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

[Peter Zijlstra]

On Tue, Jul 18, 2017 at 02:56:47PM +0800, Li, Aubrey wrote:

> 3) for tick nohz idle, we want to skip if the coming idle is short. If we can
> skip the tick nohz idle, we then skip all the items depending on it. But, there
> are two hard points:
> 
> 3.1) how to compute the period of the coming idle. My current proposal is to
> use two factors in the current idle menu governor. There are two possible
> options from Peter and Thomas, the one is to use scheduler idle estimate, which
> is task activity based, the other is to use the statistics generated from irq
> timings work.
> 
> 3.2) how to determine if the idle is short or long. My current proposal is to
> use a tunable value via /sys, while Peter prefers an auto-adjust mechanism. I
> didn't get the details of an auto-adjust mechanism yet

So the problem is that the cost of NOHZ enter/exit are for a large part
determined by (micro) architecture costs of programming timer hardware.

A single static threshold will never be the right value across all the
various machines we run Linux on.

So my suggestion was simply timing the cost of doing those functions
ever time we do them and keeping a running average of their cost. Then
use that measured cost as a basis for selecting when to skip them. For
example if the estimated idle period (by whatever estimate we end up
using) is less than 4 times the cost of doing NOHZ, don't do NOHZ.


Note how both tick_nohz_idle_{enter,exit}() already take a timestamp at
entry; all you need to do is take another one at exit and subtract.