diff mbox series

tick/rcu: fix false positive "softirq work is pending" messages on RT

Message ID 20230818200757.1808398-1-paul.gortmaker@windriver.com
State New
Headers show
Series tick/rcu: fix false positive "softirq work is pending" messages on RT | expand

Commit Message

Paul Gortmaker Aug. 18, 2023, 8:07 p.m. UTC 
From: Paul Gortmaker <paul.gortmaker@windriver.com>

In commit 0345691b24c0 ("tick/rcu: Stop allowing RCU_SOFTIRQ in idle")
the new function report_idle_softirq() was created by breaking code out
of the existing can_stop_idle_tick() for kernels v5.18 and newer.

In doing so, the code essentially went from a one conditional:

	if (a && b && c)
		warn();

to a three conditional:

	if (!a)
		return;
	if (!b)
		return;
	if (!c)
		return;
	warn();

However, it seems one of the conditionals didn't get a "!" removed.
Compare the instance of local_bh_blocked() in the old code:

-               if (ratelimit < 10 && !local_bh_blocked() &&
-                   (local_softirq_pending() & SOFTIRQ_STOP_IDLE_MASK)) {
-                       pr_warn("NOHZ tick-stop error: Non-RCU local softirq work is pending, handler #%02x!!!\n",
-                               (unsigned int) local_softirq_pending());
-                       ratelimit++;
-               }

...to the usage in the new (5.18+) code:

+       /* On RT, softirqs handling may be waiting on some lock */
+       if (!local_bh_blocked())
+               return false;

It seems apparent that the "!" should be removed from the new code.

This issue lay dormant until another fixup for the same commit was added
in commit a7e282c77785 ("tick/rcu: Fix bogus ratelimit condition").
This commit realized the ratelimit was essentially set to zero instead
of ten, and hence *no* softirq pending messages would ever be issued.

Once this commit was backported via linux-stable, both the v6.1 and v6.4
preempt-rt kernels started printing out 10 instances of this at boot:

  NOHZ tick-stop error: local softirq work is pending, handler #80!!!

Just to double check my understanding of things, I confirmed that the
v5.18-rt did print the pending-80 messages with a cherry pick of the
ratelimit fix, and then confirmed no pending softirq messages were
printed with a revert of mainline's 034569 on a v5.18-rt baseline.

Finally I confirmed it fixed the issue on v6.1-rt and v6.4-rt, and
also didn't break anything on a defconfig of mainline master of today.

Fixes: 0345691b24c0 ("tick/rcu: Stop allowing RCU_SOFTIRQ in idle")
Cc: Wen Yang <wenyang.linux@foxmail.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Paul E. McKenney <paulmck@kernel.org>
Cc: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>

