From: Masami Hiramatsu <mhiramat@kernel.org>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>,
Stephen Rothwell <sfr@canb.auug.org.au>,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Linux Next Mailing List <linux-next@vger.kernel.org>,
mhiramat@kernel.org, ast@kernel.org, hjl.tools@gmail.com,
rick.p.edgecombe@intel.com, rppt@kernel.org,
linux-toolchains@vger.kernel.org, Andrew.Cooper3@citrix.com,
ndesaulniers@google.com
Subject: Re: linux-next: build warnings after merge of the tip tree
Date: Wed, 23 Mar 2022 11:23:23 +0900 [thread overview]
Message-ID: <20220323112323.65337d76d96836e487064a99@kernel.org> (raw)
In-Reply-To: <Yjneyn8o06svJkY4@hirez.programming.kicks-ass.net>
On Tue, 22 Mar 2022 15:35:54 +0100
Peter Zijlstra <peterz@infradead.org> wrote:
> On Tue, Mar 22, 2022 at 09:12:42AM -0400, Steven Rostedt wrote:
>
> > > Suppose:
> > >
> > > notrace func_B()
> > > {
> > > ...
> > > }
> > >
> > > func_A()
> > > {
> > > ...
> > > return func_B();
> > > }
> > >
> > > then inhibiting tail calls would end up looking like:
> >
> > If we inhibit tail calls, then we do not need to make func_B notrace.
>
> Dude, you're arguing in circles :-( the notrace was a given.
>
> > > func_A:
> > > call __fentry__
> > > ...
> > > call func_B
> > > call __fexit__
> > > ret
> > >
> > > Then A is fully traced, B is invisible, as per spec. What is the
> > > problem?
> >
> > The above is fine, but then func_B is not a tail call and can also be
> > traced.
>
> Again, B is notrace as a given. This was all about how to deal with
> notrace functions.
>
> I suggested inhibiting tail-call to notrace, you said no. You now seem to
> agree that solves it.
>
> > > The problem you initially had, of doing a tail-call into a notrace, was
> > > that the __fexit__ call went missing, because notrace will obviously not
> > > have that. But that's avoided by inhibiting all tail-calls between
> > > notrace and !notrace functions (note that notrace must also not
> > > tail-call !notrace).
> >
> > I'm confused by the above. Why can't a notrace tail call a !notrace?
> > If we tail call to a
> >
> > func_B:
> > call __fentry__
> > ...
> > call __fexit__
> > ret
> >
> > then the fentry and fexit show a perfectly valid trace of func_B.
>
> Bah; I thought I had a case this morning, but now I can't seem to recall
> :/
>
> > > Your worry seems to stem about loosing visiblilty of !notrace functions,
> > > but AFAICT that doesn't happen.
> >
> > My worry is:
> >
> > func_A:
> > call __fentry__
> > ...
> > jmp func_B
> >
> > Where do we do the call __fexit__ ?
>
> In B (or wherever if B again does a tail-call).
>
> > That was the original concern, and I think the proposed solutions have
> > convoluted our thoughts about what we are trying to fix. So let's go back
> > to the beginning, and see how to deal with it.
> >
> > That is, we have:
> >
> > func_C:
> > call __fenty__
> > ...
> > call func_A:
> > ...
> > call func_B:
> > ...
> > call __fexit__
> > ret
> >
> > func_A:
> > call __fentry__
> > ...
> call __ftail__
> > jmp func_B
> >
> > func_B:
> > call __fentry__
> > ...
> > call __fexit__
> > ret
> >
> > Where the above is C calling A and B as normal functions, A calling B as a
> > tail call and B just being a normal function called by both A and C (and
> > many other functions).
>
> We need the __ftail__ thing to mark the trace-stack entry of func_A as
> complete, then any future __fexit__ will be able to pop all completed
> entries.
>
> In recap:
>
> __fentry__ -- push on trace-stack
> __ftail__ -- mark top-most entry complete
> __fexit__ -- mark top-most entry complete;
> pop all completed entries
>
> inhibit tail-calls to notrace.
>
> > And note, I do not want to limit function tracing (which does not rely on
> > __fexit__) just because we can't figure out how to handle __fexit__.
>
> I'm not following. Regular function tracing needs none of this.
>
> It's function graph tracing, kretprobes and whatever else this rethook
> stuff is about that needs this because return trampolines will stop
> working somewhere in the not too distant future.
I see the __fexit__ is needed, but why __ftail__ is needed? I guess because
func_B is notrace, in that case the __fexit__ will not be in the func_B.
Am I correct?
