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

[Steven Rostedt]

On Sun, 11 Dec 2022 00:30:36 +0100
Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:

> 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.
> 

The point of the patch is to just fix the lockdep issue. I'm happy to
remove that "no_resched" (I was planning to), but that would be a separate
change, with a different purpose, and thus a separate patch.

-- Steve