Re: [PATCH RFC V5 0/3] kvm: Improving directed yield in PLE handler

From: Raghavendra K T
Date: Sun Jul 22 2012 - 08:38:00 EST

On 07/20/2012 11:06 PM, Marcelo Tosatti wrote:
On Wed, Jul 18, 2012 at 07:07:17PM +0530, Raghavendra K T wrote:

Currently Pause Loop Exit (PLE) handler is doing directed yield to a
random vcpu on pl-exit. We already have filtering while choosing
the candidate to yield_to. This change adds more checks while choosing
a candidate to yield_to.

On a large vcpu guests, there is a high probability of
yielding to the same vcpu who had recently done a pause-loop exit.
Such a yield can lead to the vcpu spinning again.

The patchset keeps track of the pause loop exit and gives chance to a
vcpu which has:

(a) Not done pause loop exit at all (probably he is preempted lock-holder)

(b) vcpu skipped in last iteration because it did pause loop exit, and
probably has become eligible now (next eligible lock holder)

This concept also helps in cpu relax interception cases which use same handler.

Changes since V4:
- Naming Change (Avi):
struct ple ==> struct spin_loop
cpu_relax_intercepted ==> in_spin_loop
vcpu_check_and_update_eligible ==> vcpu_eligible_for_directed_yield
- mark vcpu in spinloop as not eligible to avoid influence of previous exit

Changes since V3:
- arch specific fix/changes (Christian)

Changes since v2:
- Move ple structure to common code (Avi)
- rename pause_loop_exited to cpu_relax_intercepted (Avi)
- Drop superfluous curly braces (Ingo)

Changes since v1:
- Add more documentation for structure and algorithm and Rename
plo ==> ple (Rik).
- change dy_eligible initial value to false. (otherwise very first directed
yield will not be skipped. (Nikunj)
- fixup signoff/from issue

Future enhancements:
(1) Currently we have a boolean to decide on eligibility of vcpu. It
would be nice if I get feedback on guest (>32 vcpu) whether we can
improve better with integer counter. (with counter = say f(log n )).

(2) We have not considered system load during iteration of vcpu. With
that information we can limit the scan and also decide whether schedule()
is better. [ I am able to use #kicked vcpus to decide on this But may
be there are better ideas like information from global loadavg.]

(3) We can exploit this further with PV patches since it also knows about
next eligible lock-holder.

Summary: There is a very good improvement for kvm based guest on PLE machine.
The V5 has huge improvement for kbench.

base_rik stdev patched stdev %improve
kernbench (time in sec lesser is better)
1x 49.2300 1.0171 22.6842 0.3073 117.0233 %
2x 91.9358 1.7768 53.9608 1.0154 70.37516 %

ebizzy (records/sec more is better)
1x 1129.2500 28.6793 2125.6250 32.8239 88.23334 %
2x 1892.3750 75.1112 2377.1250 181.6822 25.61596 %

Note: The patches are tested on x86.


Raghavendra K T (3):
config: Add config to support ple or cpu relax optimzation
kvm : Note down when cpu relax intercepted or pause loop exited
kvm : Choose a better candidate for directed yield
arch/s390/kvm/Kconfig | 1 +
arch/x86/kvm/Kconfig | 1 +
include/linux/kvm_host.h | 39 +++++++++++++++++++++++++++++++++++++++
virt/kvm/Kconfig | 3 +++
virt/kvm/kvm_main.c | 41 +++++++++++++++++++++++++++++++++++++++++
5 files changed, 85 insertions(+), 0 deletions(-)

Reviewed-by: Marcelo Tosatti<mtosatti@xxxxxxxxxx>

Thanks Marcelo for the review. Avi, Rik, Christian, please let me know
if this series looks good now.

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