Re: single aio thread is migrated crazily by scheduler

Ming Lei
2019-11-15T07:09:07Z

On Fri, Nov 15, 2019 at 03:56:34PM +1100, Dave Chinner wrote:
> On Fri, Nov 15, 2019 at 09:08:24AM +0800, Ming Lei wrote:
> > Hi Dave,
> >
> > On Fri, Nov 15, 2019 at 10:54:15AM +1100, Dave Chinner wrote:
> > > On Thu, Nov 14, 2019 at 07:31:53PM +0800, Ming Lei wrote:
> > > > Hi Guys,
> > > >
> > > > It is found that single AIO thread is migrated crazely by scheduler, and
> > > > the migrate period can be < 10ms. Follows the test a):
> > > >
> > > > - run single job fio[1] for 30 seconds:
> > > > ./xfs_complete 512
> > > >
> > > > - observe fio io thread migration via bcc trace[2], and the migration
> > > > times can reach 5k ~ 10K in above test. In this test, CPU utilization
> > > > is 30~40% on the CPU running fio IO thread.
> > >
> > > Using the default scheduler tunings:
> > >
> > > kernel.sched_wakeup_granularity_ns = 4000000
> > > kernel.sched_min_granularity_ns = 3000000
> > >
> > > I'm not seeing any migrations at all on a 16p x86-64 box. Even with
> > > the tunings you suggest:
> > >
> > > sysctl kernel.sched_min_granularity_ns=10000000
> > > sysctl kernel.sched_wakeup_granularity_ns=15000000
> > >
> > > There are no migrations at all.
> >
> > Looks I forget to pass $BS to the fio command line in the script posted,
> > please try the following script again and run './xfs_complete 512' first.
>
> So I ran 4kB IOs instead of 512 byte IOs. Shouldn't make any
> difference, really - it'll still be CPU bound...

In 512 block size test, the CPU utilization of fio IO thread is reduced to
40%, which is more like IO bound.

>
> <snip script>
>
> > In my test just done, the migration count is 12K in 30s fio running.
> > Sometimes the number can be quite less, but most of times, the number
> > is very big(> 5k).
>
> With my iomap-dio-overwrite patch and 512 byte IOs:
>
> $ sudo trace-cmd show |grep sched_migrate_task |wc -l
> 112
> $ sudo trace-cmd show |grep sched_migrate_task |grep fio |wc -l
> 22
>
> Without the iomap-dio-overwrite patch:
>
> $ sudo trace-cmd show |grep sched_migrate_task |wc -l
> 99
> $ sudo trace-cmd show |grep sched_migrate_task |grep fio |wc -l
> 9
> $
>
> There are -less- migrations when using the workqueue for everything.
> But it's so low in either case that it's just noise.
>
> Performance is identical for the two patches...

I can reproduce the issue with 4k block size on another RH system, and
the login info of that system has been shared to you in RH BZ.

1)
sysctl kernel.sched_min_granularity_ns=10000000
sysctl kernel.sched_wakeup_granularity_ns=15000000

2)
./xfs_complete 4k

Then you should see 1k~1.5k fio io thread migration in above test,
either v5.4-rc7(build with rhel8 config) or RHEL 4.18 kernel.

Not reproduced the issue with 512 block size on the RH system yet,
maybe it is related with my kernel config.

>
> > > > BTW, the tests are run on latest linus tree(5.4-rc7) in KVM guest, and the
> > > > fio test is created for simulating one real performance report which is
> > > > proved to be caused by frequent aio submission thread migration.
> > >
> > > What is the underlying hardware? I'm running in a 16p KVM guest on a
> > > 16p/32t x86-64 using 5.4-rc7, and I don't observe any significant
> > > CPU migration occurring at all from your test workload.
> >
> > It is a KVM guest, which is running on my Lenova T460p Fedora 29 laptop,
> > and the host kernel is 5.2.18-100.fc29.x86_64, follows the guest info:
>
> Ok, so what are all the custom distro kernel tunings that userspace
> does for the kernel?

It is standard Fedora 29.

>
> > [root@ktest-01 ~]# lscpu
> > Architecture: x86_64
> > CPU op-mode(s): 32-bit, 64-bit
> > Byte Order: Little Endian
> > CPU(s): 8
> > On-line CPU(s) list: 0-7
> > Thread(s) per core: 1
> > Core(s) per socket: 4
> > Socket(s): 2
> > NUMA node(s): 2
>
> Curious. You've configured it as two CPU sockets. If you make it a
> single socket, do your delay problems go away? The snippet of trace
> output you showed indicated it bouncing around CPUs on a single node
> (cpus 0-3), so maybe it has something to do with way the scheduler
> is interacting with non-zero NUMA distances...

I don't see that is a problem wrt. this issue, given the issue can
be reproduced on other system too.

