Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Date:   Wed, 22 Mar 2023 13:52:57 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Steven Rostedt <rostedt@goodmis.org>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux Trace Kernel <linux-trace-kernel@vger.kernel.org>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Ross Zwisler <zwisler@google.com>,
        Joel Fernandes <joel@joelfernandes.org>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Miroslav Benes <mbenes@suse.cz>
Subject: Re: [PATCH] tracing: Trace instrumentation begin and end
Message-ID: <20230322125257.GA2357380@hirez.programming.kicks-ass.net>
References: <20230321215121.71b339c5@gandalf.local.home>
 <87y1np824t.ffs@tglx>
 <20230322084834.37ed755e@gandalf.local.home>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20230322084834.37ed755e@gandalf.local.home>
Precedence: bulk

On Wed, Mar 22, 2023 at 08:48:34AM -0400, Steven Rostedt wrote:
> On Wed, 22 Mar 2023 12:19:14 +0100
> Thomas Gleixner <tglx@linutronix.de> wrote:
> 
> > Steven!
> > 
> > On Tue, Mar 21 2023 at 21:51, Steven Rostedt wrote:
> > > From: "Steven Rostedt (VMware)" <rostedt@goodmis.org>
> > > produces:
> > >
> > >  2)   0.764 us    |  exit_to_user_mode_prepare();
> > >  2)               |  /* page_fault_user: address=0x7fadaba40fd8 ip=0x7fadaba40fd8 error_code=0x14 */
> > >  2)   0.581 us    |  down_read_trylock();
> > >
> > > The "page_fault_user" event is not encapsulated around any function, which
> > > means it probably triggered and went back to user space without any trace
> > > to know how long that page fault took (the down_read_trylock() is likely to
> > > be part of the page fault function, but that's besides the point).
> > >
> > > To help bring back the old functionality, two trace points are added. One
> > > just after instrumentation begins, and one just before it ends. This way,
> > > we can see all the time that the kernel can do something meaningful, and we
> > > will trace it.  
> > 
> > Seriously? That's completely insane. Have you actually looked how many
> > instrumentation_begin()/end() pairs are in the affected code pathes?
> > 
> > Obviously not. It's a total of _five_ for every syscall and at least
> > _four_ for every interrupt/exception from user mode.
> > 
> > The number #1 design rule for instrumentation is to be as non-intrusive as
> > possible and not to be as lazy as possible.
> 
> And it still is. It still uses nops when not enabled. I could even add a
> config to only have this compiled in when the config is set, so that
> production can disable it if it wants to.
> 
> Just in case it's not obvious:
> 
> 	if (tracepoint_enabled(instrumentation_begin))
> 		call_trace_instrumentation_begin(ip, pip);
> 
> is equivalent to:
> 
> 	if (static_key_false(&__tracepoint_instrumentation_begin.key))
> 		call_trace_instrumentation_begin(ip, pip);
> 

It is still completely insane.

NAK.