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

[Song Liu <songliubraving@fb.com>]

> 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(), 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?

Thanks,
Song