linux-kernel - Re: [PATCH v2 5/6] debugfs: add debugfs_create_atomic64_t for atomic64

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Date:   Wed, 5 Apr 2023 08:26:18 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Luis Chamberlain <mcgrof@...nel.org>
Cc:     david@...hat.com, patches@...ts.linux.dev,
        linux-modules@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, pmladek@...e.com,
        petr.pavlu@...e.com, prarit@...hat.com, gregkh@...uxfoundation.org,
        rafael@...nel.org, christophe.leroy@...roup.eu, tglx@...utronix.de,
        peterz@...radead.org, song@...nel.org, rppt@...nel.org,
        dave@...olabs.net, willy@...radead.org, vbabka@...e.cz,
        mhocko@...e.com, dave.hansen@...ux.intel.com,
        colin.i.king@...il.com, jim.cromie@...il.com,
        catalin.marinas@....com, jbaron@...mai.com,
        rick.p.edgecombe@...el.com
Subject: Re: [PATCH v2 5/6] debugfs: add debugfs_create_atomic64_t for atomic64_t

On Tue, Apr 4, 2023 at 7:27 PM Luis Chamberlain <mcgrof@...nel.org> wrote:
>
> Sometimes you want to add debugfs entries for atomic counters which
> can be pretty large using atomic64_t. Add support for these.

So I realize why you use atomic64, but I really suspect you'd be
better off with just the regular "atomic_long".

This is not some debug stat that we care deeply about on 32-bit, and
"atomic64" is often really really nasty on 32-bit architectures.

For example, on x86, instead of being a single instruction, it ends up
being a cmpxchg loop. In fact, even a single atomic read is a cmpxchg
(admittedly without the need for looping).

And yeah, I realize that we don't have a "atomic_long" debugfs
interface either. But I think we could just use atomic_long for the
module code (avoiding all the horrors of 64-bit atomics on 32-bit
architectures), and then using just 'var->counter' for the value. It's
not like the debugfs stuff actually does any truly atomic updates.

So something like

        debugfs_create_ulong(... &val->counter ..);

instead of

        debugfs_create_atomic64(... &val ..);

Hmm?

I dunno. I just think this is not something that may be worth
introducing a new thing for, when it is *so* painful on 32-bit, and
doesn't seem worth it.

                   Linus