linux-kernel - Re: [PATCH v3 0/2] s390 vs. kprobes on ftrace

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Date:	Wed, 22 Oct 2014 10:26:25 +0200
From:	Heiko Carstens <heiko.carstens@...ibm.com>
To:	Vojtech Pavlik <vojtech@...e.com>
Cc:	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Anil S Keshavamurthy <anil.s.keshavamurthy@...el.com>,
	"David S. Miller" <davem@...emloft.net>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Ingo Molnar <mingo@...hat.com>, Jiri Kosina <jkosina@...e.cz>,
	Jiri Slaby <jslaby@...e.cz>,
	Steven Rostedt <rostedt@...dmis.org>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 0/2] s390 vs. kprobes on ftrace

On Tue, Oct 21, 2014 at 09:58:31PM +0200, Vojtech Pavlik wrote:
> Hello Heiko,
> 
> I can confirm that kGraft works well on top of current mainline with
> this patch added.
> 
> Another reason for a performance impact when kGraft is enabled is that
> kGraft still adds two instructions to the syscall path on s390x, as
> there is no space left for a kgraft TIF in the first eight bits of
> thread info flags. Renumbering the thread info flags such that _TIF_WORK
> occupies the first eight bits and TIF_TRACE the next eight would fix
> that problem: Do you believe it is feasible?

Hi Vojtech,

I think you're talking about the SLES12 kernel? There you can simply move
the TIF_SYSCALL bit to the same byte where your TIF_KGR bit resides.

Upstream is a bit different since the TIF_SYSCALL bit is already gone (got
replaced with an s390 specific "PIF" bit). However the free TIF bit got
already eaten up by uprobes..

However we can think of a better solution for upstream if the combined
solution of kGraft/kpatch is ready to be merged.

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