Re: [RFC] sched,livepatch: call klp_try_switch_task in __cond_resched

[Rik van Riel <riel@fb.com>]

On Sat, 2022-05-07 at 19:04 +0000, Song Liu wrote:
> > On May 7, 2022, at 11:26 AM, Rik van Riel <riel@fb.com> wrote:
> > 
> > On Sat, 2022-05-07 at 10:46 -0700, Song Liu wrote:
> > > Busy kernel threads may block the transition of livepatch. Call
> > > klp_try_switch_task from __cond_resched to make the transition
> > > easier.
> > > 
> > That seems like a useful idea given what we're seeing on
> > some systems, but I do have a nitpick with your patch :)
> > 
> > > +++ b/kernel/sched/core.c
> > > @@ -6990,6 +6990,9 @@ SYSCALL_DEFINE0(sched_yield)
> > >  #if !defined(CONFIG_PREEMPTION) ||
> > > defined(CONFIG_PREEMPT_DYNAMIC)
> > >  int __sched __cond_resched(void)
> > >  {
> > > +       if (unlikely(klp_patch_pending(current)))
> > > +               klp_try_switch_task(current);
> > > +
> > >         if (should_resched(0)) {
> > >                 preempt_schedule_common();
> > >                 return 1;
> > 
> > While should_resched and klp_patch_pending check the same
> > cache line (task->flags), now there are two separate
> > conditionals on this.
> > 
> > Would it make sense to fold the tests for TIF_NEED_RESCHED
> > and TIF_PATCH_PENDING int should_resched(), and then re-do
> > the test for TIF_PATCH_PENDING only if should_resched()
> > returns true?
> 
> x86 has a different version of should_resched(), 

Huh, I just looked at that, and the x86 should_resched()
only seems to check that we _can_ resched, not whether
we should...


/*
 * Returns true when we need to resched and can (barring IRQ state).
 */
static __always_inline bool should_resched(int preempt_offset)
{
        return unlikely(raw_cpu_read_4(__preempt_count) ==
preempt_offset);
}

I wonder if that was intended, and why, or whether
the x86 should_resched should also be checking for
TIF_NEED_RESCHED?

If the latter, the check for TIF_PATCH_PENDING could
just be merged there, too. Probably in the same macro
called from both places.


> so I am not
> quite sure what’s the right way o modify shhould_resched(). 
> OTOH, we can probably see should_resched() as-is and just 
> move klp_patch_pending, like
> 
> int __sched __cond_resched(void)
> {
>         if (should_resched(0)) {
>                 if (unlikely(klp_patch_pending(current)))
>                         klp_try_switch_task(current);
> 
>                 preempt_schedule_common();
>                 return 1;
>         }
> #ifndef CONFIG_PREEMPT_RCU
>         rcu_all_qs();
> #endif
>         return 0;
> }
> 
> Given live patch user space usually waits for many seconds, 
> I guess this should work?

That should certainly work on x86, where should_resched
seems to always return true when we can reschedule?