am335x: 5.18.x: system stalling

[Yegor Yefremov]

On Thu, Jun 2, 2022 at 12:37 PM Ard Biesheuvel <ardb at kernel.org> wrote:
>
> On Thu, 2 Jun 2022 at 12:17, Yegor Yefremov <yegorslists at googlemail.com> wrote:
> >
> > On Wed, Jun 1, 2022 at 12:50 PM Ard Biesheuvel <ardb at kernel.org> wrote:
> > >
> > > On Wed, 1 Jun 2022 at 12:46, Yegor Yefremov <yegorslists at googlemail.com> wrote:
> > > >
> > > > On Wed, Jun 1, 2022 at 12:06 PM Ard Biesheuvel <ardb at kernel.org> wrote:
> > > > >
> > > > > On Wed, 1 Jun 2022 at 12:04, Yegor Yefremov <yegorslists at googlemail.com> wrote:
> > > > > >
> > > > > > On Wed, Jun 1, 2022 at 11:28 AM Ard Biesheuvel <ardb at kernel.org> wrote:
> > > > > > >
> > > > > > > On Wed, 1 Jun 2022 at 10:08, Ard Biesheuvel <ardb at kernel.org> wrote:
> > > > > > > >
> > > > > > > > On Wed, 1 Jun 2022 at 09:59, Arnd Bergmann <arnd at arndb.de> wrote:
> > > > > > > > >
> > > > > > > > > On Wed, Jun 1, 2022 at 9:36 AM Yegor Yefremov
> > > > > > > > > <yegorslists at googlemail.com> wrote:
> > > > > > > > > > On Tue, May 31, 2022 at 5:23 PM Arnd Bergmann <arnd at arndb.de> wrote:
> > > > > > > > > > > I've pushed a modified branch now, with that fix on the broken commit,
> > > > > > > > > > > and another change to make CONFIG_IRQSTACKS user-selectable rather
> > > > > > > > > > > than always enabled. That should tell us if the problem is in the SMP
> > > > > > > > > > > patching or in the irqstacks.
> > > > > > > > > > >
> > > > > > > > > > > Can you test the top of this branch with CONFIG_IRQSTACKS disabled,
> > > > > > > > > > > and (if that still stalls) retest the fixed commit f0191ea5c2e5 ("[PART 1]
> > > > > > > > > > > ARM: implement THREAD_INFO_IN_TASK for uniprocessor systems")?
> > > > > > > > > >
> > > > > > > > > > 1. the top of this branch with CONFIG_IRQSTACKS disabled stalls
> > > > > > > > > > 2. f0191ea5c2e5 with the same config - not
> > > > > > > > >
> > > > > > > > > Ok, perfect, that does narrow down the problem quite a bit: The final
> > > > > > > > > patch has seven changes, all of which can be done individually because
> > > > > > > > > in each case the simplified version in f0191ea5c2e5 is meant to run
> > > > > > > > > the exact same instructions as the version after the change, when running
> > > > > > > > > on a uniprocessor machine such as your am335x.
> > > > > > > > >
> > > > > > > > > You have already shown earlier that the get_current() and
> > > > > > > > > __my_cpu_offset() functions are not to blame here, as reverting
> > > > > > > > > only those does not change the behavior.
> > > > > > > > >
> > > > > > > > > This leaves the is_smp() check in set_current(), and the
> > > > > > > > > four macros in <asm/assembler.h>. I don't see anything obviously
> > > > > > > > > wrong with any of those five, but I would bet on the macros
> > > > > > > > > here. Can you try bisecting into this commit, maybe reverting
> > > > > > > > > the changes to set_current and get_current first, and then
> > > > > > > > > narrowing it down to (hopefully) a single macro that causes the
> > > > > > > > > problem?
> > > > > > > > >
> > > > > > > >
> > > > > > > > set_current() is never called by the primary CPU, which is why the
> > > > > > > > is_smp() check was removed from there in 57a420435edcb0b94 ("ARM: drop
> > > > > > > > pointless SMP check on secondary startup path").
> > > > > > > >
> > > > > > > > So that leaves only the four macros in asm/assembler.h, but I don't
> > > > > > > > see anything obviously wrong with those either.
> > > > > > >
> > > > > > > I pushed a patch on top of Arnd's branch at the link below that gets
> > > > > > > rid of the subsections, and uses normal branches (and code patching)
> > > > > > > to switch between the thread ID register and the LDR to retrieve the
> > > > > > > CPU offset and the current pointer. I have no explanation whether or
> > > > > > > why it could make a difference, but I think it's worth a try.
> > > > > >
> > > > > > The link to your repo is missing.
> > > > > >
> > > > >
> > > > > Oops, sorry :-)
> > > > >
> > > > > https://git.kernel.org/pub/scm/linux/kernel/git/ardb/linux.git/log/?h=am335x-stall-test
> > > >
> > > > I have tested your branch and it stalls:
> > > >
> > >
> > > OK, thanks for verifying.
> >
> > My bisection results for f0191ea5c2e5aab29484ede0493ca385eec5472f as a base:
> >
> > percpu.h: sporadic stalls
> > current.h: always stalls
> > assembler.h: no stalls
> > smp.c: no stalls
> >
>
> So you mean that applying the changes to each of those files in
> isolation to the baseline in f0191ea5c2e5aab29484ede0493ca385eec5472f
> produces those results, right?

Right.

> That confirms my statement that smp.c cannot be the culprit, and
> appears to exonerate the pure asm pieces. I wonder if this is related
> to insufficient asm constraints on the C helpers, or just the cost
> model taking different decisions because the inline asm string is much
> longer. In any case, this opens up a couple of avenues we could
> explore to narrow this down further.
>
> As a quick check, can you try the below snippet applied onto the
> broken current.h build?
>
> --- a/arch/arm/include/asm/current.h
> +++ b/arch/arm/include/asm/current.h
> @@ -53,7 +53,8 @@ static __always_inline __attribute_const__ struct
> task_struct *get_current(void)
>             "   b       . + (2b - 0b)                           \n\t"
>             "   .popsection                                     \n\t"
>  #endif
> -           : "=r"(cur));
> +           : "=r"(cur)
> +           : "Q" (*(const unsigned long *)current_stack_pointer));

Where is the current_stack_pointer defined?

>  #elif __LINUX_ARM_ARCH__>= 7 || \
>        !defined(CONFIG_ARM_HAS_GROUP_RELOCS) || \
>        (defined(MODULE) && defined(CONFIG_ARM_MODULE_PLTS))
>
> Given that the problematic sequence appears to be in C code, could you
> please confirm whether or not the stall is reproducible when all the
> pieces that are used by the CAN stack (musb, slcan, ftdio-sio, etc)
> are built into the kernel rather than built as modules? Also, which
> GCC version are you using?

For now, the CAN stack parts are built as modules. I'll try to compile them in.

I'm using GCC 10.x

Yegor