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Date: Thu, 29 Mar 2012 08:20:31 +0200
From: Ingo Molnar <mingo@kernel.org>
To: Vaibhav Nagarnaik <vnagarnaik@google.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>, Steven Rostedt <rostedt@goodmis.org>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
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        Justin Teravest <teravest@google.com>,
        Laurent Chavey <chavey@google.com>, x86@kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 4/6] trace: trace syscall in its handler not from ptrace
 handler
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* Vaibhav Nagarnaik <vnagarnaik@google.com> wrote:

> On Wed, Mar 28, 2012 at 7:43 PM, H. Peter Anvin <hpa@zytor.com> wrote:
> > But instead you add a penalty for every syscall, even if tracing is
> > disabled.  Not cool.
> 
> I just ran a small test binary which calls syscall(SYS_getuid) in a
> tight loop and calculates the latency per syscall.
> 
> Without my patch: it is 70 ns/call
> With my patch: it is 83 ns/call
> 
> So yes, it does add a bit of latency to the syscall even if 
> tracing is disabled. I wonder if I can change the redirection 
> function so that it doesn't add so much latency.

There's a really simple rule for anything tracing/debugging 
related: and syscalls don't add *ANY* kind of latency to the 
non-tracing case. That is true of the current syscall tracing 
bits, they work via a TIF flag and don't add any latency.

Thanks,

	Ingo