From: Torsten Duwe <duwe@lst.de>
To: Michael Ellerman <mpe@ellerman.id.au>
Cc: Balbir Singh <bsingharora@gmail.com>,
Jiri Kosina <jkosina@suse.cz>, Miroslav Benes <mbenes@suse.cz>,
Petr Mladek <pmladek@suse.com>, Jessica Yu <jeyu@redhat.com>,
Steven Rostedt <rostedt@goodmis.org>,
linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org,
live-patching@vger.kernel.org
Subject: Re: [PATCH v8 4/8] ppc64 ftrace_with_regs configuration variables
Date: Tue, 16 Feb 2016 11:35:51 +0100 [thread overview]
Message-ID: <20160216103551.GA25103@lst.de> (raw)
In-Reply-To: <1455617356.31947.2.camel@ellerman.id.au>
On Tue, Feb 16, 2016 at 09:09:16PM +1100, Michael Ellerman wrote:
> On Mon, 2016-02-15 at 15:04 +0100, Torsten Duwe wrote:
> > If you use "-pg -mprofile-kernel", gcc seems to forget that, and omits the TOC
> > load, for a similar assembler calling sequence.
>
> That's by design.
Ah, ok.
> mprofile-kernel is supposed to create as little overhead as possible in the
> non-traced case. All of the burden is shifted to the trace function (_mcount).
... or its helpers, see below.
> The reason to do that is because modern distros always build with tracing, but
> most of the time tracing will not actually be active. So we want the cost of
> tracing-built-in-but-disabled to be ~zero.
Ok, that's a design goal.
> > That was the alternative I asked about; but given that the _mcount / ftrace_caller
> > trampoline hardly differs from a normal trampoline (so far), loading R2 would be the
> > general case, or an excessive special case handling would result.
>
> I'm not sure I follow what you mean there at the end.
This suggests you have not yet actively debugged this problem ;-)
> Requiring ftrace_caller() to load the kernel TOC is not a problem IMHO.
The problem is, you don't get to ftrace_caller in the first place :)
> I think I have an easier way to do it, I'll reply to the patch with that (if it
> works).
I doubt so. Either it works, _or_ it is easier ;)
To save you some work: by the design of minimal overhead you try to follow,
SQUASH_TOC_SAVE_INSN from my patches isn't sufficient. You'll need to load the
_current_ TOC _on_ the trampoline, and in turn it will be different from the regular
trampolines; and that needs to be recognised, or the normal module linker logic
won't work.
OTOH my proposed GCC change only affects a very limited number of functions...
Looking forward to your patch!
Torsten
next prev parent reply other threads:[~2016-02-16 10:35 UTC|newest]
Thread overview: 44+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-02-10 17:29 [PATCH v8 0/8] ftrace with regs + live patching for ppc64 LE (ABI v2) Torsten Duwe
2016-01-28 15:32 ` [PATCH v8 8/8] livepatch: Detect offset for the ftrace location during build Torsten Duwe
2016-02-12 16:13 ` Balbir Singh
2016-02-12 16:45 ` Petr Mladek
2016-02-13 1:33 ` Balbir Singh
2016-02-16 5:47 ` Kamalesh Babulal
2016-02-16 8:23 ` Torsten Duwe
2016-02-16 10:30 ` Kamalesh Babulal
2016-02-16 10:39 ` Torsten Duwe
2016-02-16 13:57 ` Petr Mladek
2016-02-17 3:08 ` Michael Ellerman
2016-02-23 17:00 ` Torsten Duwe
2016-02-24 6:37 ` Balbir Singh
2016-02-24 6:55 ` Balbir Singh
2016-02-24 9:23 ` Torsten Duwe
2016-02-24 11:22 ` Balbir Singh
2016-02-24 7:51 ` Kamalesh Babulal
2016-02-10 16:21 ` [PATCH v8 1/8] ppc64 (le): prepare for -mprofile-kernel Torsten Duwe
2016-02-17 10:55 ` Michael Ellerman
2016-02-17 11:30 ` Torsten Duwe
2016-02-17 11:39 ` Michael Ellerman
2016-02-10 16:22 ` [PATCH v8 2/8] ppc64le FTRACE_WITH_REGS implementation Torsten Duwe
2016-02-10 16:22 ` [PATCH v8 3/8] ppc use ftrace_modify_all_code default Torsten Duwe
2016-02-10 16:25 ` [PATCH v8 4/8] ppc64 ftrace_with_regs configuration variables Torsten Duwe
2016-02-11 7:48 ` Balbir Singh
2016-02-11 8:39 ` Kamalesh Babulal
2016-02-11 9:35 ` Balbir Singh
2016-02-11 13:00 ` Murali Sampath
2016-02-11 13:00 ` Murali Sampath
2016-02-11 8:42 ` Torsten Duwe
2016-02-11 9:34 ` Balbir Singh
2016-02-15 10:27 ` Michael Ellerman
2016-02-15 12:56 ` Jiri Kosina
2016-02-15 14:04 ` Torsten Duwe
2016-02-15 22:21 ` Torsten Duwe
2016-02-16 4:53 ` Balbir Singh
2016-02-16 10:09 ` Michael Ellerman
2016-02-16 10:35 ` Torsten Duwe [this message]
2016-02-10 16:27 ` [PATCH v8 5/8] ppc64 ftrace_with_regs: disable profiling for some files Torsten Duwe
2016-02-10 16:34 ` [PATCH v8 6/8] Implement kernel live patching for ppc64le (ABIv2) Torsten Duwe
2016-02-11 9:01 ` Miroslav Benes
2016-02-10 16:36 ` [PATCH v8 7/8] Enable LIVEPATCH to be configured on ppc64le and add livepatch.o if it is selected Torsten Duwe
2016-02-11 6:18 ` [PATCH v8 0/8] ftrace with regs + live patching for ppc64 LE (ABI v2) Balbir Singh
2016-02-11 8:38 ` Torsten Duwe
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