On 2023/8/19 04:07, paul.gortmaker@xxxxxxxxxxxxx wrote:
From: Paul Gortmaker <paul.gortmaker@xxxxxxxxxxxxx>
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@xxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Paul E. McKenney <paulmck@xxxxxxxxxx>
Cc: Frederic Weisbecker <frederic@xxxxxxxxxx>
Signed-off-by: Paul Gortmaker <paul.gortmaker@xxxxxxxxxxxxx>
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@xxxxxxxxxxx>
--
Thanks,
Wen
