of 'this_leaf' when 'sib_leaf''s cache
> level should also be checked.

I have to say, I'm a wee bit confused reading this patch - although it's
likely that I have just confused myself here.

The comment reads "For non DT/ACPI systems, assume unique level 1 caches,
system-wide shared caches for all other levels".
Does this mean all level 1 caches are unique & all level N caches are
shared with all other level N caches, but not with level M caches?
(M != N; M, N > 1)

Is this patches goal to make sure that if this_leaf is level 2 and
sib_leaf is level 1 that these are not detected as shared, since level
one caches are meant to be unique?

The previous logic checked only this_leaf's level, and declared things
shared if this_leaf is not a level 1 cache.
What happens here if this_leaf->level == 1 and sib_leaf->level == 2?
That'll be detected as shared, in a contradiction of the comment above
it, no?

As you never state the actual problem with the current code, I'm not
entirely sure if I am making a fool of myself or not here.

Probably making a fool, that's par for the course ;)

Thanks,
Conor.

> 
> Check 'sib_leaf->level'. Also update the comment as the function is
> called when populating 'shared_cpu_map'.
> 
> Fixes: f16d1becf96f ("cacheinfo: Use cache identifiers to check if the caches are shared if available")
> Signed-off-by: Pierre Gondois <pierre.gondois@....com>
> ---
>  drivers/base/cacheinfo.c | 5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/base/cacheinfo.c b/drivers/base/cacheinfo.c
> index f3903d002819..e7ad6aba5f97 100644
> --- a/drivers/base/cacheinfo.c
> +++ b/drivers/base/cacheinfo.c
> @@ -38,11 +38,10 @@ static inline bool cache_leaves_are_shared(struct cacheinfo *this_leaf,
>  {
>  	/*
>  	 * For non DT/ACPI systems, assume unique level 1 caches,
> -	 * system-wide shared caches for all other levels. This will be used
> -	 * only if arch specific code has not populated shared_cpu_map
> +	 * system-wide shared caches for all other levels.
>  	 */
>  	if (!(IS_ENABLED(CONFIG_OF) || IS_ENABLED(CONFIG_ACPI)))
> -		return !(this_leaf->level == 1);
> +		return (this_leaf->level != 1) || (sib_leaf->level != 1);
>  
>  	if ((sib_leaf->attributes & CACHE_ID) &&
>  	    (this_leaf->attributes & CACHE_ID))
> -- 
> 2.25.1
> 

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