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

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

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?