linux-kernel - Re: [BUG] msr-trace.h:42 suspicious rcu_dereference

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Message-ID: <20161121180654.GV26852@two.firstfloor.org>
Date:   Mon, 21 Nov 2016 10:06:54 -0800
From:   Andi Kleen <andi@...stfloor.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Andi Kleen <andi@...stfloor.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Jiri Olsa <jolsa@...hat.com>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
        Josh Triplett <josh@...htriplett.org>,
        Jan Stancek <jstancek@...hat.com>
Subject: Re: [BUG] msr-trace.h:42 suspicious rcu_dereference_check() usage!

> And a popf can be much more expensive than any of these. You should
> know, not all instructions are equal.
> 
> Using perf, I've seen popf take up almst 30% of a function the size of
> this.

In any case it's a small fraction of the 600+ instructions which are currently
executed for every enabled trace point.

If ftrace was actually optimized code this would make some sense, but it
clearly isn't ...

-Andi