linux-kernel - Re: [PATCH 1/1] system call notification with self

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Message-ID: <20080912143225.GA108@tv-sign.ru>
Date:	Fri, 12 Sep 2008 18:32:25 +0400
From:	Oleg Nesterov <oleg@...sign.ru>
To:	Pierre Morel <pmorel@...ux.vnet.ibm.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, Roland McGrath <roland@...hat.com>,
	Heiko Carstens <heicars2@...ux.vnet.ibm.com>,
	sameske@...ux.vnet.ibm.com,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Ingo Molnar <mingo@...e.hu>, gregkh@...e.de,
	uml-devel <user-mode-linux-devel@...ts.sourceforge.net>,
	Dave Hansen <dave@...ux.vnet.ibm.com>,
	Cedric Le Goater <clg@...ibm.com>,
	Daniel Lezcano <dlezcano@...ibm.com>
Subject: Re: [PATCH 1/1] system call notification with self_ptrace

Hello Pierre,

On 09/12, Pierre Morel wrote:
>
> You are right, the functionality can be implemented with the system call.
> But it means we have the overhead of a system call just to clear two bits,
> the TIF_SYSCALL_TRACE and the PTS_SELF.

Yes.

So you want to optimize the code for the (imho very exotic) functionality.
And again, the overhead of a system call is nothing compared to the signal
delivery. I bet this overhead won't be visible with any benchmark.

> On the other hand we have an overhead of one single "if" inside
> the handle_signal() function.

What if everyone who wants to add the new functionality will add one
single "if + code" to the core kernel just because he wants to add
a very minor optimization for his needs?

And you forgot about the maintaince overhead. You forgot that this extra
"if" uglifies/complicates the code.

This all is imho of course, and I'm not maintainer. But I promise I
will argue against this change forever ;)

> We can do the same with fork and ptrace, yes, but with a very big 
> overhead on each system call and this is why this patch is so usefull: 
> because with this patch you sit inside the thread when analysing it and 
> have a direct access to all data without the need of IPC, ptrace or any 
> task switch.
>
> I will provide a test program and plan to release a tracing tool based 
> on it.

Yes please, this would be very nice. Please do not count me, but I'm
afraid I am not alone who needs to really understand why this patch
is useful.

Oleg.

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