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Message-ID: <ZdZOYnIkjqkyfo5P@x1>
Date: Wed, 21 Feb 2024 16:26:26 -0300
From: Arnaldo Carvalho de Melo <acme@...nel.org>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc: Mike Galbraith <efault@....de>, Peter Zijlstra <peterz@...radead.org>,
Marco Elver <elver@...gle.com>, linux-rt-users@...r.kernel.org,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
Juri Lelli <juri.lelli@...hat.com>,
Clark Williams <williams@...hat.com>,
Alessandro Carminati <acarmina@...hat.com>,
Thomas Gleixner <tglx@...utronix.de>,
Artem Savkov <asavkov@...hat.com>
Subject: Re: 'perf test sigtrap' failing on PREEMPT_RT_FULL
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@...e 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@...e 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@...e 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@...e linux]$ sudo su -
> [sudo] password for acme:
> [root@...e ~]#
> [root@...e ~]# perf test sigtrap
> 68: Sigtrap : Ok
> [root@...e ~]#
> [root@...e ~]# perf probe -L perf_pending_task
> <perf_pending_task@...me/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@...e ~]# 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@...e ~]# 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@...e ~]#
> [root@...e ~]# head -5 /etc/os-release
> NAME="Red Hat Enterprise Linux"
> VERSION="9.2 (Plow)"
> ID="rhel"
> ID_LIKE="fedora"
> VERSION_ID="9.2"
> [root@...e ~]#
> 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
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