From: Josh Poimboeuf <jpoimboe@redhat.com>
To: Petr Mladek <pmladek@suse.com>
Cc: Miroslav Benes <mbenes@suse.cz>,
Peter Zijlstra <peterz@infradead.org>,
Steven Rostedt <rostedt@goodmis.org>,
Joe Lawrence <joe.lawrence@redhat.com>,
Jessica Yu <jeyu@kernel.org>,
x86@kernel.org, linux-kernel@vger.kernel.org,
mhiramat@kernel.org, bristot@redhat.com, jbaron@akamai.com,
torvalds@linux-foundation.org, tglx@linutronix.de,
mingo@kernel.org, namit@vmware.com, hpa@zytor.com,
luto@kernel.org, ard.biesheuvel@linaro.org,
live-patching@vger.kernel.org,
Randy Dunlap <rdunlap@infradead.org>
Subject: Re: [PATCH v3 5/6] x86/ftrace: Use text_poke()
Date: Tue, 28 Jan 2020 11:02:54 -0600 [thread overview]
Message-ID: <20200128170254.igb72ib5n7lvn3ds@treble> (raw)
In-Reply-To: <20200128154046.trkpkdaz7qeovhii@pathway.suse.cz>
On Tue, Jan 28, 2020 at 04:40:46PM +0100, Petr Mladek wrote:
> On Tue 2020-01-28 09:00:14, Josh Poimboeuf wrote:
> > On Tue, Jan 28, 2020 at 10:28:07AM +0100, Miroslav Benes wrote:
> > > I don't think we have something special at SUSE not generally available...
> > >
> > > ...and I don't think it is really important to discuss that and replying
> > > to the above, because there is a legitimate use case which relies on the
> > > flag. We decided to support different use cases right at the beginning.
> > >
> > > I understand it currently complicates things for objtool, but objtool is
> > > sensitive to GCC code generation by definition. "Issues" appear with every
> > > new GCC version. I see no difference here and luckily it is not so
> > > difficult to fix it.
> > >
> > > I am happy to help with acting on those objtool warning reports you
> > > mentioned in the other email. Just Cc me where appropriate. We will take a
> > > look.
> >
> > As I said, the objtool warnings aren't even the main issue.
>
> Great.
>
> Anyway, I think that we might make your life easier with using
> the proposed -Wsuggest-attribute=noreturn.
Maybe. Though if I understand correctly, this doesn't help for any of
the new warnings because they're for static functions, and this only
warns about global functions.
> Also it might be possible to create the list of global
> noreturn functions using some gcc tool. Similar way that we get
> the list of functions that need to be livepatched explicitly
> because of the problematic optimizations.
>
> It sounds like a win-win approach.
I don't quite get how that could be done in an automated way, but ideas
about how to implement it would certainly be welcome.
> > There are N users[*] of CONFIG_LIVEPATCH, where N is perhaps dozens.
> > For N-1 users, they have to suffer ALL the drawbacks, with NONE of the
> > benefits.
>
> You wrote in the other mail:
>
> > The problems associated with it: performance, LTO incompatibility,
> > clang incompatibility (I think?), the GCC dead code issue.
>
> SUSE performance team did extensive testing and did not found
> any real performance issues. It was discussed when the option
> was enabled upstream.
>
> Are the other problems affecting real life usage, please?
> Could you be more specific about them, please?
The original commit mentioned 1-3% scheduler degradation. And I'd
expect things to worsen over time as interprocedural optimizations
improve.
Also, LTO is coming whether we like it or not. As is Clang. Those are
real-world things which will need to work with livepatching sooner or
later.
> > And, even if they wanted those benefits, they have no idea how to get
> > them because the patch creation process isn't documented.
>
> I do not understand this. All the sample modules and selftests are
> using source based livepatches.
We're talking in circles. Have you read the thread?
The samples are a (dangerous) joke. With or without -flive-patching.
> It is actually the only somehow documented way. Sure, the
> documentation might get improved. Patches are welcome.
Are you suggesting for *me* to send documentation for how *you* build
patches?
> The option is not currently needed by the selftests only because there
> is no selftest for this type of problems. But the problems are real.
