[v2 0/6] KVM: arm64: implement vcpu_is_preempted check
Usama Arif
usama.arif at bytedance.com
Wed Nov 9 11:38:55 PST 2022
On 07/11/2022 18:24, Punit Agrawal wrote:
> Hi Usama,
>
> Usama Arif <usama.arif at bytedance.com> writes:
>
>> This patchset adds support for vcpu_is_preempted in arm64, which allows the guest
>> to check if a vcpu was scheduled out, which is useful to know incase it was
>> holding a lock. vcpu_is_preempted can be used to improve
>> performance in locking (see owner_on_cpu usage in mutex_spin_on_owner,
>> mutex_can_spin_on_owner, rtmutex_spin_on_owner and osq_lock) and scheduling
>> (see available_idle_cpu which is used in several places in kernel/sched/fair.c
>> for e.g. in wake_affine to determine which CPU can run soonest):
>>
>> This patchset shows improvement on overcommitted hosts (vCPUs > pCPUS), as waiting
>> for preempted vCPUs reduces performance.
>>
>> This patchset is inspired from the para_steal_clock implementation and from the
>> work originally done by Zengruan Ye:
>> https://lore.kernel.org/linux-arm-kernel/20191226135833.1052-1-yezengruan@huawei.com/.
>>
>> All the results in the below experiments are done on an aws r6g.metal instance
>> which has 64 pCPUs.
>>
>> The following table shows the index results of UnixBench running on a 128 vCPU VM
>> with (6.0.0+vcpu_is_preempted) and without (6.0.0 base) the patchset.
>> TestName 6.0.0 base 6.0.0+vcpu_is_preempted % improvement for vcpu_is_preempted
>> Dhrystone 2 using register variables 187761 191274.7 1.871368389
>> Double-Precision Whetstone 96743.6 98414.4 1.727039308
>> Execl Throughput 689.3 10426 1412.548963
>> File Copy 1024 bufsize 2000 maxblocks 549.5 3165 475.978162
>> File Copy 256 bufsize 500 maxblocks 400.7 2084.7 420.2645371
>> File Copy 4096 bufsize 8000 maxblocks 894.3 5003.2 459.4543218
>> Pipe Throughput 76819.5 78601.5 2.319723508
>> Pipe-based Context Switching 3444.8 13414.5 289.4130283
>> Process Creation 301.1 293.4 -2.557289937
>> Shell Scripts (1 concurrent) 1248.1 28300.6 2167.494592
>> Shell Scripts (8 concurrent) 781.2 26222.3 3256.669227
>> System Call Overhead 3426 3729.4 8.855808523
>>
>> System Benchmarks Index Score 3053 11534 277.7923354
>>
>> This shows a 277% overall improvement using these patches.
>>
>> The biggest improvement is in the shell scripts benchmark, which forks a lot of processes.
>> This acquires rwsem lock where a large chunk of time is spent in base 6.0.0 kernel.
>> This can be seen from one of the callstack of the perf output of the shell
>> scripts benchmark on 6.0.0 base (pseudo NMI enabled for perf numbers below):
>> - 33.79% el0_svc
>> - 33.43% do_el0_svc
>> - 33.43% el0_svc_common.constprop.3
>> - 33.30% invoke_syscall
>> - 17.27% __arm64_sys_clone
>> - 17.27% __do_sys_clone
>> - 17.26% kernel_clone
>> - 16.73% copy_process
>> - 11.91% dup_mm
>> - 11.82% dup_mmap
>> - 9.15% down_write
>> - 8.87% rwsem_down_write_slowpath
>> - 8.48% osq_lock
>>
>> Just under 50% of the total time in the shell script benchmarks ends up being
>> spent in osq_lock in the base 6.0.0 kernel:
>> Children Self Command Shared Object Symbol
>> 17.19% 10.71% sh [kernel.kallsyms] [k] osq_lock
>> 6.17% 4.04% sort [kernel.kallsyms] [k] osq_lock
>> 4.20% 2.60% multi. [kernel.kallsyms] [k] osq_lock
>> 3.77% 2.47% grep [kernel.kallsyms] [k] osq_lock
>> 3.50% 2.24% expr [kernel.kallsyms] [k] osq_lock
>> 3.41% 2.23% od [kernel.kallsyms] [k] osq_lock
>> 3.36% 2.15% rm [kernel.kallsyms] [k] osq_lock
>> 3.28% 2.12% tee [kernel.kallsyms] [k] osq_lock
>> 3.16% 2.02% wc [kernel.kallsyms] [k] osq_lock
>> 0.21% 0.13% looper [kernel.kallsyms] [k] osq_lock
>> 0.01% 0.00% Run [kernel.kallsyms] [k] osq_lock
>>
>> and this comes down to less than 1% total with 6.0.0+vcpu_is_preempted kernel:
>> Children Self Command Shared Object Symbol
>> 0.26% 0.21% sh [kernel.kallsyms] [k] osq_lock
>> 0.10% 0.08% multi. [kernel.kallsyms] [k] osq_lock
>> 0.04% 0.04% sort [kernel.kallsyms] [k] osq_lock
>> 0.02% 0.01% grep [kernel.kallsyms] [k] osq_lock
>> 0.02% 0.02% od [kernel.kallsyms] [k] osq_lock
>> 0.01% 0.01% tee [kernel.kallsyms] [k] osq_lock
>> 0.01% 0.00% expr [kernel.kallsyms] [k] osq_lock
>> 0.01% 0.01% looper [kernel.kallsyms] [k] osq_lock
>> 0.00% 0.00% wc [kernel.kallsyms] [k] osq_lock
>> 0.00% 0.00% rm [kernel.kallsyms] [k] osq_lock
>>
>> To make sure, there is no change in performance when vCPUs < pCPUs, UnixBench
>> was run on a 32 CPU VM. The kernel with vcpu_is_preempted implemented
>> performed 0.9% better overall than base kernel, and the individual benchmarks
>> were within +/-2% improvement over 6.0.0 base.
