APPLIED: [PATCH][SRU][Xenial] Fix for LP:#1724614

[Khaled Elmously]

Applied to Xenial

On 2018-01-26 17:41:23 , Victor Tapia wrote:
> BugLink: https://bugs.launchpad.net/bugs/1724614
> 
> [SRU Justification]
> 
> [Impact]
> 
> We've identified a constant high (~90%) system time load at the host level
> when a VCPU in a KVM guest remains or switches/resumes in/from halt/idle state
> in a constant frequency, usually for a slightly smaller time than the default polling
> period.
> The halt polling mechanism has the intention to reduce latency in the cases
> on which the guest is quickly resumed saving a call to the scheduler.
> We've performed some testing by adjusting the /sys/module/kvm/parameters/halt_poll_ns
> value which defines the max time that should be spend polling before calling the
> scheduler to allow it to run other tasks (which defaults to 400000 ns in Ubuntu).
> With the default value the tests shows that the load remains nearly on 90% on a
> VCPU that has a single task in the run queue.
> We've also tested altering the halt_poll_ns value to 200000 ns and the results
> seems to drop the system time usage from 90% to ~5%.
> 
> root at porygon:/home/ubuntu# echo 200000 > /sys/module/kvm/parameters/halt_poll_ns
> root at porygon:/home/ubuntu# mpstat 1 -P 6 5
> Linux 4.4.0-112-generic (porygon) 	01/24/2018 	_x86_64_	(64 CPU)
> 
> 02:06:08 PM  CPU    %usr   %nice    %sys %iowait    %irq   %soft  %steal  %guest  %gnice   %idle
> ...
> Average:       6    0.00    0.00    4.26    0.00    0.00    0.00    0.00   17.83    0.00   77.91
> 
> root at porygon:/home/ubuntu# echo 400000 > /sys/module/kvm/parameters/halt_poll_ns
> root at porygon:/home/ubuntu# mpstat 1 -P 6 5
> Linux 4.4.0-112-generic (porygon) 	01/24/2018 	_x86_64_	(64 CPU)
> 
> 02:06:20 PM  CPU    %usr   %nice    %sys %iowait    %irq   %soft  %steal  %guest  %gnice   %idle
> ...
> Average:       6    0.00    0.00   89.59    0.00    0.00    0.00    0.00    8.45    0.00    1.96
> 
> [Test]
> 
> 1) Configure a KVM guest with a single pinned VCPU.
> 2) Run the following program (http://pastebin.ubuntu.com/25731919/) at the KVM guest.
> $ gcc test.c -lpthread -o test && ./test 250 0
> 3) Run mpstat at the host on the pinned CPU and compare the stats
> $ sudo mpstat 1 -P 6 5
> 
> [Fix]
> 
> Change the halt polling max time to half of the current value.
> In some fio benchmarks, halt_poll_ns=400000 caused CPU utilization to
> increase heavily even in cases where the performance improvement was
> small. In particular, bandwidth divided by CPU usage was as much as
> 60% lower.
> To some extent this is the expected effect of the patch, and the
> additional CPU utilization is only visible when running the
> benchmarks. However, halving the threshold also halves the extra
> CPU utilization (from +30-130% to +20-70%) and has no negative
> effect on performance.
> Signed-off-by: Paolo Bonzini <pbonzini at redhat.com>
> * https://github.com/torvalds/linux/commit/b401ee0b85a53e89739ff68a5b1a0667d664afc9
> 
> Paolo Bonzini (1):
>   KVM: x86: lower default for halt_poll_ns
> 
>  arch/x86/include/asm/kvm_host.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> -- 
> 2.7.4
> 
> 
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