Comments

Wen Yang Aug. 20, 2023, 5:23 p.m. UTC | #1 
On 2023/8/19 04:07, paul.gortmaker@windriver.com wrote:
> From: Paul Gortmaker <paul.gortmaker@windriver.com>
>
> In commit 0345691b24c0 ("tick/rcu: Stop allowing RCU_SOFTIRQ in idle")
> the new function report_idle_softirq() was created by breaking code out
> of the existing can_stop_idle_tick() for kernels v5.18 and newer.
>
> In doing so, the code essentially went from a one conditional:
>
> 	if (a && b && c)
> 		warn();
>
> to a three conditional:
>
> 	if (!a)
> 		return;
> 	if (!b)
> 		return;
> 	if (!c)
> 		return;
> 	warn();
>
> However, it seems one of the conditionals didn't get a "!" removed.
> Compare the instance of local_bh_blocked() in the old code:
>
> -               if (ratelimit < 10 && !local_bh_blocked() &&
> -                   (local_softirq_pending() & SOFTIRQ_STOP_IDLE_MASK)) {
> -                       pr_warn("NOHZ tick-stop error: Non-RCU local softirq work is pending, handler #%02x!!!\n",
> -                               (unsigned int) local_softirq_pending());
> -                       ratelimit++;
> -               }
>
> ...to the usage in the new (5.18+) code:
>
> +       /* On RT, softirqs handling may be waiting on some lock */
> +       if (!local_bh_blocked())
> +               return false;
>
> It seems apparent that the "!" should be removed from the new code.
>
> This issue lay dormant until another fixup for the same commit was added
> in commit a7e282c77785 ("tick/rcu: Fix bogus ratelimit condition").
> This commit realized the ratelimit was essentially set to zero instead
> of ten, and hence *no* softirq pending messages would ever be issued.
>
> Once this commit was backported via linux-stable, both the v6.1 and v6.4
> preempt-rt kernels started printing out 10 instances of this at boot:
>
>    NOHZ tick-stop error: local softirq work is pending, handler #80!!!
>
> Just to double check my understanding of things, I confirmed that the
> v5.18-rt did print the pending-80 messages with a cherry pick of the
> ratelimit fix, and then confirmed no pending softirq messages were
> printed with a revert of mainline's 034569 on a v5.18-rt baseline.
>
> Finally I confirmed it fixed the issue on v6.1-rt and v6.4-rt, and
> also didn't break anything on a defconfig of mainline master of today.
>
> Fixes: 0345691b24c0 ("tick/rcu: Stop allowing RCU_SOFTIRQ in idle")
> Cc: Wen Yang <wenyang.linux@foxmail.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Paul E. McKenney <paulmck@kernel.org>
> Cc: Frederic Weisbecker <frederic@kernel.org>
> Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
>
> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> index 2b865cb77feb..b52e1861b913 100644
> --- a/kernel/time/tick-sched.c
> +++ b/kernel/time/tick-sched.c
> @@ -1050,7 +1050,7 @@ static bool report_idle_softirq(void)
>   		return false;
>   
>   	/* On RT, softirqs handling may be waiting on some lock */
> -	if (!local_bh_blocked())
> +	if (local_bh_blocked())
>   		return false;
>   
>   	pr_warn("NOHZ tick-stop error: local softirq work is pending, handler #%02x!!!\n",


Good catch!

