Re: [PATCH v2 5/6] entry,hrtimer: Push reprogramming timers into the interrupt return path

[Thomas Gleixner <tglx@kernel.org>]

On Mon, Feb 02 2026 at 17:33, Peter Zijlstra wrote:
> On Mon, Feb 02, 2026 at 03:37:13PM +0100, Thomas Gleixner wrote:
>> On Wed, Jan 21 2026 at 17:20, Peter Zijlstra wrote:
>> >  	while (ti_work & EXIT_TO_USER_MODE_WORK_LOOP) {
>> >  
>> > +		/*
>> > +		 * If hrtimer need re-arming, do so before enabling IRQs,
>> > +		 * except when a reschedule is needed, in that case schedule()
>> > +		 * will do this.
>> > +		 */
>> > +		if ((ti_work & (_TIF_NEED_RESCHED |
>> > +				_TIF_NEED_RESCHED_LAZY |
>> > +				_TIF_HRTIMER_REARM)) == _TIF_HRTIMER_REARM)
>> > +			hrtimer_rearm();
>> 
>> Two things I'm not convinced that they are handled correctly:
>> 
>>     1) Interrupts
>> 
>>        After reenabling interrupts and before reaching schedule() an
>>        interrupt comes in and runs soft interrupt processing for a while
>>        on the way back, which delays the update until that processing
>>        completes.
>
> So the basic thing looks like:
>
>       <USER-MODE>
> 	irqentry_enter()
> 	run_irq_on_irqstack_cond()
> 	  if (user_mode() || hardirq_stack_inuse)
> 	    irq_enter_rcu();
> 	    func_c();
> 	    irq_exit_rcu()
> 	      __irq_exit_rcu()
> 	        invoke_softirq()
> 	irqentry_exit()
> 	  irqentry_exit_to_user_mode()
> 	    irqentry_exit_to_user_mode_prepare()
> 	      __exit_to_user_mode_prepare()
> 	        exit_to_user_mode_loop() 
> 		  ...here...
>
> So a nested IRQ at this point will have !user_mode(), but I think it can
> still end up in softirqs due to that hardirq_stack_inuse. Should we
> perhaps make sure only user_mode() ends up in softirqs?

All interrupts independent of the mode they hit are ending up in
irq_exit_rcu() and therefore in __irq_exit_rcu()

 	run_irq_on_irqstack_cond()
 	  if (user_mode() || hardirq_stack_inuse)
            // Stay on user or hardirq stack
 	    irq_enter_rcu();
 	    func_c();
 	    irq_exit_rcu()
         else
            // MAGIC ASM to switch to hardirq stack
            call irq_enter_rcu
            call func_c
            call irq_exit_rcu

The only reason why invoke_softirq() won't be called is when the
interrupt hits into the softirq processing region of the previous
interrupt, which means it's already on the hardirq stack.

But looking at this there is already a problem without interrupt
nesting:

        irq_enter_rcu();
        timer_interrupt()
          hrtimer_interrupt()
            delay_rearm();
        irq_exit_rcu()
          __irq_exit_rcu()
            invoke_softirq()  <- Here

Soft interrupts can run for quite some time, which means this already
can cause timers being delayed for way too long. I think in
__irq_exit_rcu() you want to do:

       if (!in_interrupt() && local_softirq_pending()) {
         hrtimer_rearm();
         invoke_softirq();
       }

Thanks,

        tglx