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Fri, 23 Aug 2024 10:56:12 -0700 (PDT) Date: Fri, 23 Aug 2024 10:56:12 -0700 (PDT) From: "Christoph Lameter (Ampere)" To: Will Deacon cc: Catalin Marinas , Peter Zijlstra , Ingo Molnar , Waiman Long , Boqun Feng , Linus Torvalds , linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-arch@vger.kernel.org Subject: Re: [PATCH v2] Avoid memory barrier in read_seqcount() through load acquire In-Reply-To: <20240823103205.GA31866@willie-the-truck> Message-ID: <6427b071-40de-cad5-d1e5-e45f84ae837a@gentwo.org> References: <20240819-seq_optimize-v2-1-9d0da82b022f@gentwo.org> <20240823103205.GA31866@willie-the-truck> MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII X-Rspam-User: X-Rspamd-Queue-Id: A56F14001D X-Rspamd-Server: rspam01 X-Stat-Signature: c3xqz9m64eakg775qpmfxkfwiq1n3uu9 X-HE-Tag: 1724436265-248348 X-HE-Meta: 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 /58wCvGM e2sSKFBtwjnFISvZTYI/8QuqiEFpXjkeVsd5eW5gz0LOvuxG68eerFy0l7f6WPYSfPPogICcpSkSkslEIXO2J0GQOh5HYVj9rmemBRgqeqKARfrtWct9W9Z3jOXKGd5sp4Vn73ip8LFYDbhxkKUyuAFoZh6LNX6JxPJcr6VL8RXIST8nNb/T/Spm4nINDyt5A8WDMdE1HAaFqUyRzD0OE5ahlJ0x9FwW7ct1tQe2U+qk5VHuemJeoHLWXg9tqhr8PE9nR1l7APVS214/9BRl+NgzWeL6Xs+VI5//A+Fsrf8OEABdkEd0Pa/217qPYAp/4jMrNahfu5kytzjY= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Fri, 23 Aug 2024, Will Deacon wrote: > On Mon, Aug 19, 2024 at 11:30:15AM -0700, Christoph Lameter via B4 Relay wrote: > > +static __always_inline unsigned \ > > +__seqprop_##lockname##_sequence_acquire(const seqcount_##lockname##_t *s) \ > > +{ \ > > + unsigned seq = smp_load_acquire(&s->seqcount.sequence); \ > > + \ > > + if (!IS_ENABLED(CONFIG_PREEMPT_RT)) \ > > + return seq; \ > > + \ > > + if (preemptible && unlikely(seq & 1)) { \ > > + __SEQ_LOCK(lockbase##_lock(s->lock)); \ > > + __SEQ_LOCK(lockbase##_unlock(s->lock)); \ > > + \ > > + /* \ > > + * Re-read the sequence counter since the (possibly \ > > + * preempted) writer made progress. \ > > + */ \ > > + seq = smp_load_acquire(&s->seqcount.sequence); \ > > We could probably do even better with LDAPR here, as that should be > sufficient for this. It's a can of worms though, as it's not implemented > on all CPUs and relaxing smp_load_acquire() might introduce subtle > breakage in places where it's used to build other types of lock. Maybe > you can hack something to see if there's any performance left behind > without it? I added the following patch. Kernel booted fine. No change in the cycles of read_seq() LDAPR --------------------------- Test Single 2 CPU 4 CPU 8 CPU 16 CPU 32 CPU 64 CPU ALL write seq : 13 98 385 764 1551 3043 6259 11922 read seq : 8 8 8 8 8 8 9 10 rw seq : 8 101 247 300 467 742 1384 2101 LDA --------------------------- Test Single 2 CPU 4 CPU 8 CPU 16 CPU 32 CPU 64 CPU ALL write seq : 13 90 343 785 1533 3032 6315 11073 read seq : 8 8 8 8 8 8 9 11 rw seq : 8 79 227 313 423 755 1313 2220 Index: linux/arch/arm64/include/asm/barrier.h =================================================================== --- linux.orig/arch/arm64/include/asm/barrier.h +++ linux/arch/arm64/include/asm/barrier.h @@ -167,22 +167,22 @@ do { \ kasan_check_read(__p, sizeof(*p)); \ switch (sizeof(*p)) { \ case 1: \ - asm volatile ("ldarb %w0, %1" \ + asm volatile (".arch_extension rcpc\nldaprb %w0, %1" \ : "=r" (*(__u8 *)__u.__c) \ : "Q" (*__p) : "memory"); \ break; \ case 2: \ - asm volatile ("ldarh %w0, %1" \ + asm volatile (".arch_extension rcpc\nldaprh %w0, %1" \ : "=r" (*(__u16 *)__u.__c) \ : "Q" (*__p) : "memory"); \ break; \ case 4: \ - asm volatile ("ldar %w0, %1" \ + asm volatile (".arch_extension rcpc\nldapr %w0, %1" \ : "=r" (*(__u32 *)__u.__c) \ : "Q" (*__p) : "memory"); \ break; \ case 8: \ - asm volatile ("ldar %0, %1" \ + asm volatile (".arch_extension rcpc\nldapr %0, %1" \ : "=r" (*(__u64 *)__u.__c) \ : "Q" (*__p) : "memory"); \ break; \