RE: [PATCH 1/3] [POC] test implementaion of rt-signals

[Johannes Kirchmair <johannes.kirchmair@sigmatek.at>]

Hello, 

> -----Original Message-----
> From: Jan Kiszka <jan.kiszka@siemens.com>
> Sent: Montag, 15. Mai 2023 12:38
> To: Johannes Kirchmair <johannes.kirchmair@sigmatek.at>;
> xenomai@lists.linux.dev; Schaffner, Tobias <tobias.schaffner@siemens.com>
> Subject: Re: [PATCH 1/3] [POC] test implementaion of rt-signals
> 
> CAUTION: External E-Mail !
> 
> On 15.05.23 08:50, Johannes Kirchmair wrote:
> >> -----Original Message-----
> >> From: Jan Kiszka <jan.kiszka@siemens.com>
> >> Sent: Freitag, 12. Mai 2023 19:39
> >> To: Johannes Kirchmair <johannes.kirchmair@sigmatek.at>;
> >> xenomai@lists.linux.dev; Schaffner, Tobias <tobias.schaffner@siemens.com>
> >> Subject: Re: [PATCH 1/3] [POC] test implementaion of rt-signals
> >>
> >> CAUTION: External E-Mail !
> >>
> >> On 09.05.23 15:17, Johannes Kirchmair wrote:
> >>> Hello Jan and Tobias,
> >>>
> >>> I just dropped the patches on the mailing list.
> >>> For the dovetail functions I took a rather naive approach, using most of the
> >> frame setup code provided by Linux.
> >>> I just tested it for 32bit and 64bit x86 Applications and it seems to work.
> >>>
> >>> I also added a tool rt_signal_hist that runs in a cycle and measures the time an
> >> exception handling takes.
> >>> It is afterwards put into an txt file that reassembles an histogram of the
> timings.
> >>>
> >>> Unfortunately as a 32bit application the rt_signal_hist tool does not run to
> >> completion, printing the following:
> >>> " Xenomai/cobalt: sleeping while holding mutex
> >>> CPU time limit exceeded"
> >>>
> >>> Maybe the naive approach is not so reliable ;-/
> >>> Hope I find the time the look into it.
> >>>
> >>
> >> I looked into the x86 signalframe helpers before and found some traces
> >> of locking that make them incompatible - if I rember correctly. After
> >> that, I dropped the idea of just removing the "static".
> > Do you remember where you saw the locking in the code, was it rather obvious
> or hidden?
> > When I had a look I did not see locking in the functions I used, so I thought
> dropping the "static" would be fine.
> >
> > Would you still try to use some of the functions Linux provides?
> >
> 
> I don't recall too many details, specficially not which kernel version I
> looked at. Maybe part of my concerns was that I started from
> setup_rt_frame (rseq_signal_deliver), rather than x64_setup_rt_frame & Co.
> 
> I was now digging into 6.3, and one area that is sensitive is the FPU
> handling. We end up in fault_in_readable, and I'm not sure yet if that
> is ok. The whole FPU save/restore needs a careful check, or more, if we
> are not violating the assumptions of OOB.
> 
> But did you run your code with full lock debugging enabled already? Did
> you stress faulting sigframe setups/restores? That would be needed as well.

