Messages in this thread |  | | Subject | Re: UML time-travel warning from __run_timers | From | Johannes Berg <> | Date | Sat, 02 Apr 2022 16:09:29 +0200 |
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Hi Vincent,
> [10737482.720000][ C0] ------------[ cut here ]------------ > [10737482.720000][ C0] WARNING: CPU: 0 PID: 0 at kernel/time/timer.c:1729 __run_timers+0x36d/0x380 >
[for those new on the thread, full message and config here: https://lore.kernel.org/r/20220330110156.GA9250@axis.com]
I think maybe you found a bug in the timers code?
Your config has CONFIG_NO_HZ_COMMON, so we have both BASE_STD and BASE_DEF.
Evidently, in your config, we *never* have any timer with TIMER_DEFERRABLE, which would put it into BASE_DEF.
(I put a WARN_ON into get_timer_cpu_base() and get_timer_this_cpu_base() in the if, and it never triggered; I guess my config has something that creates a deferrable timer, so it didn't trigger, but I didn't check that now.)
Therefore, base->next_expiry never changes or something?
At init, we get
init_timer_cpu(0) base 0 clk=0xffff8ad0, next_expiry=0x13fff8acf init_timer_cpu(0) base 1 clk=0xffff8ad0, next_expiry=0x13fff8acf
which makes sense, jiffies is set up to wrap very quickly after boot.
The warning triggers when we have jiffies=0x13fff9600, so it's just after the "next_expiry", so in this code:
static inline void __run_timers(struct timer_base *base) { struct hlist_head heads[LVL_DEPTH]; int levels;
if (time_before(jiffies, base->next_expiry)) return;
we no longer return. Previously, we've *never* executed past that if for BASE_DEF.
But we never touched this timer base nor did we ever want to recalc it I guess, so
WARN_ON_ONCE(!levels && !base->next_expiry_recalc);
triggers.
I thought about changing that condition to
if (time_before(...) || !base->timers_pending) return;
and that *does* make the splat go away, but I fear that might make it not recalculate when needed, so perhaps in the condition we should have
if (time_before(...) || (!base->timers_pending && !base->next_expiry_recalc)) return;
or something? (which also avoids hitting the warning)
But I really don't know anything about this code, so adding a few CCs. Can you help?
Thanks, johannes
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