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From:   Thomas Gleixner <tglx@linutronix.de>
To:     Steven Rostedt <rostedt@goodmis.org>,
        "Paul E. McKenney" <paulmck@kernel.org>
Cc:     linux-kernel@vger.kernel.org,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Karol Herbst <karolherbst@gmail.com>,
        Pekka Paalanen <ppaalanen@gmail.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Andy Lutomirski <luto@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@kernel.org>, Borislav Petkov <bp@alien8.de>
Subject: Re: [for-next][PATCH 13/25] x86/mm/kmmio: Use
 rcu_read_lock_sched_notrace()
In-Reply-To: <20221210133425.4657985e@gandalf.local.home>
References: <20221210135750.425719934@goodmis.org>
 <20221210135825.241167123@goodmis.org>
 <20221210174753.GD4001@paulmck-ThinkPad-P17-Gen-1>
 <20221210133425.4657985e@gandalf.local.home>
Date:   Sun, 11 Dec 2022 00:30:36 +0100
Message-ID: <87359mdeg3.ffs@tglx>
MIME-Version: 1.0
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On Sat, Dec 10 2022 at 13:34, Steven Rostedt wrote:
> On Sat, 10 Dec 2022 09:47:53 -0800 "Paul E. McKenney" <paulmck@kernel.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.

Thanks,

        tglx