Re: [patch V9 04/39] x86/entry: Provide idtentry_entry/exit_cond_rcu()

[Paul E. McKenney]

On Thu, May 21, 2020 at 10:05:17PM +0200, Thomas Gleixner wrote:
> From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> 
> After a lengthy discussion [1] it turned out that RCU does not need a full
> rcu_irq_enter/exit() when RCU is already watching. All it needs if
> NOHZ_FULL is active is to check whether the tick needs to be restarted.
> 
> This allows to avoid a separate variant for the pagefault handler which
> cannot invoke rcu_irq_enter() on a kernel pagefault which might sleep.
> 
> The cond_rcu argument is only temporary and will be removed once the
> existing users of idtentry_enter/exit() have been cleaned up. After that
> the code can be significantly simplified.

And yes, I see that the sole call to rcu_irq_enter_check_tick() does
have instrumentation turned on, so good!  ;-)

							Thanx, Paul