Thank you,
--
Masami Hiramatsu <mhiramat@kernel.org>
next prev parent reply other threads:[~2022-03-23 2:23 UTC|newest]
Thread overview: 93+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-03-21 3:03 linux-next: build warnings after merge of the tip tree Stephen Rothwell
2022-03-21 12:55 ` Peter Zijlstra
2022-03-21 13:04 ` Peter Zijlstra
2022-03-21 13:08 ` Peter Zijlstra
2022-03-21 13:45 ` Peter Zijlstra
2022-03-21 14:19 ` Mark Rutland
2022-03-21 15:28 ` Peter Zijlstra
2022-03-21 15:45 ` Peter Zijlstra
2022-03-21 16:37 ` Linus Torvalds
2022-03-21 16:44 ` Peter Zijlstra
2022-03-21 16:52 ` Linus Torvalds
2022-03-21 22:05 ` Stephen Rothwell
2022-03-21 22:12 ` Alexei Starovoitov
2022-03-21 22:46 ` Stephen Rothwell
2022-03-21 22:50 ` Alexei Starovoitov
2022-03-21 22:55 ` Steven Rostedt
2022-03-22 4:51 ` Masami Hiramatsu
2022-03-22 4:53 ` Alexei Starovoitov
2022-03-22 7:42 ` Peter Zijlstra
2022-03-22 4:38 ` Masami Hiramatsu
2022-03-21 15:28 ` Steven Rostedt
2022-03-21 16:04 ` Peter Zijlstra
2022-03-21 16:12 ` Steven Rostedt
2022-03-21 16:15 ` Steven Rostedt
2022-03-21 16:22 ` Steven Rostedt
2022-03-21 16:39 ` Steven Rostedt
2022-03-21 16:40 ` Peter Zijlstra
2022-03-21 16:45 ` Steven Rostedt
2022-03-21 16:50 ` Peter Zijlstra
2022-03-21 16:54 ` Steven Rostedt
2022-03-22 7:54 ` Peter Zijlstra
2022-03-22 13:12 ` Steven Rostedt
2022-03-22 14:35 ` Peter Zijlstra
2022-03-22 15:04 ` Steven Rostedt
2022-03-22 15:19 ` Peter Zijlstra
2022-03-22 15:48 ` Peter Zijlstra
2022-03-22 16:17 ` Steven Rostedt
2022-03-23 2:23 ` Masami Hiramatsu [this message]
2022-03-23 2:42 ` Steven Rostedt
2022-03-23 6:28 ` Masami Hiramatsu
2022-03-22 14:25 ` Masami Hiramatsu
2022-03-21 16:48 ` Peter Zijlstra
2022-03-22 5:31 ` Masami Hiramatsu
2022-03-22 8:08 ` Peter Zijlstra
2022-03-22 9:14 ` Masami Hiramatsu
2022-03-22 12:07 ` Peter Zijlstra
2022-03-22 12:17 ` Peter Zijlstra
2022-03-22 12:46 ` Masami Hiramatsu
2022-03-22 13:22 ` Steven Rostedt
2022-03-22 13:15 ` Mark Rutland
2022-03-22 13:51 ` Masami Hiramatsu
2022-03-22 10:46 ` Peter Zijlstra
2022-03-22 10:59 ` Peter Zijlstra
-- strict thread matches above, loose matches on Subject: below --
2024-02-02 3:59 Stephen Rothwell
2023-12-01 0:29 Stephen Rothwell
2023-12-01 12:09 ` Uros Bizjak
2023-12-04 4:08 ` Stephen Rothwell
2023-12-04 7:02 ` Uros Bizjak
2023-12-11 5:19 ` Stephen Rothwell
2023-12-11 7:06 ` Uros Bizjak
2023-06-02 3:12 Stephen Rothwell
2022-11-21 7:41 Stephen Rothwell
2022-05-20 7:49 Stephen Rothwell
2022-04-27 0:10 Stephen Rothwell
2022-04-27 11:31 ` Borislav Petkov
2022-04-27 13:43 ` Tom Lendacky
2022-03-22 3:51 Stephen Rothwell
2022-03-22 21:52 ` Peter Zijlstra
2022-03-22 23:11 ` Stephen Rothwell
2021-12-17 3:40 Stephen Rothwell
2022-01-21 23:58 ` Stephen Rothwell
2022-03-15 2:32 ` Stephen Rothwell
2022-04-04 3:26 ` Stephen Rothwell
2021-10-12 10:20 Stephen Rothwell
2021-10-12 13:58 ` André Almeida
2020-11-30 7:05 Stephen Rothwell
2020-11-30 10:17 ` Borislav Petkov
2020-11-30 21:56 ` Ernst, Justin
2020-11-30 22:18 ` Borislav Petkov
2020-11-23 7:19 Stephen Rothwell
2020-11-23 23:03 ` Jarkko Sakkinen
2017-11-02 2:53 Stephen Rothwell
2017-11-03 21:00 ` Masami Hiramatsu
2017-11-04 8:01 ` Ingo Molnar
2017-11-04 12:16 ` Masami Hiramatsu
2017-11-13 11:31 ` Stephen Rothwell
2017-06-23 4:19 Stephen Rothwell
2016-07-14 3:49 Stephen Rothwell
2016-07-14 3:37 Stephen Rothwell
2016-07-14 4:18 ` Stephen Rothwell
2012-10-12 5:11 Stephen Rothwell
2011-01-31 4:27 Stephen Rothwell
2011-01-31 5:08 ` Jaswinder Singh
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