linux-kernel - RE: [PATCH 2/2] intel_idle: Introduce 'states

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Message-ID: <6cf71f6964c6433abeaf445847c97611@AcuMS.aculab.com>
Date:   Fri, 31 Jan 2020 11:54:13 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     "'artem.bityutskiy@...ux.intel.com'" 
        <artem.bityutskiy@...ux.intel.com>,
        "'Rafael J. Wysocki'" <rjw@...ysocki.net>,
        Linux PM <linux-pm@...r.kernel.org>
CC:     Linux ACPI <linux-acpi@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Len Brown <len.brown@...el.com>,
        Zhang Rui <rui.zhang@...el.com>,
        David Box <david.e.box@...ux.intel.com>,
        "Rafael J. Wysocki" <rafael@...nel.org>
Subject: RE: [PATCH 2/2] intel_idle: Introduce 'states_off' module parameter

From: Artem Bityutskiy >
> Sent: 31 January 2020 11:24
> On Fri, 2020-01-31 at 11:07 +0000, David Laight wrote:
> > Unless you know exactly which cpu table is being used the
> > only constraint a user can request is the latency.
> 
> Hi David,
> 
> in all my use-cases I always know what is the CPU I am dealing with and
> what are the C-states. Simply because in my view they are always CPU-
> dependent in terms of what they do and how are they named.
> 
> What you say sounds to me like you would want to disable some C-states
> without knowing anything (or much) about the CPU you are dealing with
> and the C-state names.
> 
> If so, could you please share examples of such use-cases?

Dunno, but clearly you want to disable (say) C3 while leaving C6
enabled.

I was trying to find why it was taking 600+us for a RT process
to get rescheduled when it had only been sleeping for a few us.

I found where it was sleeping, but that didn't help at all.
Someone pointed me at a 'random' pdf that referred to /dev/cpu_dma_latency.
Setting that to a small value (eg 20) helps no end.
But there are no references in the code or man pages to that.

	David

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