Reviewed-by: Wen Yang <wenyang.linux@foxmail.com>

--
Thanks,
Wen
Paul E. McKenney Aug. 21, 2023, 10:03 p.m. UTC | #2 
On Mon, Aug 21, 2023 at 01:23:15AM +0800, Wen Yang wrote:
> 
> On 2023/8/19 04:07, paul.gortmaker@windriver.com wrote:
> > From: Paul Gortmaker <paul.gortmaker@windriver.com>
> > 
> > In commit 0345691b24c0 ("tick/rcu: Stop allowing RCU_SOFTIRQ in idle")
> > the new function report_idle_softirq() was created by breaking code out
> > of the existing can_stop_idle_tick() for kernels v5.18 and newer.
> > 
> > In doing so, the code essentially went from a one conditional:
> > 
> > 	if (a && b && c)
> > 		warn();
> > 
> > to a three conditional:
> > 
> > 	if (!a)
> > 		return;
> > 	if (!b)
> > 		return;
> > 	if (!c)
> > 		return;
> > 	warn();
> > 
> > However, it seems one of the conditionals didn't get a "!" removed.
> > Compare the instance of local_bh_blocked() in the old code:
> > 
> > -               if (ratelimit < 10 && !local_bh_blocked() &&
> > -                   (local_softirq_pending() & SOFTIRQ_STOP_IDLE_MASK)) {
> > -                       pr_warn("NOHZ tick-stop error: Non-RCU local softirq work is pending, handler #%02x!!!\n",
> > -                               (unsigned int) local_softirq_pending());
> > -                       ratelimit++;
> > -               }
> > 
> > ...to the usage in the new (5.18+) code:
> > 
> > +       /* On RT, softirqs handling may be waiting on some lock */
> > +       if (!local_bh_blocked())
> > +               return false;
> > 
> > It seems apparent that the "!" should be removed from the new code.
> > 
> > This issue lay dormant until another fixup for the same commit was added
> > in commit a7e282c77785 ("tick/rcu: Fix bogus ratelimit condition").
> > This commit realized the ratelimit was essentially set to zero instead
> > of ten, and hence *no* softirq pending messages would ever be issued.
> > 
> > Once this commit was backported via linux-stable, both the v6.1 and v6.4
> > preempt-rt kernels started printing out 10 instances of this at boot:
> > 
> >    NOHZ tick-stop error: local softirq work is pending, handler #80!!!
> > 
> > Just to double check my understanding of things, I confirmed that the
> > v5.18-rt did print the pending-80 messages with a cherry pick of the
> > ratelimit fix, and then confirmed no pending softirq messages were
> > printed with a revert of mainline's 034569 on a v5.18-rt baseline.
> > 
> > Finally I confirmed it fixed the issue on v6.1-rt and v6.4-rt, and
> > also didn't break anything on a defconfig of mainline master of today.
> > 
> > Fixes: 0345691b24c0 ("tick/rcu: Stop allowing RCU_SOFTIRQ in idle")
> > Cc: Wen Yang <wenyang.linux@foxmail.com>
> > Cc: Thomas Gleixner <tglx@linutronix.de>
> > Cc: Peter Zijlstra <peterz@infradead.org>
> > Cc: Paul E. McKenney <paulmck@kernel.org>
> > Cc: Frederic Weisbecker <frederic@kernel.org>
> > Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
> > 
> > diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> > index 2b865cb77feb..b52e1861b913 100644
> > --- a/kernel/time/tick-sched.c
> > +++ b/kernel/time/tick-sched.c
> > @@ -1050,7 +1050,7 @@ static bool report_idle_softirq(void)
> >   		return false;
> >   	/* On RT, softirqs handling may be waiting on some lock */
> > -	if (!local_bh_blocked())
> > +	if (local_bh_blocked())
> >   		return false;
> >   	pr_warn("NOHZ tick-stop error: local softirq work is pending, handler #%02x!!!\n",
> 
> Good catch!
> 
> Reviewed-by: Wen Yang <wenyang.linux@foxmail.com>

Frederic would normally take this, but he appears to be out.  So I am
(probably only temporarily) queueing this in -rcu for more testing
coverage.

							Thanx, Paul
Ahmad Fatoum Aug. 24, 2023, 4 p.m. UTC | #3 
On 18.08.23 22:07, paul.gortmaker@windriver.com wrote:
> From: Paul Gortmaker <paul.gortmaker@windriver.com>
> 
> In commit 0345691b24c0 ("tick/rcu: Stop allowing RCU_SOFTIRQ in idle")
> the new function report_idle_softirq() was created by breaking code out
> of the existing can_stop_idle_tick() for kernels v5.18 and newer.
> 
> In doing so, the code essentially went from a one conditional:
> 
> 	if (a && b && c)
> 		warn();
> 
> to a three conditional:
> 
> 	if (!a)
> 		return;
> 	if (!b)
> 		return;
> 	if (!c)
> 		return;
> 	warn();
> 
> However, it seems one of the conditionals didn't get a "!" removed.
> Compare the instance of local_bh_blocked() in the old code:
> 
> -               if (ratelimit < 10 && !local_bh_blocked() &&
> -                   (local_softirq_pending() & SOFTIRQ_STOP_IDLE_MASK)) {
> -                       pr_warn("NOHZ tick-stop error: Non-RCU local softirq work is pending, handler #%02x!!!\n",
> -                               (unsigned int) local_softirq_pending());
> -                       ratelimit++;
> -               }
> 
> ...to the usage in the new (5.18+) code:
> 
> +       /* On RT, softirqs handling may be waiting on some lock */
> +       if (!local_bh_blocked())
> +               return false;
> 
> It seems apparent that the "!" should be removed from the new code.
> 
> This issue lay dormant until another fixup for the same commit was added
> in commit a7e282c77785 ("tick/rcu: Fix bogus ratelimit condition").
> This commit realized the ratelimit was essentially set to zero instead
> of ten, and hence *no* softirq pending messages would ever be issued.
> 
> Once this commit was backported via linux-stable, both the v6.1 and v6.4
> preempt-rt kernels started printing out 10 instances of this at boot:
> 
>   NOHZ tick-stop error: local softirq work is pending, handler #80!!!
> 
> Just to double check my understanding of things, I confirmed that the
> v5.18-rt did print the pending-80 messages with a cherry pick of the
> ratelimit fix, and then confirmed no pending softirq messages were
> printed with a revert of mainline's 034569 on a v5.18-rt baseline.
> 
> Finally I confirmed it fixed the issue on v6.1-rt and v6.4-rt, and
> also didn't break anything on a defconfig of mainline master of today.
> 
> Fixes: 0345691b24c0 ("tick/rcu: Stop allowing RCU_SOFTIRQ in idle")
> Cc: Wen Yang <wenyang.linux@foxmail.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Paul E. McKenney <paulmck@kernel.org>
> Cc: Frederic Weisbecker <frederic@kernel.org>
> Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>

