linux-kernel - Re: [PATCH v6 5/6] tracing: Show inode and device major:minor in deferred user space stacktrace

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Message-ID: <20250829204459.6ea62c31@gandalf.local.home>
Date: Fri, 29 Aug 2025 20:44:59 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Arnaldo Carvalho de Melo <arnaldo.melo@...il.com>, Steven Rostedt
 <rostedt@...nel.org>, linux-kernel@...r.kernel.org,
 linux-trace-kernel@...r.kernel.org, bpf@...r.kernel.org, x86@...nel.org,
 Masami Hiramatsu <mhiramat@...nel.org>, Mathieu Desnoyers
 <mathieu.desnoyers@...icios.com>, Josh Poimboeuf <jpoimboe@...nel.org>,
 Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...nel.org>, Jiri
 Olsa <jolsa@...nel.org>, Arnaldo Carvalho de Melo <acme@...nel.org>,
 Namhyung Kim <namhyung@...nel.org>, Thomas Gleixner <tglx@...utronix.de>,
 Andrii Nakryiko <andrii@...nel.org>, Indu Bhagat <indu.bhagat@...cle.com>,
 "Jose E. Marchesi" <jemarch@....org>, Beau Belgrave
 <beaub@...ux.microsoft.com>, Jens Remus <jremus@...ux.ibm.com>, Andrew
 Morton <akpm@...ux-foundation.org>, Florian Weimer <fweimer@...hat.com>,
 Sam James <sam@...too.org>, Kees Cook <kees@...nel.org>, "Carlos O'Donell"
 <codonell@...hat.com>
Subject: Re: [PATCH v6 5/6] tracing: Show inode and device major:minor in
 deferred user space stacktrace

On Fri, 29 Aug 2025 19:09:35 -0400
Steven Rostedt <rostedt@...dmis.org> wrote:

> Yeah, we could add an optimization to store vma's in the callchain walk to
> see if the next call chain belongs to a previous one. Could even just cache
> the previous vma, as it's not as common to have one library calling into
> another and back again.

Although, it does happen with libc. :-p

cookie=300000004
 =>  <000000000008f687> : 0x666220af
 =>  <0000000000014560> : 0x88512fee
 =>  <000000000001f94a> : 0x88512fee
 =>  <000000000001fc9e> : 0x88512fee
 =>  <000000000001fcfa> : 0x88512fee
 =>  <000000000000ebae> : 0x88512fee
 =>  <0000000000029ca8> : 0x666220af

The 0x666220af is libc, where the first item is (according to objdump):

000000000008f570 <__libc_alloca_cutoff@@GLIBC_PRIVATE>:

And the last one (top of the stack) is:

0000000000029c20 <__libc_init_first@@GLIBC_2.2.5>:

Of course libc starts the application, and then the application will likely
call back into libc. We could optimize for this case with:

  first_vma = NULL;
  vma = NULL;
  foreach addr in callchain
    if (!first_vma)
      vma = first_vma = vma_alloc()
    else if (addr in range of first_vma)
      vma = first_vma
    else (addr not in range of vma)
      vma = vma_lookup(addr);

-- Steve