+ linux-kernel
I just retested with 5.12 final:
As long as I give the VM more than one cpu, I can easily reproduce the problem with:
> while true; do cat linux-image-5.12.0-rc8-dbg_5.12.0-rc8-22_amd64.deb > /dev/null; done
> and
> pidofcat=$(pidof cat); echo $pidofcat; trace-cmd record -e all -P $pidofcat
On a single cpu VM it doesn't seem to trigger.
Is anyone else able to reproduce this?
Any ideas how to debug this?
It's really annoying to fear a machine freeze when trying to trace problems
on recent kernels...
Thanks for any possible hint...
metze
Am 22.04.21 um 16:26 schrieb Stefan Metzmacher:
>
> Hi Steven, hi Ingo,
>
> I recently tried to analyze the performance of Samba using io-uring.
>
> I was using ubuntu 20.04 with the 5.10.0-1023-oem kernel, which is based on v5.10.25, see:
> https://kernel.ubuntu.com/git/kernel-ppa/mirror/ubuntu-oem-5.10-focal.git/log/?h=oem-5.10-prep
> trace-cmd is at version 2.8.3-4build1.
>
> In order to find the bottleneck I tried to use (trace-cmd is at version 2.8.3-4build1):
>
> trace-cmd -e all -P ${pid_of_io_uring_worker}
>
> As a result the server was completely dead immediately.
>
> I tried to reproduce this in a virtual machine (inside virtualbox).
>
> I used a modified 'io_uring-cp' that loops forever, see:
> https://github.com/metze-samba/liburing/commit/5e98efed053baf03521692e786c1c55690b04d8e
>
> When I run './io_uring-cp-forever link-cp.c file',
> I see a 'io_wq_manager' and a 'io_wqe_worker-0' kernel thread,
> while './io_uring-cp-forever link-cp.c file' as well as 'io_wqe_worker-0'
> consume about 25% cpu each.
>
> When I run 'trace-cmd -e all -P $pid' for 'io_wqe_worker-0' or 'io_wq_manager'
> I can reproduce the problem, then I found that the same seems to happen for
> also for other kernel threads e.g. '[kworker/1:1-events]', it seems that
> it happens for all kernel threads, which are not completely idle.
>
> Which this:
>
> From 'top':
> 1341 root 20 0 2512 576 508 S 33,4 0,1 0:10.39 io_uring-cp-for
> 1343 root 20 0 0 0 0 R 29,8 0,0 0:08.43 io_wqe_worker-0
> 7 root 20 0 0 0 0 I 0,3 0,0 0:00.31 kworker/0:1-events
>
> PID 5 is [kworker/0:0-ata_sff]
>
> # trace-cmd record -e all -P 5'
> Hit Ctrl^C to stop recording
> ^CCPU0 data recorded at offset=0x7b8000
> 0 bytes in size
> CPU1 data recorded at offset=0x7b8000
> 69632 bytes in size
>
> # But
> # trace-cmd record -e all -P 7
> => machine unresponsive (no blinking cursor on the console anymore)
> On the host 'top' shows that the VirtualBoxVM cpu emulator thread 'EMT-1'
> uses 100% cpu, so I guess the guest kernel is in something like an endless
> recursion loop. Maybe a trace function recursing to itself?
>
> On the same VM I tried a 5.12rc8 kernel and there I can also reproduce the
> problem.
>
> I also managed to reproduce the problem without io-uring, just using:
>
> while true; do cat linux-image-5.12.0-rc8-dbg_5.12.0-rc8-22_amd64.deb > /dev/null; done
>
> in order to keep some kernel threads moving.
> This happens with 5.12rc8 and 5.10.0-1023-oem, but I was not able to
> reproduce any of this using the 5.8.0-50-generic kernel, see
> https://kernel.ubuntu.com/git/ubuntu/ubuntu-focal.git/log/?h=Ubuntu-hwe-5.8-5.8.0-50.56_20.04.1
>
> I was also able to reproduce this with a ubuntu 21.04 vm using
> the 5.11.0-14-generic kernel, see:
> https://kernel.ubuntu.com/git/ubuntu/ubuntu-hirsute.git/log/?h=Ubuntu-5.11.0-14.15
> On this one I only managed to reproduce the problem with
> './io_uring-cp-forever link-cp.c file', but not with
> 'while true; do cat linux-image-5.12.0-rc8-dbg_5.12.0-rc8-22_amd64.deb > /dev/null; done'
>
>
> So it seems the problem was introduced after 5.8 and is not really related to
> io-uring. And it may not be purely related to kernel threads.
>
> With this on 5.12-rc8 (again):
>
> └─tmux: server,903
> ├─bash,904
> │ └─io_uring-cp-for,925 link-cp.c file
> │ ├─{iou-mgr-925},926
> │ └─{iou-wrk-925},927
> └─bash,929
> └─pstree,938 -a -t -p
>
> I was able to to trace once:
>
> root@ub1704-166:~# trace-cmd record -e all -P 925
> Hit Ctrl^C to stop recording
> ^CCPU0 data recorded at offset=0x7b8000
> 10842112 bytes in size
> CPU1 data recorded at offset=0x120f000
> 36450304 bytes in size
>
> But the 2nd run reproduced the problem:
> root@ub1704-166:~# trace-cmd record -e all -P 925
>
> I was also able to reproduce it with:
>
> while true; do cat linux-image-5.12.0-rc8-dbg_5.12.0-rc8-22_amd64.deb > /dev/null; done
> and
> pidofcat=$(pidof cat); echo $pidofcat; trace-cmd record -e all -P $pidofcat
>
> So it seems any busy thread (user or kernel) triggers the problem.
>
> Any ideas what has changed after 5.8?
>
> Thanks!
> metze
>
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