linux-kernel - Re: [for-next][PATCH 13/25] x86/mm/kmmio: Use rcu_read_lock_sched

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Message-ID: <20221210185555.7abc4de4@gandalf.local.home>
Date:   Sat, 10 Dec 2022 18:55:55 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     "Paul E. McKenney" <paulmck@...nel.org>,
        linux-kernel@...r.kernel.org,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Karol Herbst <karolherbst@...il.com>,
        Pekka Paalanen <ppaalanen@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>
Subject: Re: [for-next][PATCH 13/25] x86/mm/kmmio: Use
 rcu_read_lock_sched_notrace()

On Sun, 11 Dec 2022 00:30:36 +0100
Thomas Gleixner <tglx@...utronix.de> 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@...nel.org> 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