linux-kernel - Re: [patch V6 12/37] x86/entry: Provide idtentry_entry/exit_cond

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Date:   Tue, 19 May 2020 23:20:19 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Andy Lutomirski <luto@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        "Paul E. McKenney" <paulmck@...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 V6 12/37] x86/entry: Provide idtentry_entry/exit_cond_rcu()

Andy Lutomirski <luto@...nel.org> writes:
> On Tue, May 19, 2020 at 1:20 PM Thomas Gleixner <tglx@...utronix.de> wrote:
>> Thomas Gleixner <tglx@...utronix.de> writes:
>> It's about this:
>>
>> rcu_nmi_enter()
>> {
>>         if (!rcu_is_watching()) {
>>             make it watch;
>>         } else if (!in_nmi()) {
>>             do_magic_nohz_dyntick_muck();
>>         }
>>
>> So if we do all irq/system vector entries conditional then the
>> do_magic() gets never executed. After that I got lost...
>
> I'm also baffled by that magic, but I'm also not suggesting doing this
> to *all* entries -- just the not-super-magic ones that use
> idtentry_enter().
>
> Paul, what is this code actually trying to do?

Citing Paul from IRC:

  "The way things are right now, you can leave out the rcu_irq_enter()
   if this is not a nohz_full CPU.

   Or if this is a nohz_full CPU, and the tick is already
   enabled, in that case you could also leave out the rcu_irq_enter().

   Or even if this is a nohz_full CPU and it does not have the tick
   enabled, if it has been in the kernel less than a few tens of
   milliseconds, still OK to avoid invoking rcu_irq_enter()

   But my guess is that it would be a lot simpler to just always call
   it.

Hope that helps.

Thanks,

        tglx