> They would actually deserve selftests. Again, patches are welcome.
>
> My understanding is that the source based livepatches is the future.
I think that still remains to be seen.
> N-1 users are just waiting until the 1 user develops more helper tools
> for this.
No. N-1 users have no idea how to make (safe) source-based patches in
the first place. And if *you* don't need the tools, why would anyone
else? Why not document the process and encourage the existence of other
users so they can get involved and help with the tooling?
> I would really like to hear about some serious problems
> before we do this step back in upstream.
Sometimes you need to take 1 step back before you can take 2 steps
forward. I regret ACKing the original patch. It was too early.
--
Josh
next prev parent reply other threads:[~2020-01-28 17:03 UTC|newest]
Thread overview: 45+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <20191007081945.10951536.8@infradead.org>
[not found] ` <20191008104335.6fcd78c9@gandalf.local.home>
[not found] ` <20191009224135.2dcf7767@oasis.local.home>
[not found] ` <20191010092054.GR2311@hirez.programming.kicks-ass.net>
[not found] ` <20191010091956.48fbcf42@gandalf.local.home>
[not found] ` <20191010140513.GT2311@hirez.programming.kicks-ass.net>
[not found] ` <20191010115449.22044b53@gandalf.local.home>
[not found] ` <20191010172819.GS2328@hirez.programming.kicks-ass.net>
[not found] ` <20191011125903.GN2359@hirez.programming.kicks-ass.net>
[not found] ` <20191015130739.GA23565@linux-8ccs>
[not found] ` <20191015135634.GK2328@hirez.programming.kicks-ass.net>
2019-10-15 14:13 ` [PATCH v3 5/6] x86/ftrace: Use text_poke() Miroslav Benes
2019-10-15 15:06 ` Joe Lawrence
2019-10-15 15:31 ` Jessica Yu
2019-10-15 22:17 ` Joe Lawrence
2019-10-15 22:27 ` Steven Rostedt
2019-10-16 7:42 ` Peter Zijlstra
2019-10-16 10:15 ` Miroslav Benes
2019-10-21 15:05 ` Josh Poimboeuf
2020-01-20 16:50 ` Josh Poimboeuf
2020-01-21 8:35 ` Miroslav Benes
2020-01-21 16:10 ` Josh Poimboeuf
2020-01-22 10:09 ` Miroslav Benes
2020-01-22 21:42 ` Josh Poimboeuf
2020-01-28 9:28 ` Miroslav Benes
2020-01-28 15:00 ` Josh Poimboeuf
2020-01-28 15:40 ` Petr Mladek
2020-01-28 17:02 ` Josh Poimboeuf [this message]
2020-01-29 0:46 ` Jiri Kosina
2020-01-29 2:17 ` Josh Poimboeuf
2020-01-29 3:14 ` Jiri Kosina
2020-01-29 12:28 ` Miroslav Benes
2020-01-29 15:59 ` Josh Poimboeuf
2020-01-30 9:53 ` Petr Mladek
2020-01-30 14:17 ` Josh Poimboeuf
2020-01-31 7:17 ` Petr Mladek
2020-01-22 12:15 ` Miroslav Benes
2020-01-22 15:05 ` Miroslav Benes
2020-01-22 22:03 ` Josh Poimboeuf
2020-01-23 10:19 ` Martin Jambor
2019-10-16 7:49 ` Peter Zijlstra
2019-10-16 10:20 ` Miroslav Benes
2019-10-16 13:29 ` Miroslav Benes
2019-10-18 13:03 ` Jessica Yu
2019-10-18 13:40 ` Petr Mladek
2019-10-21 14:14 ` Jessica Yu
2019-10-21 15:31 ` Josh Poimboeuf
2019-10-22 8:27 ` Miroslav Benes
2019-10-22 14:31 ` Josh Poimboeuf
2019-10-23 9:04 ` Miroslav Benes
2019-10-16 6:51 ` Miroslav Benes
2019-10-16 9:23 ` Peter Zijlstra
2019-10-16 9:36 ` Jessica Yu
2019-10-16 9:51 ` Peter Zijlstra
2019-10-16 12:39 ` Peter Zijlstra
2019-10-22 8:45 ` Miroslav Benes
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