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

David Sharp
2012-03-29T22:45:09Z

On Thu, Mar 29, 2012 at 12:12 PM, H. Peter Anvin <hpa@zytor.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.