>> Hence the patches have no negative affect when vCPUs < pCPUs.
>>
>>
>> The other method discussed in https://lore.kernel.org/linux-arm-kernel/20191226135833.1052-1-yezengruan@huawei.com/
>> was pv conditional yield by Marc Zyngier and Will Deacon to reduce vCPU reschedule
>> if the vCPU will exit immediately.
>> (https://git.kernel.org/pub/scm/linux/kernel/git/maz/arm-platforms.git/log/?h=kvm-arm64/pvcy).
>> The patches were ported to 6.0.0 kernel and tested with UnixBench with a 128 vCPU VM:
>>
>> TestName 6.0.0 base 6.0.0+pvcy % improvement for pvcy
>> Dhrystone 2 using register variables 187761 183128 -2.467498575
>> Double-Precision Whetstone 96743.6 96645 -0.101918887
>> Execl Throughput 689.3 1019.8 47.9471928
>> File Copy 1024 bufsize 2000 maxblocks 549.5 2029.7 269.3721565
>> File Copy 256 bufsize 500 maxblocks 400.7 1439.4 259.2213626
>> File Copy 4096 bufsize 8000 maxblocks 894.3 3434.1 283.9986582
>> Pipe Throughput 76819.5 74268.8 -3.320380893
>> Pipe-based Context Switching 3444.8 5963.3 73.11019508
>> Process Creation 301.1 163.2 -45.79873796
>> Shell Scripts (1 concurrent) 1248.1 1859.7 49.00248378
>> Shell Scripts (8 concurrent) 781.2 1171 49.89759345
>> System Call Overhead 3426 3194.4 -6.760070053
>>
>> System Benchmarks Index Score 3053 4605 50.83524402
>>
>> pvcy shows a smaller overall improvement (50%) compared to vcpu_is_preempted (277%).
>> Host side flamegraph analysis shows that ~60% of the host time when using pvcy
>> is spent in kvm_handle_wfx, compared with ~1.5% when using vcpu_is_preempted,
>> hence vcpu_is_preempted shows a larger improvement.
>>
>> It might be that pvcy can be used in combination with vcpu_is_preempted, but this
>> series is to start a discussion on vcpu_is_preempted as it shows a much bigger
>> improvement in performance and its much easier to review vcpu_is_preempted standalone.
>
> Looking at both the patchsets - this one and the pvcy, it looks to me
> that vcpu_is_preempted() and the pvcy patches are somewhat
> orthogonal. The former is optimizing mutex and rwsem in their optimistic
> spinning phase while the latter is going after spinlocks (via wfe).
>
> Unless I'm missing something the features are not necessarily comparable
> on the same workloads - unixbench is probably mutex heavy and doesn't
> show as much benefit with just the pvcy changes. I wonder if it's easy
> to have both the features enabled to see this in effect.
>
> I've left some comments on the patches; but no need to respin just
> yet. Let's see if there is any other feedback.
>
> Thanks,
> Punit
>
There was a small bug in v2, where pv_lock_init was called too early in
the boot in setup_arch, hence pvlock_vcpu_state was not initialized for
vCPU 0 (the state was initialized for vCPUs 1-127 during secondary core
boot, hence the rest of the vCPUs were using pvlock correctly). I will
send the next revision making it an early_initcall along with addressing
Punits' comments, but will wait for further comments on v2 before
sending it. I have tested it with early_initcall and didnt see a
significant change in performance (which is expected as only 1 out of
128 vCPUs wasnt using pvlock correctly).
I tried pvcy+vcpu_is_preempted patches together and I see a slight
reduction in performance over pvcy only.
As a summary, with the above changes to move to early_initcall included
the overall Unixbench score improvements are:
Change over 6.0.0 base kernel % improvement over base
vcpu_is_preempted 279%
pvcy 51%
pvcy+vcpu_is_preempted 37%
Thanks,
Usama
> [...]
>
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