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SubjectRe: UML time-travel warning from __run_timers
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:]

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

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))

we no longer return. Previously, we've *never* executed past that if for

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);


I thought about changing that condition to

if (time_before(...) || !base->timers_pending)

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))

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?


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