>
> > Vendor ID: GenuineIntel
> > CPU family: 6
> > Model: 94
> > Model name: Intel(R) Core(TM) i7-6820HQ CPU @ 2.70GHz
> > Stepping: 3
> > CPU MHz: 2712.000
> > BogoMIPS: 5424.00
> > Virtualization: VT-x
> > Hypervisor vendor: KVM
> > Virtualization type: full
> > L1d cache: 32K
> > L1i cache: 32K
> > L2 cache: 4096K
> > L3 cache: 16384K
> > NUMA node0 CPU(s): 0-3
> > NUMA node1 CPU(s): 4-7
> > Flags: fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmxp
>
> That seems like a very minimal set of CPU flags - looks like you are
> not actually passing the actual host CPU capabilities through to the
> guest. That means it will be doing the slowest, most generic
> spectre/meltdown mitigations, right?

The above line is just trunated by the console terminal.

>
> Also, shouldn't lscpu be telling us all the CPU bug mitigations in
> place?
>
> From my test system:
>
> Architecture: x86_64
> CPU op-mode(s): 32-bit, 64-bit
> Byte Order: Little Endian
> Address sizes: 40 bits physical, 48 bits virtual
> CPU(s): 16
> On-line CPU(s) list: 0-15
> Thread(s) per core: 1
> Core(s) per socket: 1
> Socket(s): 16
> NUMA node(s): 1
> Vendor ID: GenuineIntel
> CPU family: 6
> Model: 45
> Model name: Intel(R) Xeon(R) CPU E5-4620 0 @ 2.20GHz
> Stepping: 7
> CPU MHz: 2199.998
> BogoMIPS: 4399.99
> Virtualization: VT-x
> Hypervisor vendor: KVM
> Virtualization type: full
> L1d cache: 512 KiB
> L1i cache: 512 KiB
> L2 cache: 64 MiB
> L3 cache: 256 MiB
> NUMA node0 CPU(s): 0-15
> Vulnerability L1tf: Mitigation; PTE Inversion; VMX flush not necessary, SMT disabled
> Vulnerability Mds: Mitigation; Clear CPU buffers; SMT Host state unknown
> Vulnerability Meltdown: Vulnerable
> Vulnerability Spec store bypass: Mitigation; Speculative Store Bypass disabled via prctl and seccomp
> Vulnerability Spectre v1: Mitigation; usercopy/swapgs barriers and __user pointer sanitization
> Vulnerability Spectre v2: Vulnerable, IBPB: disabled, STIBP: disabled
> Flags: fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 ss syscall nx pdpe1gb rdtscp l
> m constant_tsc arch_perfmon rep_good nopl xtopology cpuid tsc_known_freq pni pclmulqdq vmx ssse3 cx16 pcid sse4_1 sse4_2 x2apic
> popcnt tsc_deadline_timer aes xsave avx hypervisor lahf_lm cpuid_fault ssbd ibrs ibpb stibp tpr_shadow vnmi flexpriority ept vpi
> d tsc_adjust xsaveopt arat umip md_clear arch_capabilities
>
> So, to rule out that it has something to do with kernel config,
> I just ran up a kernel built with your config.gz, and the problem
> does not manifest. The only difference was a few drivers I needed to
> boot my test VMs, and I was previously not using paravirt spinlocks.
>
> So, I still can't reproduce the problem. Indeed, the workload gets
> nowhere near single CPU bound with your config - it's using half the
> CPU for the same performance:
>
> %Cpu2 : 19.8 us, 28.2 sy, 0.0 ni, 0.0 id, 52.0 wa, 0.0 hi, 0.0 %si, 0.0 st
>
> Basically, it's spending half it's time waiting on IO. If I wind the
> delay down to 1000ns:
>
> %Cpu1 : 42.2 us, 42.2 sy, 0.0 ni, 0.0 id, 0.0 wa, 0.0 hi, 15.6 %si, 0.0 st
>
> it spends an awful lot of time in soft-interrupt, but is back to
> being CPU bound.
>
> Despite this, I still don't see any significant amount of task
> migration. In fact, I see a lot less with your kernel config that I
> do with my original kernel config, because the CPU load was far
> lower.
>
> > Just run a quick test several times after applying the above patch, and looks it
> > does make a big difference in test './xfs_complete 512' wrt. fio io thread migration.
>
> There's something very different about your system, and it doesn't
> appear to be a result of the kernel code itself. I think you're
> going to have to do all the testing at the moment, Ming, because
> it's clear that my test systems do not show up the problems even
> when using the same kernel config as you do...
>
> If you reconfig you kvm setup to pass all the native host side cpu
> flags through to the guest, does the problem go away? I think adding
> "-cpu host" to your qemu command line will do that...

Please login to the RH system I shared to you, and you will see the
issue.

Thanks,
Ming