linux-kernel - Re: [PATCH 2/2] convert to syscall tracepoints

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Date:	Tue, 9 Jun 2009 00:00:13 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Jason Baron <jbaron@...hat.com>
Cc:	"H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
	fweisbec@...il.com, laijs@...fujitsu.com, rostedt@...dmis.org,
	peterz@...radead.org, mathieu.desnoyers@...ymtl.ca,
	jiayingz@...gle.com, mbligh@...gle.com, roland@...hat.com,
	fche@...hat.com
Subject: Re: [PATCH 2/2] convert to syscall tracepoints

* Jason Baron <jbaron@...hat.com> wrote:

> Yes, this can easily be done....but that wasn't the problem I was 
> interested in solving. I wanted a per syscall tracepoint site. I 
> thought I had been making that clear all along...

We can do that, but _not in the syscall_.

There's absolutely nothing wrong with having an array of 
tracepoints/callbacks that is being called by the TIF_FTRACE code. 
We already have per syscall attributes - that can include a callback 
too.

> [...] Please notice that the implementation I've proposed obtains 
> the syscall number, and then jumps to the appropriate tracepoint 
> and then exits. Its quite efficient. In fact, I've enabled all of 
> the syscalls using my proposed method and running tbench I'm able 
> to get more throughput then using the current syscall method. I've 
> also done 'getpid()' loops and seen no performance difference 
> between the approaches. I'm happy to run any other benchmarks...

If you cannot see a performance difference in getpid() loop that's 
possibly because glibc caches getpid() results ...

Try getppid().

Anyway - bloating every syscall with a tracepoint check is out of 
question and unnecessary. We already have the TIF_ flag based 
machinery to call back on syscalls - and what we need is the proper 
type/field info extracted and /debug/tracing/syscalls/* entries 
created.

	Ingo
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