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

[Rik van Riel <riel@redhat.com>]

On 07/22/2012 08:34 AM, Raghavendra K T wrote:
> 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)
>>> - add config HAVE_KVM_CPU_RELAX_INTERCEPT (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.
>>>
>>> Links
>>> V4: https://lkml.org/lkml/2012/7/16/80
>>> V3: https://lkml.org/lkml/2012/7/12/437
>>> V2: https://lkml.org/lkml/2012/7/10/392
>>> V1: https://lkml.org/lkml/2012/7/9/32
>>>
>>> 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@redhat.com>
>>
>
> Thanks Marcelo for the review. Avi, Rik, Christian, please let me know
> if this series looks good now.

The series looks good to me.

Reviewed-by: Rik van Riel <riel@redhat.com>

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