Re: [for-next][PATCH 13/25] x86/mm/kmmio: Use rcu_read_lock_sched_notrace()

[Thomas Gleixner]

On Sat, Dec 10 2022 at 13:34, Steven Rostedt wrote:
> On Sat, 10 Dec 2022 09:47:53 -0800 "Paul E. McKenney" <paulmck@xxxxxxxxxx> wrote:
>> This does mess with preempt_count() redundantly, but the overhead from
>> that should be way down in the noise.
>
> I was going to remove it, but then I realized that it would be a functional
> change, as from the comment above, it uses "preempt_enable_no_resched(),
> which there is not a rcu_read_unlock_sched() variant.

preempt_enable_no_resched() in this context is simply garbage.

preempt_enable_no_resched() tries to avoid the overhead of checking
whether rescheduling is due after decrementing preempt_count() because
the code which it this claims to know that it is _not_ the outermost one
which brings preempt count back to preemtible state.

I concede that there are hot paths which actually can benefit, but this
code has exactly _ZERO_ benefit from that. Taking that tracing exception
and handling it is orders of magnitudes more expensive than a regular
preempt_enable().

So just get rid of it and don't proliferate cargo cult programming.

Thanks,

        tglx