From owner-freebsd-acpi@FreeBSD.ORG  Tue Oct  2 17:18:53 2012
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From: Sean Bruno <seanbru@yahoo-inc.com>
To: Andriy Gapon <avg@freebsd.org>
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Date: Tue, 02 Oct 2012 10:18:33 -0700
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Cc: "freebsd-acpi@freebsd.org" <freebsd-acpi@freebsd.org>
Subject: Re: notify userland about C-state changes
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> The following patch adds only per-CPU notifications.
> 
>     acpi_cpu: explicitly notify userland about c-state changes
> 
> diff --git a/sys/dev/acpica/acpi_cpu.c b/sys/dev/acpica/acpi_cpu.c
> index 82e204a..15201f9 100644
> --- a/sys/dev/acpica/acpi_cpu.c
> +++ b/sys/dev/acpica/acpi_cpu.c
> @@ -1054,6 +1054,8 @@ acpi_cpu_notify(ACPI_HANDLE h, UINT32 notify, void *context)
>      ACPI_SERIAL_BEGIN(cpu);
>      acpi_cpu_set_cx_lowest(sc);
>      ACPI_SERIAL_END(cpu);
> +
> +    acpi_UserNotify("PROCESSOR", sc->cpu_handle, notify);
>  }
> 
>  static int
> 

So quick question, does this happen a lot on a system with a sporadic
workload?  Does this introduce overhead to the system to service the
notification requests?

Sean