Re: + lazy-tlb-fix-hotplug-exit-race-with-mmu_lazy_tlb_shootdown.patch added to mm-hotfixes-unstable branch

[Nicholas Piggin]

On Sun Jun 11, 2023 at 5:29 AM AEST, Thomas Gleixner wrote:
> On Thu, May 25 2023 at 13:52, Andrew Morton wrote:
>
> Replying here as I wasn't cc'ed on the patch.
>
> > @@ -1030,6 +1031,8 @@ static int take_cpu_down(void *_param)
> >  	enum cpuhp_state target = max((int)st->target, CPUHP_AP_OFFLINE);
> >  	int err, cpu = smp_processor_id();
> >  
> > +	idle_task_prepare_exit();
> > +
> >  	/* Ensure this CPU doesn't handle any more interrupts. */
> >  	err = __cpu_disable();
> >  	if (err < 0)
> > --- a/kernel/sched/core.c~lazy-tlb-fix-hotplug-exit-race-with-mmu_lazy_tlb_shootdown
> > +++ a/kernel/sched/core.c
> > @@ -9373,19 +9373,33 @@ void sched_setnuma(struct task_struct *p
> >   * Ensure that the idle task is using init_mm right before its CPU goes
> >   * offline.
> >   */
> > -void idle_task_exit(void)
> > +void idle_task_prepare_exit(void)
>
> This function name along with the above comment is completely
> misleading. It suggests this is about the idle task itself instead of
> making it clear that this ensures that the kernel threads of the
> outgoing CPU are not longer using a mm which is not init_mm.
>
> The callsite is arbitratily chosen too. Why does this have to be done
> from stomp machine context?

It's the minimalish fix. My patch didn't change what that idle_task_exit
is attempting to do.

> There is zero reason to do so. The last hotplug state before teardown is
> CPUHP_AP_SCHED_WAIT_EMPTY. It invokes sched_cpu_wait_empty() in the
> context of the CPU hotplug thread of the outgoing CPU.
>
> sched_cpu_wait_empty() guarantees that there are no temporarily pinned
> (via migrate disable) user space tasks on that CPU anymore. The
> scheduler guarantees that there won't be user space tasks woken up on or
> migrated to that CPU because the CPU is not in the cpu_active mask.
>
> The stopper thread has absolutely nothing to do with that.
>
> So sched_cpu_wait_empty() is the obvious place to handle that:
>
> int sched_cpu_wait_empty(unsigned int cpu)
> {
> 	balance_hotplug_wait();
> +	sched_force_init_mm();
> 	return 0;
> }
>
> And then have:
>
> /*
>  * Invoked on the outgoing CPU in context of the CPU hotplug thread
>  * after ensuring that there are no user space tasks left on the CPU.
>  *
>  * If there is a lazy mm in use on the hotplug thread, drop it and
>  * switch to init_mm.
>  *
>  * The reference count on init_mm is dropped in finish_cpu().
>  */
> static void sched_force_init_mm(void)
> {
>
> No?

It could be done in many places. Peter touched it last and it's
been in the tree since prehistoric times.

> >  {
> >  	struct mm_struct *mm = current->active_mm;
> >  
> > -	BUG_ON(cpu_online(smp_processor_id()));
> > -	BUG_ON(current != this_rq()->idle);
> > +	WARN_ON(!irqs_disabled());
> >  
> >  	if (mm != &init_mm) {
> > -		switch_mm(mm, &init_mm, current);
> > +		mmgrab_lazy_tlb(&init_mm);
> > +		current->active_mm = &init_mm;
> > +		switch_mm_irqs_off(mm, &init_mm, current);
> >  		finish_arch_post_lock_switch();
> > +		mmdrop_lazy_tlb(mm);
> >  	}
> > +	/* finish_cpu() will mmdrop the init_mm ref after this CPU stops */
> > +}
> > +
> > +/*
> > + * After the CPU is offline, double check that it was previously switched to
> > + * init_mm. This call can be removed because the condition is caught in
> > + * finish_cpu() as well.
>
> So why adding it in the first place?
>
> The changelog mumbles something about reducing churn, but I fail to see
> that reduction. This adds 10 lines of pointless code and comments for
> zero value.

Not sure what you're talking about. The patch didn't add it. Removing it
requires removing it from all archs, which is the churn.

Thanks,
Nick