Yes, thought about activating debugging in kernel, yesterday. Resulting in the following kernel output:
[   83.338288] ------------[ cut here ]------------
[   83.338292] WARNING: CPU: 3 PID: 268 at mm/memory.c:5640 __might_fault+0x28/0x30
[   83.338299] Modules linked in:
[   83.338301] CPU: 3 PID: 268 Comm: signal_test_tas Tainted: G        W          6.3.0+ #7
[   83.338304] Hardware name: Default string Default string/Default string, BIOS 5.19 04/28/2022
[   83.338305] IRQ stage: Xenomai
[   83.338306] RIP: 0010:__might_fault+0x28/0x30
[   83.338308] Code: 90 90 f3 0f 1e fa 0f 1f 44 00 00 55 65 8b 05 57 11 24 6a 48 89 e5 a9 00 00 00 02 75 06 5d c3 cc cc cc cc 9c 58 f6 c4 02 75 f3 <0f> 0b 5d c3 cc cc cc cc 90 90 90 90 90 90 90 90 90 90 90 90 90 90
[   83.338311] RSP: 0000:ffffafbc417d3cb8 EFLAGS: 00010046
[   83.338313] RAX: 0000000000000006 RBX: 00000000df53adbc RCX: ffffafbc417d3d68
[   83.338314] RDX: 00000000000002b4 RSI: 0000000000000060 RDI: ffffffff9782ead0
[   83.338315] RBP: ffffafbc417d3cb8 R08: ffffafbc417d3e48 R09: 0000000000000001
[   83.338316] R10: 0000000000000001 R11: 0000000000000000 R12: 00000000df53af40
[   83.338317] R13: ffffafbc417d3db8 R14: 0000000002000000 R15: 0000000000000000
[   83.338318] FS:  0000000000000000(0000) GS:ffff966e2e000000(0063) knlGS:00000000df53bb40
[   83.338320] CS:  0010 DS: 002b ES: 002b CR0: 0000000080050033
[   83.338321] CR2: 00000000f7ee7080 CR3: 0000000006550000 CR4: 0000000000350ee0
[   83.338323] Call Trace:
[   83.338325]  <TASK>
[   83.338328]  copy_fpstate_to_sigframe+0xaa/0x410
[   83.338335]  get_sigframe+0xdc/0x2e0
[   83.338339]  ia32_setup_rt_frame+0x67/0x2f0
[   83.338343]  dovetail_setup_rt_signal_frame+0x43/0x50
[   83.338346]  xnthread_handle_rt_signals+0xf2/0x130
[   83.338349]  ? handle_head_syscall+0x179/0x550
[   83.338352]  ? __pfx_CoBaLt_sigaction+0x10/0x10
[   83.338355]  ? handle_head_syscall+0x179/0x550
[   83.338357]  ? __pfx_CoBaLt_sigaction+0x10/0x10
[   83.338359]  handle_oob_trap_entry+0x1b6/0x3c0
[   83.338362]  ? syscall_enter_from_user_mode_work+0x31/0xb0
[   83.338366]  __oob_trap_notify+0x2f/0x40
[   83.338368]  exc_invalid_op+0xb7/0x110
[   83.338371]  asm_exc_invalid_op+0x1f/0x30
[   83.338374] RIP: 0023:0x5659868a
[   83.338376] Code: Unable to access opcode bytes at 0x56598660.
[   83.338377] RSP: 002b:00000000df53b1c0 EFLAGS: 00010246
[   83.338378] RAX: 00000000f453ff4d RBX: 000000005659b000 RCX: 0000000000000000
[   83.338379] RDX: 0000000000000012 RSI: 00000000f453ff4d RDI: 00000000f7ed3ac8
[   83.338380] RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000000
[   83.338381] R10: 0000000000000000 R11: 0000000000000296 R12: 0000000000000000
[   83.338382] R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000
[   83.338386]  </TASK>
[   83.338387] irq event stamp: 0
[   83.338388] hardirqs last  enabled at (0): [<0000000000000000>] 0x0
[   83.338389] hardirqs last disabled at (0): [<ffffffff95cb68eb>] copy_process+0x7fb/0x21b0
[   83.338393] softirqs last  enabled at (0): [<ffffffff95cb68ef>] copy_process+0x7ff/0x21b0
[   83.338395] softirqs last disabled at (0): [<0000000000000000>] 0x0
[   83.338396] ---[ end trace 0000000000000000 ]---

Do you think, this is related to the message I get on execution.

Other question: By "stress faulting" you mean running tests while generating a lot of cpu load?

  Johannes
> 
> >>
> >>> Sorry for not putting more effort into this so fare.
> >>>
> >>> The next most important step in my opinion would be to define a clean
> interface
> >> into dovetail for the signal frame setup and restoring.
> >>> Actually that's the task I consider one of the harder and I am not so sure how
> to
> >> approach this.
> >>>
> >>
> >> Exactly, and we need to involve Philippe now soon. Let me medidate over
> >> some interface options as well. Will pick up if you don't find the time
> >> soon.
> > At the moment, I am not event sure, what the next task would be to continue
> with this feature.
> > As I am not sure how to define the interface for dovetail, I hesitate a little bit to
> change the code as it is right now.
> >
> > What I could do is have a look why my hist-tool breaks in the 32bit user space
> case.
> >
> 
> Agree, this needs to be understood.
> 
> > I also could try to provide an idea for the interface, but I am not so sure if it is of
> good use, as my knowledge of the Kernels internal workings is still rather limited.
> 
> In general, requests from the co-kernel core to switch to a signal stack
> (signal setup) or restore from it (sigreturn) sounds like a reasonable
> abstraction to me for now.
> 
> Jan
> 
> --
> Siemens AG, Technology
> Competence Center Embedded Linux