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Message-ID: <D9KXE2YX8R2M.3L7Q6NVIXKPE9@google.com>
Date: Thu, 01 May 2025 15:22:55 +0000
From: Brendan Jackman <jackmanb@...gle.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Christoph Hellwig <hch@....de>, <chenlinxuan@...ontech.com>, 
	Keith Busch <kbusch@...nel.org>, Jens Axboe <axboe@...nel.dk>, Sagi Grimberg <sagi@...mberg.me>, 
	Andrew Morton <akpm@...ux-foundation.org>, Yishai Hadas <yishaih@...dia.com>, 
	Jason Gunthorpe <jgg@...pe.ca>, Shameer Kolothum <shameerali.kolothum.thodi@...wei.com>, 
	Kevin Tian <kevin.tian@...el.com>, Alex Williamson <alex.williamson@...hat.com>, 
	Peter Huewe <peterhuewe@....de>, Jarkko Sakkinen <jarkko@...nel.org>, 
	Masahiro Yamada <masahiroy@...nel.org>, Nathan Chancellor <nathan@...nel.org>, 
	Nicolas Schier <nicolas.schier@...ux.dev>, 
	Nick Desaulniers <nick.desaulniers+lkml@...il.com>, Bill Wendling <morbo@...gle.com>, 
	Justin Stitt <justinstitt@...gle.com>, Vlastimil Babka <vbabka@...e.cz>, 
	Suren Baghdasaryan <surenb@...gle.com>, Michal Hocko <mhocko@...e.com>, 
	Johannes Weiner <hannes@...xchg.org>, Zi Yan <ziy@...dia.com>, 
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, "Paul E. McKenney" <paulmck@...nel.org>, 
	Boqun Feng <boqun.feng@...il.com>, Dmitry Vyukov <dvyukov@...gle.com>, 
	Andrey Konovalov <andreyknvl@...il.com>, Juergen Gross <jgross@...e.com>, 
	Boris Ostrovsky <boris.ostrovsky@...cle.com>, Thomas Gleixner <tglx@...utronix.de>, 
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, <x86@...nel.org>, 
	"H. Peter Anvin" <hpa@...or.com>, <linux-nvme@...ts.infradead.org>, 
	<linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>, <kvm@...r.kernel.org>, 
	<virtualization@...ts.linux.dev>, <linux-integrity@...r.kernel.org>, 
	<linux-kbuild@...r.kernel.org>, <llvm@...ts.linux.dev>, 
	Winston Wen <wentao@...ontech.com>, <kasan-dev@...glegroups.com>, 
	<xen-devel@...ts.xenproject.org>, Changbin Du <changbin.du@...el.com>, 
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH RFC v3 0/8] kernel-hacking: introduce CONFIG_NO_AUTO_INLINE

On Thu May 1, 2025 at 3:02 PM UTC, Peter Zijlstra wrote:
> On Thu, May 01, 2025 at 02:19:47PM +0000, Brendan Jackman wrote:
>> On Tue Apr 29, 2025 at 12:35 PM UTC, Christoph Hellwig wrote:
>> > On Tue, Apr 29, 2025 at 12:06:04PM +0800, Chen Linxuan via B4 Relay wrote:
>> >> This series introduces a new kernel configuration option NO_AUTO_INLINE,
>> >> which can be used to disable the automatic inlining of functions.
>> >> 
>> >> This will allow the function tracer to trace more functions
>> >> because it only traces functions that the compiler has not inlined.
>> >
>> > This still feels like a bad idea because it is extremely fragile.
>> 
>> Can you elaborate on that - does it introduce new fragility?
>
> given it needs to sprinkle __always_inline around where it wasn't needed
> before, yeah.

Right, I guess I just wouldn't have associated that with the word
"fragility", but that's a reasonable complaint!

> Also, why would you want this? function tracer is already too much
> output. Why would you want even more?

Yes, tracing every function is already too noisy, this would make it
even more too-noisy, not sure "too noisy" -> "way too noisy" is a
particularly meaningful degradation.

Whereas enlarging the pool of functions that you can _optionally target_
for tracing, or nice reliable breakpoints in GDB, and disasm that's
easier to mentally map back to C, seems like a helpful improvement for
test builds. Personally I sometimes spam a bunch of `noinline` into code
I'm debugging so this seems like a way to just slap that same thing on
the whole tree without dirtying the code, right?

Not that I have a strong opinion on the cost/benefit here, but the
benefit seems nonzero to me.

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