linux-kernel - Re: [PATCH v1 0/3] arch_topology: Correct CPU capacity scaling

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Date:   Mon, 14 Mar 2022 18:10:58 +0000
From:   Ionela Voinescu <ionela.voinescu@....com>
To:     Leo Yan <leo.yan@...aro.org>
Cc:     Sudeep Holla <sudeep.holla@....com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Bryan O'Donoghue <bryan.odonoghue@...aro.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 0/3] arch_topology: Correct CPU capacity scaling

Hi Leo,

On Sunday 13 Mar 2022 at 13:55:09 (+0800), Leo Yan wrote:
> This patch set is to address issues for CPU capacity scaling.
> 
> "capacity-dmips-mhz" property might be absent in all CPU nodes, and in
> another situation, DT might have inconsistent binding issue, e.g. some
> CPU nodes have "capacity-dmips-mhz" property and some nodes miss the
> property.  Current code mixes these two cases and always rollback to CPU
> capacity 1024 for these two cases.
> 
> Patches 01 and 02 in this set are used to distinguish the two different
> DT binding cases, and for the inconsistent binding issue, it rolls back
> to 1024 without CPU capacity scaling.
> 
> Patch 03 is to handle the case for absenting "capacity-dmips-mhz"
> property in CPU nodes, the patch proceeds to do CPU capacity scaling based
> on CPU maximum capacity.  Thus it can reflect the correct CPU capacity for
> Arm platforms with "fast" and "slow" clusters (CPUs in two clusters have
> the same raw capacity but with different maximum frequencies).
> 

In my opinion it's difficult to handle absent "capacity-dmips-mhz"
properties, as they can be a result of 3 scenarios: potential..
 1. bug in DT
 2. unwillingness to fill this information in DT
 3. suggestion that we're dealing with CPUs with same u-arch
    (same capacity-dmips-mhz)

I'm not sure it's up to us to interpret suggestions in the code so I
believe treating missing information as error is the right choice, which
is how we're handling this now.

For 3. (and patch 03), isn't it easier to populate capacity-dmips-mhz to
the same value (say 1024) in DT? That is a clear message that we're
dealing with CPUs with the same u-arch.

Thanks,
Ionela.


> This patch set is applied on the mainline kernel with the latest commit
> 68453767131a ("ARM: Spectre-BHB: provide empty stub for non-config").
> And tested on Arm64 Hikey960 platform (with a bit hacking to emulate
> fast and slow clusters).
> 
> 
> Leo Yan (3):
>   arch_topology: Correct semantics for 'cap_parsing_failed'
>   arch_topology: Handle inconsistent binding of CPU raw capacity
>   arch_topology: Scale CPU capacity if without CPU raw capacity
> 
>  drivers/base/arch_topology.c | 42 +++++++++++++++++++++++++++++-------
>  1 file changed, 34 insertions(+), 8 deletions(-)
> 
> -- 
> 2.25.1
> 
>