linux-kernel - Re: UML time-travel warning from __run

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Date:   Sun, 03 Apr 2022 19:19:17 +0200
From:   Johannes Berg <johannes@...solutions.net>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Vincent Whitchurch <vincent.whitchurch@...s.com>
Cc:     linux-um@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Anna-Maria Gleixner <anna-maria@...utronix.de>,
        Frederic Weisbecker <frederic@...nel.org>
Subject: Re: UML time-travel warning from __run_timers

On Sun, 2022-04-03 at 19:13 +0200, Johannes Berg wrote:
> On Sun, 2022-04-03 at 18:18 +0200, Thomas Gleixner wrote:
> > On Sat, Apr 02 2022 at 16:09, Johannes Berg wrote:
> > > 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:
> > 
> > which does not make sense.
> 
> If you say so, I have no idea :)
> 
> > If next_expiry is 0x13fff8acf and jiffies
> > advanced to 0x13fff9600 when the warning triggered, then either it
> > missed to expire the timer at 0x13fff8acf or it failed to recalculate
> > next_expiry.
> 
> There was no timer. If there's ever a timer on this base (BASE_DEF) then
> this doesn't happen.
> 
> So it has to be the latter, but I'm trying to understand in the code
> where it would*ever* recalculate next_expiry if it in fact never
> expires?
> 

Actually, in a sense, this *is* the case of (just) recalculating
next_expiry, no? We just never set next_expiry_recalc since there was
never any timer on this?

So actually this also makes the warning go away:

--- a/kernel/time/timer.c
+++ b/kernel/time/timer.c
@@ -1729,6 +1733,7 @@ static inline void __run_timers(struct timer_base *base)
 		WARN_ON_ONCE(!levels && !base->next_expiry_recalc);
 		base->clk++;
 		base->next_expiry = __next_timer_interrupt(base);
+		base->next_expiry_recalc = !levels;
 
 		while (levels--)
 			expire_timers(base, heads + levels);
@@ -2005,6 +2010,7 @@ static void __init init_timer_cpu(int cpu)
 		raw_spin_lock_init(&base->lock);
 		base->clk = jiffies;
 		base->next_expiry = base->clk + NEXT_TIMER_MAX_DELTA;
+		base->next_expiry_recalc = true;
 		timer_base_init_expiry_lock(base);
 	}
 }


johannes