Tested-by: Ahmad Fatoum <a.fatoum@pengutronix.de>

Thanks,
Ahmad

> 
> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> index 2b865cb77feb..b52e1861b913 100644
> --- a/kernel/time/tick-sched.c
> +++ b/kernel/time/tick-sched.c
> @@ -1050,7 +1050,7 @@ static bool report_idle_softirq(void)
>  		return false;
>  
>  	/* On RT, softirqs handling may be waiting on some lock */
> -	if (!local_bh_blocked())
> +	if (local_bh_blocked())
>  		return false;
>  
>  	pr_warn("NOHZ tick-stop error: local softirq work is pending, handler #%02x!!!\n",
Sebastian Andrzej Siewior Aug. 31, 2023, 1:32 p.m. UTC | #4 
On 2023-08-28 17:03:39 [+0200], Frederic Weisbecker wrote:
> > Frederic would normally take this, but he appears to be out.  So I am
> > (probably only temporarily) queueing this in -rcu for more testing
> > coverage.
> 
> I'm back, I should relay this to Thomas to avoid conflicts with
> timers changes.

I somehow missed this thread and I do see this if I enable NO_HZ. I lost
it…
Anyway, I'm going to pick it up for RT and ping the timer department
after the merge window.

> Thanks all of you, clearly I wasn't thinking much the day I wrote this
> patch.
:)

Sebastian
Thomas Gleixner Sept. 1, 2023, 9:56 a.m. UTC | #5 
On Thu, Aug 31 2023 at 15:32, Sebastian Andrzej Siewior wrote:

> On 2023-08-28 17:03:39 [+0200], Frederic Weisbecker wrote:
>> > Frederic would normally take this, but he appears to be out.  So I am
>> > (probably only temporarily) queueing this in -rcu for more testing
>> > coverage.
>> 
>> I'm back, I should relay this to Thomas to avoid conflicts with
>> timers changes.
>
> I somehow missed this thread and I do see this if I enable NO_HZ. I lost
> it…
> Anyway, I'm going to pick it up for RT and ping the timer department
> after the merge window.

It's queued in timers/urgent and will hit Linus tree before rc1
diff mbox series

Patch

diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 2b865cb77feb..b52e1861b913 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -1050,7 +1050,7 @@  static bool report_idle_softirq(void)
 		return false;
 
 	/* On RT, softirqs handling may be waiting on some lock */
-	if (!local_bh_blocked())
+	if (local_bh_blocked())
 		return false;
 
 	pr_warn("NOHZ tick-stop error: local softirq work is pending, handler #%02x!!!\n",