linux-kernel - Re: [PATCH v2] Avoid memory barrier in read

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Message-ID: <8dcd8772-2c0c-20af-86c4-18f32c07d1e9@gentwo.org>
Date: Fri, 23 Aug 2024 12:38:05 -0700 (PDT)
From: "Christoph Lameter (Ampere)" <cl@...two.org>
To: Will Deacon <will@...nel.org>
cc: Catalin Marinas <catalin.marinas@....com>, 
    Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>, 
    Waiman Long <longman@...hat.com>, Boqun Feng <boqun.feng@...il.com>, 
    Linus Torvalds <torvalds@...ux-foundation.org>, linux-mm@...ck.org, 
    linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org, 
    linux-arch@...r.kernel.org
Subject: Re: [PATCH v2] Avoid memory barrier in read_seqcount() through load
 acquire

On Fri, 23 Aug 2024, Will Deacon wrote:

> > +#ifdef CONFIG_ARCH_HAS_ACQUIRE_RELEASE
> > +#define raw_read_seqcount_begin(s)					\
> > +({									\
> > +	unsigned _seq;							\
> > +									\
> > +	while ((_seq = seqprop_sequence_acquire(s)) & 1)		\
> > +		cpu_relax();						\
>
> It would also be interesting to see whether smp_cond_load_acquire()
> performs any better that this loop in the !RT case.

The hack to do this follows. Kernel boots but no change in cycles. Also
builds a kernel just fine.

Another benchmark may be better. All my synthetic tests do is run the
function calls in a loop in parallel on multiple cpus.

The main effect here may be the reduction of power since the busyloop is
no longer required. I would favor a solution like this. But the patch is
not clean given the need to get rid of the const attribute with a cast.


Index: linux/include/linux/seqlock.h
===================================================================
--- linux.orig/include/linux/seqlock.h
+++ linux/include/linux/seqlock.h
@@ -325,9 +325,9 @@ SEQCOUNT_LOCKNAME(mutex,        struct m
 #define raw_read_seqcount_begin(s)					\
 ({									\
 	unsigned _seq;							\
+	seqcount_t *e = seqprop_ptr((struct seqcount_spinlock *)s);	\
 									\
-	while ((_seq = seqprop_sequence_acquire(s)) & 1)		\
-		cpu_relax();						\
+	_seq = smp_cond_load_acquire(&e->sequence, ((e->sequence & 1) == 0));	\
 									\
 	kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX);			\
 	_seq;								\