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SubjectRe: [Patch] idle governor: Avoid lock acquisition to read pm_qos before entering idle
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On Wed, 2011-02-09 at 17:21 -0800, Tim Chen wrote:
> I noticed that before entering idle state, the menu idle governor will
> look up the current pm_qos value according to the list of qos request
> received. This look up currently needs the acquisition of a lock to go
> down a list of qos requests to find the qos value, slowing down the
> entrance into idle state due to contention by multiple cpus to traverse
> this list. The contention is severe when there are a lot of cpus waking
> and going into idle. For example, for a simple workload that has 32
> pair of processes ping ponging messages to each other, where 64 cores
> cores are active in test system, I see the following profile:
>
> - 37.82% swapper [kernel.kallsyms] [k]
> _raw_spin_lock_irqsave
> - _raw_spin_lock_irqsave
> - 95.65% pm_qos_request
> menu_select
> cpuidle_idle_call
> - cpu_idle
> 99.98% start_secondary
>
> Perhaps a better approach will be to cache the updated pm_qos value so
> reading it does not require lock acquisition as in the patch below.
>
> Tim
>
> Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>

What you say is true as long as the value is 32 bits ... perhaps a note
of that should be made somewhere?

Otherwise, it looks like a good lockless optimisation on the read path,
so you can add my ack.

James




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