From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To: torvalds@linux-foundation.org
Cc: akpm@linux-foundation.org, bigeasy@linutronix.de,
boqun.feng@gmail.com, bp@alien8.de, linux-kernel@vger.kernel.org,
longman@redhat.com, mingo@kernel.org, peterz@infradead.org,
tglx@linutronix.de, will@kernel.org,
Dennis Zhou <dennis@kernel.org>, Tejun Heo <tj@kernel.org>,
Christoph Lameter <cl@linux.com>
Subject: [PATCH 0/3] Remove volatile from arch_raw_cpu_ptr() and revert the hacks.
Date: Thu, 24 Mar 2022 18:39:24 +0100 [thread overview]
Message-ID: <20220324173927.2230447-1-bigeasy@linutronix.de> (raw)
In-Reply-To: <CAHk-=wh81PAKxapicYOby3WWOFJV2htR5KWCRdKNgw68mOc0Zg@mail.gmail.com>
On 2022-03-23 10:17:08 [-0700], Linus Torvalds wrote:
> I get the feeling that the real problem is that on x86, we have this:
>
> #define arch_raw_cpu_ptr(ptr) \
> ({ \
> unsigned long tcp_ptr__; \
> asm volatile("add " __percpu_arg(1) ", %0" \
> : "=r" (tcp_ptr__) \
> : "m" (this_cpu_off), "0" (ptr)); \
> (typeof(*(ptr)) __kernel __force *)tcp_ptr__; \
> })
>
> and that "volatile" is just *WRONG*.
>
> That volatile is what literally tells the compiler "you can't remove
> this if it isn't used".
It is indeed just x86. After double checking arm/mips removes that code
properly.
> But there's no point to that.
>
> So how about we
>
> (a) just revert commit 9983a9d577db4
>
> (b) remove that bogus 'volatile'
>
> Doesn't that fix the problem?
The following series does that. The assembly code looks okay. In a few
simple test cases the this_cpu_ptr() usage is always created and is not
moved passed preempt_enable() statement.
The resulting vmlinux shrunk a bit. The test config lost ~2KiB:
text data bss dec hex filename
22533901 10722831 13963496 47220228 2d08604 vmlinux.volatile
22531589 10722831 13971688 47226108 2d09cfc vmlinux.patched
after looking at it it was sometimes due avoiding this_cpu_ptr(),
sometimes it looked that the compiler made other decisions at the
earlier resulting to be more beneficial later on.
Sebastian
next prev parent reply other threads:[~2022-03-24 17:39 UTC|newest]
Thread overview: 49+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-03-21 11:11 [GIT PULL] locking changes for v5.18 Ingo Molnar
2022-03-22 22:05 ` Linus Torvalds
2022-03-22 22:19 ` Borislav Petkov
2022-03-22 22:58 ` Linus Torvalds
2022-03-23 7:11 ` Sebastian Andrzej Siewior
2022-03-23 11:09 ` [PATCH] locking/local_lock: Pretend to use the per-CPU variable if not needed Sebastian Andrzej Siewior
2022-03-23 17:17 ` Linus Torvalds
2022-03-24 17:39 ` Sebastian Andrzej Siewior [this message]
2022-03-24 17:39 ` [PATCH 1/3] x86/percpu: Remove volatile from arch_raw_cpu_ptr() Sebastian Andrzej Siewior
2022-03-24 17:39 ` [PATCH 2/3] Revert "locking/local_lock: Make the empty local_lock_*() function a macro." Sebastian Andrzej Siewior
2022-03-24 17:39 ` [PATCH 3/3] Revert "mm/page_alloc: mark pagesets as __maybe_unused" Sebastian Andrzej Siewior
2022-03-24 18:28 ` [PATCH 0/3] Remove volatile from arch_raw_cpu_ptr() and revert the hacks Linus Torvalds
2022-03-28 13:55 ` Peter Zijlstra
2022-03-28 14:59 ` Sebastian Andrzej Siewior
2022-03-28 14:58 ` [PATCH v2 " Sebastian Andrzej Siewior
2022-03-28 14:58 ` [PATCH v2 1/3] x86/percpu: Remove volatile from arch_raw_cpu_ptr() Sebastian Andrzej Siewior
2022-04-05 8:28 ` [tip: locking/urgent] " tip-bot2 for Sebastian Andrzej Siewior
2022-03-28 14:58 ` [PATCH v2 2/3] Revert "locking/local_lock: Make the empty local_lock_*() function a macro." Sebastian Andrzej Siewior
2022-04-05 8:28 ` [tip: locking/urgent] " tip-bot2 for Sebastian Andrzej Siewior
2022-03-28 14:58 ` [PATCH v2 3/3] Revert "mm/page_alloc: mark pagesets as __maybe_unused" Sebastian Andrzej Siewior
2022-04-05 8:28 ` [tip: locking/urgent] " tip-bot2 for Sebastian Andrzej Siewior
2022-03-23 11:21 ` [PATCH] x86/defconfig: Enable WERROR Borislav Petkov
2022-03-23 17:19 ` Linus Torvalds
2022-03-23 17:33 ` Borislav Petkov
2022-03-24 8:31 ` [PATCH] x86/config: Make the x86 defconfigs a bit more usable Ingo Molnar
2022-03-24 9:12 ` David Laight
2022-03-24 15:47 ` Nathan Chancellor
2022-03-25 11:52 ` Andy Shevchenko
2022-03-27 19:04 ` Ingo Molnar
2022-03-27 19:03 ` Ingo Molnar
2022-03-28 15:41 ` Nathan Chancellor
2022-09-02 8:50 ` Ingo Molnar
2022-09-02 9:18 ` Masahiro Yamada
2022-09-04 9:48 ` Ingo Molnar
2022-09-05 2:16 ` Masahiro Yamada
2022-09-05 9:54 ` Ingo Molnar
[not found] ` <CAK7LNAQyiNpbLuVjjQ8-GOQECtfQZqsNS8xH0E2ZkLAHYtXt7A@mail.gmail.com>
2022-09-10 17:28 ` Linus Torvalds
2022-03-24 8:16 ` [tip: x86/urgent] x86/defconfig: Enable WERROR tip-bot2 for Borislav Petkov
2022-03-25 11:41 ` [GIT PULL] locking changes for v5.18 Andy Shevchenko
2022-03-25 12:23 ` Peter Zijlstra
2022-03-25 13:06 ` Andy Shevchenko
2022-03-25 17:29 ` Linus Torvalds
2022-03-25 17:53 ` Andy Shevchenko
2022-03-22 22:38 ` Peter Zijlstra
2022-03-22 23:26 ` Linus Torvalds
2022-03-24 8:40 ` Ingo Molnar
2022-03-24 10:19 ` Borislav Petkov
2022-03-24 23:19 ` Nick Desaulniers
2022-03-22 23:27 ` pr-tracker-bot
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