Unhandled level 2 translation fault (11) at 0x000000b8, esr 0x92000046, rpi3 (aarch64)

[Bas van Tiel]

On Wed, Jan 11, 2017 at 4:33 PM, Dave Martin <Dave.Martin at arm.com> wrote:
> On Wed, Jan 11, 2017 at 02:49:03PM +0000, Catalin Marinas wrote:
>> On Tue, Jan 10, 2017 at 12:14:23PM +0000, Catalin Marinas wrote:
>> > On Mon, Jan 09, 2017 at 07:06:19PM +0100, Bas van Tiel wrote:
>> > > > I defined STACKSIZE to the kernel's SIGSTKSZ (16384) and it seems to run
>> > > > fine, though I'll leave it longer/overnight (on a Juno board). With the
>> > > > 4K signal stack it was crashing shortly after start.
>> > >
>> > > I tried the STACKSIZE of 16384 for both the RPI3 and the PINEA64 board
>> > > and still see the same behaviour of crashing. Sometimes the process
>> > > is also blocked for a long time before it crashes.
>> > >
>> > > Setting the interval to 200 usec [5 Khz] will help to crash it faster.
>> > >
>> > > To further isolate the issue I will create a kernel module (based on a
>> > > hrtimer) that will sent a periodic signal to the registered process
>> > > and execute the same sighandler logic to check if the problem is still
>> > > there.
>> >
>> > I lowered the interval to 100us (it was 100ms in the original file) and
>> > I can indeed trigger segfault easily on Juno. But it doesn't fail in the
>> > same way every time, I sometimes get permission fault, other times bad
>> > frame.
>>
>> With 100us interval, it segfaults on x86 fairly quickly as well, so I
>> don't think it's a kernel issue.
>
> To be able to take a signal at all, stacks need to be at least SIGSTKSZ
> bytes in practice:
>
>
> diff --git a/context_demo.c b/context_demo.c
> index 2cc63f7..b1f3bbc 100644
> --- a/context_demo.c
> +++ b/context_demo.c
> @@ -22,7 +22,7 @@
>
>
>  #define NUMCONTEXTS 10              /* how many contexts to make */
> -#define STACKSIZE 4096              /* stack size */
> +#define STACKSIZE SIGSTKSZ          /* stack size */
>  #define INTERVAL 100                /* timer interval in nanoseconds */
>
>  sigset_t set;                       /* process wide signal mask */
>
>
> The other issue looks a bit subtler, to do with signal masking.
>
> SIGALRM will be masked on entry to timer_interrupt() and restored on
> return, due to and absence of SA_NODEFER from sa_flags when calling
> sigaction.  (Setting SIGALRM in sa_mask also has this effect, but this
> is redundant without SA_NODEFER.)
>
> However, by explicitly clearing this signal from
> signal_context.uc_sigmask, we'll enter scheduler() with SIGALRM
> unmasked.  If a new SIGALRM is taken before scheduler() has called
> setcontext(), we'll pile up another signal on signal_stack and call
> schedule() again, still on signal_stack ... and this can repeat
> indefinitely.
>
> There's no need to clear SIGALRM from the signal mask: it will be
> cleared when timer_interrupt() returns after resuming an interrupted
> task (as part of the signal frame restore work done by rt_sigreturn).
> So:
>
> @@ -61,7 +61,6 @@ timer_interrupt(int j, siginfo_t *si, void *old_context)
>      signal_context.uc_stack.ss_sp = signal_stack;
>      signal_context.uc_stack.ss_size = STACKSIZE;
>      signal_context.uc_stack.ss_flags = 0;
> -    sigemptyset(&signal_context.uc_sigmask);
>      makecontext(&signal_context, scheduler, 1);
>
>      /* save running thread, jump to scheduler */
>
> For me, this seems to fix the problem.
>
> It also makes sense of what we've seen: we need either short timer
> intervals, slow machines, or high system load (or some combination) in
> order to take enough extra signals in scheduler() to cause a stack
> overflow.

so the piling up of the signals is because the handling time of the
signal takes more than the influx of signals related to the interval
time => userspace issue (programming error), thank you for the
explanation and pointing this out to userspace.

In my case the unmasking of SIGALRM cannot be done, because that would
hide the fact that the processing is too slow compared to the influx.

I did a rerun of the program on the 64-bit rpi3 and was able to have
an interval of 13 [us] in which the contexts are executing
a while(1) with a few nops for 6 hours (isolcpus, sched_fifo,
prio:99). Going below below 13 [us] the segfault occurs.

>
> I can't see the purpose of running scheduler() in its own context here,
> except so that it doesn't contribute stack overhead to the thread
> stacks (which hardly seems worthwhile, since its overhead is probably a
> lot smaller than the signal overhead anyway -- maybe I'm missing
> something).

The reason is to have an HPC usecase with context scheduling in 1
process on a dedicated isolated core (CONFIG_NO_HZ_FULL). The signal
can be seen as an HW/SW-IRQ that can be elevated above the
main-process at all times when it occurs.

> makeconext() and swapcontext() are obsoleted by POSIX.1-2008 and
> considered non-portable (see makecontext(3), swapcontext(3)).  Really,
> the ucontext API should not be used for anything except cooperative
> switching now (certainly this covered the vast majority or real-world
> usage the last time I looked into it).

agree

> For anything else, pthreads
> almost certainly do it better.
>

agree

--
Bas