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Date:   Thu, 21 May 2020 14:06:10 -0700
From:   "Paul E. McKenney" <>
To:     Thomas Gleixner <>
Cc:     LKML <>,
        Andy Lutomirski <>,
        Andrew Cooper <>,
        X86 ML <>,
        Alexandre Chartre <>,
        Frederic Weisbecker <>,
        Paolo Bonzini <>,
        Sean Christopherson <>,
        Masami Hiramatsu <>,
        Petr Mladek <>,
        Steven Rostedt <>,
        Joel Fernandes <>,
        Boris Ostrovsky <>,
        Juergen Gross <>,
        Brian Gerst <>,
        Mathieu Desnoyers <>,
        Josh Poimboeuf <>,
        Will Deacon <>,
        Tom Lendacky <>,
        Wei Liu <>,
        Michael Kelley <>,
        Jason Chen CJ <>,
        Zhao Yakui <>,
        "Peter Zijlstra (Intel)" <>
Subject: Re: [patch V9 04/39] x86/entry: Provide

On Thu, May 21, 2020 at 10:05:17PM +0200, Thomas Gleixner wrote:
> From: Thomas Gleixner <>
> 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

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