Re: [v2 0/6] KVM: arm64: implement vcpu_is_preempted check
From: Usama Arif
Date: Wed Nov 09 2022 - 14:39:05 EST
On 07/11/2022 18:24, Punit Agrawal wrote:
Hi Usama,
Usama Arif <usama.arif@xxxxxxxxxxxxx> 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@xxxxxxxxxx/.
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@xxxxxxxxxx/
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
[...]