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

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Date:	Thu, 29 Mar 2012 12:02:04 -0700
From:	Vaibhav Nagarnaik <vnagarnaik@...gle.com>
To:	Ingo Molnar <mingo@...nel.org>
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

On Wed, Mar 28, 2012 at 11:20 PM, Ingo Molnar <mingo@...nel.org> wrote:
>
> * 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 for the feedback. I had missed this added latency due to this
patch when tracing is disabled.

To fix that, instead of a TIF flag, I am using a flag in the
current->trace bitmap. I check that flag before jumping to the tracing
function. That reduces the latency from 83 ns/call to 74 ns/call.


Thanks

Vaibhav Nagarnaik
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