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

From: Ionela Voinescu <ionela.voinescu@arm.com>
To: Leo Yan <leo.yan@linaro.org>
Cc: Sudeep Holla <sudeep.holla@arm.com>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	"Rafael J. Wysocki" <rafael@kernel.org>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	Bryan O'Donoghue <bryan.odonoghue@linaro.org>,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH v1 0/3] arch_topology: Correct CPU capacity scaling
Date: Mon, 14 Mar 2022 18:10:58 +0000	[thread overview]
Message-ID: <Yi+FMrG9NyBnMX0i@arm.com> (raw)
In-Reply-To: <20220313055512.248571-1-leo.yan@linaro.org>

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
> 
> 