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

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Message-ID: <20200127121542.05a124d7@gandalf.local.home>
Date:   Mon, 27 Jan 2020 12:15:42 -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 16:56:12 +0000
David Laight <David.Laight@...LAB.COM> wrote:

> >From Steven Rostedt
> > Sent: 27 January 2020 15:50
> > 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.  
> 
> With idle=poll the delays seem to be minimal.
> 
> Is there any way to limit the C state entered by mwait?

I believe you can dynamically change C state. There's a pdf about it:

  https://wiki.ntb.ch/infoportal/_media/embedded_systems/ethercat/controlling_processor_c-state_usage_in_linux_v1.1_nov2013.pdf

-- Steve