linux-kernel - Re: [RFC PATCH] x86/syscalls: allow tracing of __do_sys

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Message-ID: <Yymds595qthVSnt0@hirez.programming.kicks-ass.net>
Date:   Tue, 20 Sep 2022 13:02:11 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Nadav Amit <nadav.amit@...il.com>
Cc:     "Steven Rostedt (Google)" <rostedt@...dmis.org>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Borislav Petkov <bp@...en8.de>, Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [RFC PATCH] x86/syscalls: allow tracing of __do_sys_[syscall]
 functions

On Mon, Sep 19, 2022 at 07:35:42PM -0700, Nadav Amit wrote:

> 1. What is the reason that inline functions are marked with notrace?

IIRC the concern is that a notrace function using an inline function;
GCC deciding to not inline and then still hitting tracing.

For noinstr we've mandated __always_inline to avoid this problem. The
direct advantage is that those inlined into instrumented code get, well,
instrumented.

> 2. Is probing function that is called from do_idle() supposed to work, or
>    should the kernel prevent it?

Should work for some :-) Specifically it doesn't work for those that
disable RCU, and that's (largely) being fixed here:

  https://lore.kernel.org/all/20220919095939.761690562@infradead.org/T/#u

Although looking at it just now, I think I missed a spot.. lemme go fix
;-)

I'm failing to find this callchain; where is
tick_nohz_get_sleep_length() calling to elfcorehdr_read() ?!?

> [ 2381.892478]  elfcorehdr_read+0x40/0x40
> [ 2381.896681]  tick_nohz_get_sleep_length+0x9d/0xc0
> [ 2381.901955]  menu_select+0x4bb/0x630
> [ 2381.905965]  cpuidle_select+0x16/0x20
> [ 2381.910069]  do_idle+0x1d2/0x270
> [ 2381.913689]  cpu_startup_entry+0x20/0x30
> [ 2381.918086]  start_secondary+0x118/0x150
> [ 2381.922484]  secondary_startup_64_no_verify+0xc3/0xcb
> [ 2381.928147]  </TASK>
> [ 2381.931535] Modules linked in: zram
> [ 2381.936365] CR2: ffffc90077cb6e4b
> [ 2381.940998] ---[ end trace 0000000000000000 ]---
>