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Message-ID: <20241025032253.GN29862@gate.crashing.org>
Date: Thu, 24 Oct 2024 22:22:53 -0500
From: Segher Boessenkool <segher@...nel.crashing.org>
To: Michael Ellerman <mpe@...erman.id.au>
Cc: "Nysal Jan K.A." <nysal@...ux.ibm.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
linuxppc-dev@...ts.ozlabs.org, Nathan Chancellor <nathan@...nel.org>,
Nick Desaulniers <ndesaulniers@...gle.com>,
Bill Wendling <morbo@...gle.com>,
Justin Stitt <justinstitt@...gle.com>,
Vlastimil Babka <vbabka@...e.cz>,
Kent Overstreet <kent.overstreet@...ux.dev>,
Rick Edgecombe <rick.p.edgecombe@...el.com>,
Roman Gushchin <roman.gushchin@...ux.dev>,
linux-kernel@...r.kernel.org, llvm@...ts.linux.dev
Subject: Re: [PATCH] sched/membarrier: Fix redundant load of membarrier_state
Hi!
On Fri, Oct 25, 2024 at 11:29:38AM +1100, Michael Ellerman wrote:
> [To += Mathieu]
>
> "Nysal Jan K.A." <nysal@...ux.ibm.com> writes:
> > From: "Nysal Jan K.A" <nysal@...ux.ibm.com>
> >
> > On architectures where ARCH_HAS_SYNC_CORE_BEFORE_USERMODE
> > is not selected, sync_core_before_usermode() is a no-op.
> > In membarrier_mm_sync_core_before_usermode() the compiler does not
> > eliminate redundant branches and the load of mm->membarrier_state
> > for this case as the atomic_read() cannot be optimized away.
>
> I was wondering if this was caused by powerpc's arch_atomic_read() which
> uses asm volatile.
>
> But replacing arch_atomic_read() with READ_ONCE() makes no difference,
> presumably because the compiler still can't see that the READ_ONCE() is
> unnecessary (which is kind of by design).
Exactly.
> > GCC 12.2.1:
> > -----------
> > add/remove: 0/0 grow/shrink: 0/1 up/down: 0/-32 (-32)
> > Function old new delta
> > finish_task_switch.isra 852 820 -32
>
> GCC 12 is a couple of years old, I assume GCC 14 behaves similarly?
GCC 12 is still being actively developed. There will be a 12.5
probably halfway next year (and that will be the last 12.x release,
yes). The GCC homepage (<https://gcc.gnu.org>) will tell you what
releases are still maintained/supported, and sometimes you can derive
our planned plans from there as well :-)
But yes, 14 is similar (I did not test, but I feel confident making that
assertion :-) )
> > static inline void membarrier_mm_sync_core_before_usermode(struct mm_struct *mm)
> > {
> > + if (!IS_ENABLED(CONFIG_ARCH_HAS_SYNC_CORE_BEFORE_USERMODE))
> > + return;
> > if (current->mm != mm)
> > return;
> > if (likely(!(atomic_read(&mm->membarrier_state) &
>
> The other option would be to have a completely separate stub, eg:
>
> #ifdef CONFIG_ARCH_HAS_SYNC_CORE_BEFORE_USERMODE
> static inline void membarrier_mm_sync_core_before_usermode(struct mm_struct *mm)
> {
> if (current->mm != mm)
> return;
> if (likely(!(atomic_read(&mm->membarrier_state) &
> MEMBARRIER_STATE_PRIVATE_EXPEDITED_SYNC_CORE)))
> return;
> sync_core_before_usermode();
> }
> #else
> static inline void membarrier_mm_sync_core_before_usermode(struct mm_struct *mm) { }
> #endif
>
> Not sure what folks prefer.
>
> In either case I think it's probably worth a short comment explaining
> why it's worth the trouble (ie. that the atomic_read() prevents the
> compiler from doing DCE).
Since you ask, I like the proposed change (the inline one) best. But
yeah, comment please!
(And it is not about DCE -- just the definition of __READ_ONCE makes it
directly impossible to CSE any expressions with this, it (standards-
violatingly) casts the pointers to pointers to volatile, and you cannot
CSE any accesses to volatile objects!)
So what are the actual semantics the kernel wants from its READ_ONCE,
and from its atomics in general? GCC has perfectly fine in-compiler
support for such things, there is no need for making a second rate
manual implementation of parts of this, when you can use a good
implementation of everything instead!
Segher
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