linux-kernel - Re: [PATCH 4/6] trace: trace syscall in its handler not from ptrace handler

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Message-ID: <20120329062031.GB1376@gmail.com>
Date:	Thu, 29 Mar 2012 08:20:31 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Vaibhav Nagarnaik <vnagarnaik@...gle.com>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	David Sharp <dhsharp@...gle.com>,
	Justin Teravest <teravest@...gle.com>,
	Laurent Chavey <chavey@...gle.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/6] trace: trace syscall in its handler not from ptrace
 handler


* Vaibhav Nagarnaik <vnagarnaik@...gle.com> wrote:

> On Wed, Mar 28, 2012 at 7:43 PM, H. Peter Anvin <hpa@...or.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
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