linux-kernel - Re: [PATCH v2] arch_topology: Make cpu

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Date:   Wed, 27 Mar 2019 10:56:42 +0000
From:   Quentin Perret <quentin.perret@....com>
To:     Lingutla Chandrasekhar <clingutla@...eaurora.org>
Cc:     sudeep.holla@....com, dietmar.eggemann@....com,
        gregkh@...uxfoundation.org, will.deacon@....com,
        catalin.marinas@....com, morten.rasmussen@....com,
        linux-arm-kernel@...ts.infradead.org, jeremy.linton@....com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] arch_topology: Make cpu_capacity sysfs node as
 ready-only

Hi,

On Friday 08 Mar 2019 at 18:08:48 (+0530), Lingutla Chandrasekhar wrote:
> If user updates any cpu's cpu_capacity, then the new value is going to
> be applied to all its online sibling cpus. But this need not to be correct
> always, as sibling cpus (in ARM, same micro architecture cpus) would have
> different cpu_capacity with different performance characteristics.
> So updating the user supplied cpu_capacity to all cpu siblings
> is not correct.
> 
> And another problem is, current code assumes that 'all cpus in a cluster
> or with same package_id (core_siblings), would have same cpu_capacity'.
> But with commit '5bdd2b3f0f8 ("arm64: topology: add support to remove
> cpu topology sibling masks")', when a cpu hotplugged out, the cpu
> information gets cleared in its sibling cpus. So user supplied
> cpu_capacity would be applied to only online sibling cpus at the time.
> After that, if any cpu hot plugged in, it would have different cpu_capacity
> than its siblings, which breaks the above assumption.
> 
> So instead of mucking around the core sibling mask for user supplied
> value, use device-tree to set cpu capacity. And make the cpu_capacity
> node as read-only to know the assymetry between cpus in the system.
> While at it, remove cpu_scale_mutex usage, which used for sysfs write
> protection.
> 
> Tested-by: Dietmar Eggemann <dietmar.eggemann@....com>
> Acked-by: Sudeep Holla <sudeep.holla@....com>
> Signed-off-by: Lingutla Chandrasekhar <clingutla@...eaurora.org>

Reviewed-by: Quentin Perret <quentin.perret@....com>
Tested-by: Quentin Perret <quentin.perret@....com>

Thanks for doing this,
Quentin