Re: 'perf test sigtrap' failing on PREEMPT_RT_FULL

[Arnaldo Carvalho de Melo <acme@kernel.org>]

In Thu, 4 Jan 2024 19:35:57 -0300, Arnaldo Carvalho de Melo wrote:
> Em Fri, Jul 28, 2023 at 05:07:18PM +0200, Sebastian Andrzej Siewior escreveu:
> > On 2023-07-26 08:10:45 [+0200], Mike Galbraith wrote:
> > > > [   52.848925] BUG: scheduling while atomic: perf/6549/0x00000002

> > > Had bf9ad37dc8a not been reverted due to insufficient beauty, you could
> > > trivially make the sigtrap test a happy camper (wart tested in tip-rt).
 
> > Thank you for the pointer Mike.
 
> > I guess we need this preempt_disable_notrace() in perf_pending_task()
> > due to context accounting in get_recursion_context(). Would a
> > migrate_disable() be sufficient or could we send the signal outside of
> > the preempt-disabled block?

> I got back to this, need to go again over all the callers of
> perf_swevent_get_recursion_context(), from the first quick glance there
> are other places with preempt_disable()/enable(), but doing just the
> switch to migrate disable/enable on perf_pending_task() makes this
> specific test to work:

> [acme@nine linux]$ git log --oneline -5
> 086dab66d504 (HEAD -> linux-rt-devel/linux-6.7.y-rt/send_sig_perf.fix, tag: v6.7-rc5-rt5, linux-rt-devel/linux-6.7.y-rt) v6.7-rc5-rt5
> 29e0d951f39b printk: Update the printk series.
> 2308ecc8ce88 (tag: v6.7-rc5-rt4) v6.7-rc5-rt4
> 10d5f3551216 Merge tag 'v6.7-rc5' into linux-6.7.y-rt
> a39b6ac3781d (tag: v6.7-rc5, linux-rt-devel/master, linux-rt-devel/linux-6.7.y) Linux 6.7-rc5
> [acme@nine linux]$ git diff
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index c9d123e13b57..a9b9ef60f6b3 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -6801,7 +6801,7 @@ static void perf_pending_task(struct callback_head *head)
>         * If we 'fail' here, that's OK, it means recursion is already disabled
>         * and we won't recurse 'further'.
>         */
>-       preempt_disable_notrace();
>+       migrate_disable();
>        rctx = perf_swevent_get_recursion_context();

Pardon my ignorance, is it safe to call preempt_count() with preemption
enabled on PREEMPT_RT, or at least in the context being discussed here?

Because:

	 perf_swevent_get_recursion_context()
	     get_recursion_context()
                 interrupt_context_level()
                     preempt_count()	

And:

int perf_swevent_get_recursion_context(void)
{
        struct swevent_htable *swhash = this_cpu_ptr(&swevent_htable);

        return get_recursion_context(swhash->recursion);
}

>         if (event->pending_work) {
> @@ -6812,7 +6812,7 @@ static void perf_pending_task(struct callback_head *head)
 
>        if (rctx >= 0)
>                 perf_swevent_put_recursion_context(rctx);
> -       preempt_enable_notrace();
> +       migrate_enable();
 
>         put_event(event);
>  }
> [acme@nine linux]$ uname -a
> Linux nine 6.7.0-rc5-rt5.sigtrap-fix-dirty #2 SMP PREEMPT_RT Thu Jan  4 18:11:44 -03 2024 x86_64 x86_64 x86_64 GNU/Linux
> [acme@nine linux]$ sudo su -
> [sudo] password for acme: 
> [root@nine ~]# 
> [root@nine ~]# perf test sigtrap
>  68: Sigtrap                                                         : Ok
> [root@nine ~]# 
> [root@nine ~]# perf probe -L perf_pending_task
> <perf_pending_task@/home/acme/git/linux/kernel/events/core.c:0>
>       0  static void perf_pending_task(struct callback_head *head)
>          {
>       2         struct perf_event *event = container_of(head, struct perf_event, pending_task);
>       3         int rctx;
       
>                 /*
>                  * If we 'fail' here, that's OK, it means recursion is already disabled
>                  * and we won't recurse 'further'.
>                  */
>                 migrate_disable();
>      10         rctx = perf_swevent_get_recursion_context();
>         
>      12         if (event->pending_work) {
>      13                 event->pending_work = 0;
>      14                 perf_sigtrap(event);
>      15                 local_dec(&event->ctx->nr_pending);
>                 }
>         
>      18         if (rctx >= 0)
>      19                 perf_swevent_put_recursion_context(rctx);
>      20         migrate_enable();
     
>      22         put_event(event);
>          }
         
>          #ifdef CONFIG_GUEST_PERF_EVENTS

> [root@nine ~]# perf probe perf_pending_task
> Added new event:
>   probe:perf_pending_task (on perf_pending_task)

> You can now use it in all perf tools, such as:

> 	perf record -e probe:perf_pending_task -aR sleep 1

> [root@nine ~]# perf trace --max-events=1 -e probe:perf_pending_task/max-stack=6/ perf test sigtrap 
>  68: Sigtrap                                                         : Ok
>      0.000 :9608/9608 probe:perf_pending_task(__probe_ip: -2064408784)
>                                        perf_pending_task ([kernel.kallsyms])
>                                        task_work_run ([kernel.kallsyms])
>                                        exit_to_user_mode_loop ([kernel.kallsyms])
>                                        exit_to_user_mode_prepare ([kernel.kallsyms])
>                                        irqentry_exit_to_user_mode ([kernel.kallsyms])
>                                        asm_sysvec_irq_work ([kernel.kallsyms])
> [root@nine ~]#

> [root@nine ~]# head -5 /etc/os-release
> NAME="Red Hat Enterprise Linux"
> VERSION="9.2 (Plow)"
> ID="rhel"
> ID_LIKE="fedora"
> VERSION_ID="9.2"
> [root@nine ~]#

> I did the test without the above patch and the original problem is
> reproduced.
 
> > This is also used in perf_pending_irq() and on PREEMPT_RT this is
> > invoked from softirq context which is preemptible.

Humm, and then when going thru perf_pending_irq() we don't hit that
scheduling on atomic.

- Arnaldo