On Tue, Mar 31, 2020 at 05:40:55PM +0530, Gautham R. Shenoy wrote:
> From: "Gautham R. Shenoy" <ego@xxxxxxxxxxxxxxxxxx>
>
>
> *** RFC Only. Not intended for inclusion ************
>
> Motivation
> ~~~~~~~~~~~~~~~
>
> The POWER ISA v3.0 allows stop instruction to be executed from a Guest
> Kernel (HV=0,PR=0) context. If the hypervisor has cleared
> PSSCR[ESL|EC] bits, then the stop instruction thus executed will cause
> the vCPU thread to "pause", thereby donating its cycles to the other
> threads in the core until the paused thread is woken up by an
> interrupt. If the hypervisor has set the PSSCR[ESL|EC] bits, then
> execution of the "stop" instruction will raise a Hypervisor Facility
> Unavailable exception.
>
> The stop idle state in the guest (henceforth referred to as stop0lite)
> when enabled
>
> * has a very small wakeup latency (1-3us) comparable to that of
> snooze and considerably better compared the Shared CEDE state
> (25-30us). Results are provided below for wakeup latency measured
> by waking up an idle CPU in a given state using a timer as well as
> using an IPI.
>
> ======================================================================
> Wakeup Latency measured using a timer (in ns) [Lower is better]
> ======================================================================
> Idle state | Nr samples | Min | Max | Median | Avg | Stddev|
> ======================================================================
> snooze | 60 | 787 | 1059 | 938 | 937.4 | 42.27 |
> ======================================================================
> stop0lite | 60 | 770 | 1182 | 948 | 946.4 | 67.41 |
> ======================================================================
> Shared CEDE| 60 | 9550 | 36694 | 29219 |28564.1|3545.9 |
> ======================================================================
>
Posted two copies of Wakeup latency measured by timer. Here is the
wakeup latency measured using an IPI.
======================================================================
Wakeup latency measured using an IPI (in ns) [Lower is better]
======================================================================
Idle state | Nr | Min | Max | Median | Avg | Stddev |
|samples | | | | | |
----------------------------------------------------------------------
snooze | 60 | 2089| 4228| 2259| 2342.31| 316.56|
----------------------------------------------------------------------
stop0lite | 60 | 1947| 3674| 2653| 2610.57| 266.73|
----------------------------------------------------------------------
Shared CEDE| 60 | 20147| 36305| 21827| 26762.65| 6875.01|
======================================================================
--
Thanks and Regards
gautham.
