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 15:44:41 -0700
From:	David Sharp <dhsharp@...gle.com>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Vaibhav Nagarnaik <vnagarnaik@...gle.com>,
	Ingo Molnar <mingo@...nel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.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 Thu, Mar 29, 2012 at 12:12 PM, H. Peter Anvin <hpa@...or.com> wrote:
> On 03/29/2012 12:02 PM, Vaibhav Nagarnaik wrote:
>>
>> 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.
>>
>
> ANY increase to the fastpath is unacceptable, period.

I think the last 4 ns would probably be eliminated if we could figure
out how to call the inline function trace_sys_enter directly instead
of through the out-of-line wrapper we had to add to work around the
preprocessor magic.
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