linux-kernel - Re: [RFC PATCH v1 00/11] Create fast idle path for short idle periods

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Date:   Mon, 10 Jul 2017 10:27:05 -0700
From:   Andi Kleen <ak@...ux.intel.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Aubrey Li <aubrey.li@...el.com>, tglx@...utronix.de,
        len.brown@...el.com, rjw@...ysocki.net, tim.c.chen@...ux.intel.com,
        arjan@...ux.intel.com, paulmck@...ux.vnet.ibm.com,
        yang.zhang.wz@...il.com, x86@...nel.org,
        linux-kernel@...r.kernel.org, Aubrey Li <aubrey.li@...ux.intel.com>
Subject: Re: [RFC PATCH v1 00/11] Create fast idle path for short idle periods

On Mon, Jul 10, 2017 at 06:42:06PM +0200, Peter Zijlstra wrote:
> On Mon, Jul 10, 2017 at 07:46:09AM -0700, Andi Kleen wrote:
> > > So how much of the gain is simply due to skipping NOHZ? Mike used to
> > > carry a patch that would throttle NOHZ. And that is a _far_ smaller and
> > > simpler patch to do.
> > 
> > Have you ever looked at a ftrace or PT trace of the idle entry?
> > 
> > There's just too much stuff going on there. NOHZ is just the tip
> > of the iceberg.
> 
> I have, and last time I did the actual poking at the LAPIC (to make NOHZ
> happen) was by far the slowest thing happening.

That must have been a long time ago because modern systems use TSC deadline
for a very long time ...  

It's still slow, but not as slow as the LAPIC.

> Data to indicate what hurts how much would be a very good addition to
> the Changelogs. Clearly you have some, you really should have shared.

Aubrey?

-Andi