linux-kernel - Re: sched/fair: Long delays starting RT processes on idle cpu.

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Message-ID: <20200127104948.59eac75a@gandalf.local.home>
Date:   Mon, 27 Jan 2020 10:49:48 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     David Laight <David.Laight@...LAB.COM>
Cc:     'linux-kernel' <linux-kernel@...r.kernel.org>
Subject: Re: sched/fair: Long delays starting RT processes on idle cpu.

On Mon, 27 Jan 2020 15:39:24 +0000
David Laight <David.Laight@...LAB.COM> wrote:

> I'd have thought that the processor should wake up much faster than that.
> I can't see the memory write that is paired with the monitor/mwait.
> Does it need a strong barrier?

You may want to prevent the CPU from going into a deep C state. 90us is
something I would expect if the CPU is in a deep C state (I've seen
much longer wake up times due to deep C state).

Boot the kernel with idle=poll and see if you can trigger the same
latency. If not, then you know it's the CPU going into a deep C state
that is causing your latency.

-- Steve