[PATCH v2 0/3] cpuidle-pseries: Parse extended CEDE information for idle.

[Gautham R. Shenoy]

From: "Gautham R. Shenoy" <ego@xxxxxxxxxxxxxxxxxx>

Hi,

This is a v2 of the patch series to parse the extended CEDE
information in the pseries-cpuidle driver.

The v1 of this patchset can be found here :
https://lore.kernel.org/linuxppc-dev/1594120299-31389-1-git-send-email-ego@xxxxxxxxxxxxxxxxxx/

The change from v1 --> v2 :

 * Dropped Patches 4 and 5 which would expose extended idle-states,
   that wakeup on external interrupts, to cpuidle framework.  These
   were RFC patches in v1. Dropped them because currently the only
   extended CEDE state that wakesup on external interrupts is CEDE(1)
   which adds no signifcant value over CEDE(0).
   
 * Rebased the patches onto powerpc/merge.
 
 * No changes in code for Patches 1-3.

Motivation:
===========
On pseries Dedicated Linux LPARs, apart from the polling snooze idle
state, we currently have the CEDE idle state which cedes the CPU to
the hypervisor with latency-hint = 0.

However, the PowerVM hypervisor supports additional extended CEDE
states, which can be queried through the "ibm,get-systems-parameter"
rtas-call with the CEDE_LATENCY_TOKEN. The hypervisor maps these
extended CEDE states to appropriate platform idle-states in order to
provide energy-savings as well as shifting power to the active
units. On existing pseries LPARs today we have extended CEDE with
latency-hints {1,2} supported.

The patches in this patchset, adds code to parse the CEDE latency
records provided by the hypervisor. We use this information to
determine the wakeup latency of the regular CEDE (which we have been
so far hardcoding to 10us while experimentally it is much lesser ~
1us), by looking at the wakeup latency provided by the hypervisor for
Extended CEDE states. Since the platform currently advertises Extended
CEDE 1 to have wakeup latency of 2us, we can be sure that the wakeup
latency of the regular CEDE is no more than this.

With Patches 1-3, we see an improvement in the single-threaded
performance on ebizzy.

2 ebizzy threads bound to the same big-core. 25% improvement in the
avg records/s (higher the better) with patches 1-3.
x without_patches
* with_patches
    N           Min           Max        Median           Avg        Stddev
x  10       2491089       5834307       5398375       4244335     1596244.9
*  10       2893813       5834474       5832448     5327281.3     1055941.4

We do not observe any major regression in either the context_switch2
benchmark or the schbench benchmark

context_switch2 across CPU0 CPU1 (Both belong to same big-core, but different
small cores). We observe a minor 0.14% regression in the number of
context-switches (higher is better).
x without_patch
* with_patch
    N           Min           Max        Median           Avg        Stddev
x 500        348872        362236        354712     354745.69      2711.827
* 500        349422        361452        353942      354215.4     2576.9258

context_switch2 across CPU0 CPU8 (Different big-cores). We observe a 0.37%
improvement in the number of context-switches (higher is better).
x without_patch
* with_patch
    N           Min           Max        Median           Avg        Stddev
x 500        287956        294940        288896     288977.23     646.59295
* 500        288300        294646        289582     290064.76     1161.9992

schbench:
No major difference could be seen until the 99.9th percentile.

Without-patch
Latency percentiles (usec)
	50.0th: 29
	75.0th: 39
	90.0th: 49
	95.0th: 59
	*99.0th: 13104
	99.5th: 14672
	99.9th: 15824
	min=0, max=17993

With-patch:
Latency percentiles (usec)
	50.0th: 29
	75.0th: 40
	90.0th: 50
	95.0th: 61
	*99.0th: 13648
	99.5th: 14768
	99.9th: 15664
	min=0, max=29812

Gautham R. Shenoy (3):
  cpuidle-pseries: Set the latency-hint before entering CEDE
  cpuidle-pseries: Add function to parse extended CEDE records
  cpuidle-pseries : Fixup exit latency for CEDE(0)

 drivers/cpuidle/cpuidle-pseries.c | 167 +++++++++++++++++++++++++++++++++++++-
 1 file changed, 165 insertions(+), 2 deletions(-)

-- 
1.9.4