linux-kernel - Re: [patch V9 04/39] x86/entry: Provide idtentry_entry/exit_cond

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Message-ID: <20200521210610.GA25708@paulmck-ThinkPad-P72>
Date:   Thu, 21 May 2020 14:06:10 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        X86 ML <x86@...nel.org>,
        Alexandre Chartre <alexandre.chartre@...cle.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        Brian Gerst <brgerst@...il.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Will Deacon <will@...nel.org>,
        Tom Lendacky <thomas.lendacky@....com>,
        Wei Liu <wei.liu@...nel.org>,
        Michael Kelley <mikelley@...rosoft.com>,
        Jason Chen CJ <jason.cj.chen@...el.com>,
        Zhao Yakui <yakui.zhao@...el.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>
Subject: Re: [patch V9 04/39] x86/entry: Provide
 idtentry_entry/exit_cond_rcu()

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