From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 21F2451D520 for ; Mon, 7 Sep 2026 16:40:43 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788799245; cv=none; b=qF6J1MWOjH/FJ1IkyCbkk8/KVgpuGSlz4cHm1MlEWmRonn+w/DMEgvYC8au7Ba1s1JYyah4ZoNCSK5SsnQejjJBMm6VzLSq60RZxcaJMSh9WkdhG56ubrR7f9WZlC/7dbyuD+1wo2SHbsKmyB254mcqJqg7sJaRxtO9AKxA4rP0= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1788799245; c=relaxed/simple; bh=F/eIKVxoRM7+eCCK+fTCtxM0onowSoTzMu5Ye07uNJA=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=ROK6rZkwP2nHtr3Tj8ilr1H/dw22HGOsOSgeruJA2WHYY+mwqH+Vq02jUHkRCVFvZIr90ulpToTsqZMGAF5OyMhm1EFM4DJU40VtWCrW4n839CGmfxplcLu4RXItjtMxMoE7tz49d1lZS0fK/r4mDBVFgATJekA3mS0pAxvHNdk= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=GqQiwLF6; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="GqQiwLF6" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 7A86F1F00A3D; Mon, 7 Sep 2026 16:40:41 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1788799243; bh=PaWEgmueo8IVck4dwZ9rseBcFsOQjJHuo9EpQo9RZ2Q=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=GqQiwLF6RV4PV/kPo9LX/Uy9wh7QegNKRRoE7fAt79YnY2JfLZFFZy60iRpZOp5WB tdjgrwZcdhoE1wd/pTZSJfNdGTo8O5ppaUQI4ffmQFR4/9ummYcWOwiO4PJ6CBotNx sHVV8Kg5d3WlkKtpMfEcyWchmhitnmtbqd/ff8eG02835Scub+c5eC+InTP87TQY75 unamOUxBVHNQI1zhqs1SffAPEJF/g1zbbRegcGwI72bhowr2OPT9SgceLYEdAUYk9q vtGYxNdK6m5Fk8UumgaEWP2N9XxlKr2NLJmo4G+Ov2xx7JSi3QK25FPrFpgERLqK3a KEBuXf0R5I3dQ== From: Will Deacon To: linux-arm-kernel@lists.infradead.org Cc: linux-kernel@vger.kernel.org, Will Deacon , Thomas Gleixner , Catalin Marinas , Borislav Petkov , Lorenzo Pieralisi , Jinjie Ruan , Mark Rutland , David Woodhouse , Peter Zijlstra , Marc Zyngier Subject: [PATCH 03/19] cpu/hotplug: Avoid busy-polling on archs where cpu_relax() is a no-op Date: Mon, 7 Sep 2026 17:40:06 +0100 Message-ID: <20260907164024.17164-4-will@kernel.org> X-Mailer: git-send-email 2.47.3 In-Reply-To: <20260907164024.17164-1-will@kernel.org> References: <20260907164024.17164-1-will@kernel.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit On some architectures (such as arm64), cpu_relax() is effectively a NOP and so isn't particularly efficient when used in a tight polling loop such as the CPU state synchronisation in cpuhp_wait_for_sync_state(). Once an incoming CPU has reached the SYNC_STATE_ALIVE state, we know that it is executing within the kernel and so we can use the more efficient polling mechanism provided by the atomic_cond_read* API. Extend the generic implementation of arch_cpuhp_sync_state_poll() to take the state details as additional parameters and polling using atomic_cond_read_relaxed() instead of cpu_relax() once we have reached the alive state. No change on x86. Signed-off-by: Will Deacon --- arch/x86/kernel/smpboot.c | 2 +- include/linux/cpuhotplug.h | 2 +- kernel/cpu.c | 13 +++++++++---- 3 files changed, 11 insertions(+), 6 deletions(-) diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c index ba01a9e919b7..362f85cbdbaf 100644 --- a/arch/x86/kernel/smpboot.c +++ b/arch/x86/kernel/smpboot.c @@ -1138,7 +1138,7 @@ void arch_cpuhp_cleanup_dead_cpu(unsigned int cpu) pr_info("CPU %u is now offline\n", cpu); } -void arch_cpuhp_sync_state_poll(void) +void arch_cpuhp_sync_state_poll(atomic_t *st, int old) { if (smp_ops.poll_sync_state) smp_ops.poll_sync_state(); diff --git a/include/linux/cpuhotplug.h b/include/linux/cpuhotplug.h index feb32949aeea..bbcee650155f 100644 --- a/include/linux/cpuhotplug.h +++ b/include/linux/cpuhotplug.h @@ -509,7 +509,7 @@ static inline void cpuhp_online_idle(enum cpuhp_state state) { } struct task_struct; void cpuhp_ap_sync_alive(void); -void arch_cpuhp_sync_state_poll(void); +void arch_cpuhp_sync_state_poll(atomic_t *st, int old); void arch_cpuhp_cleanup_kick_cpu(unsigned int cpu); int arch_cpuhp_kick_ap_alive(unsigned int cpu, struct task_struct *tidle); bool arch_cpuhp_init_parallel_bringup(void); diff --git a/kernel/cpu.c b/kernel/cpu.c index 97a9bfe4edad..d9fe204f02cb 100644 --- a/kernel/cpu.c +++ b/kernel/cpu.c @@ -303,7 +303,13 @@ static inline void cpuhp_ap_update_sync_state(enum cpuhp_sync_state state) (void)atomic_xchg(st, state); } -void __weak arch_cpuhp_sync_state_poll(void) { cpu_relax(); } +void __weak arch_cpuhp_sync_state_poll(atomic_t *st, int old) +{ + if (old < SYNC_STATE_ALIVE) + cpu_relax(); + else + atomic_cond_read_relaxed(st, VAL != old); +} static bool cpuhp_wait_for_sync_state(unsigned int cpu, enum cpuhp_sync_state state, enum cpuhp_sync_state next_state) @@ -328,7 +334,7 @@ static bool cpuhp_wait_for_sync_state(unsigned int cpu, enum cpuhp_sync_state st return false; } else if (now - start < NSEC_PER_MSEC) { /* Poll for one millisecond */ - arch_cpuhp_sync_state_poll(); + arch_cpuhp_sync_state_poll(st, sync); } else { usleep_range(USEC_PER_MSEC, 2 * USEC_PER_MSEC); } @@ -395,8 +401,7 @@ void cpuhp_ap_sync_alive(void) cpuhp_ap_update_sync_state(SYNC_STATE_ALIVE); /* Wait for the control CPU to release it. */ - while (atomic_read(st) != SYNC_STATE_SHOULD_ONLINE) - cpu_relax(); + atomic_cond_read_acquire(st, VAL == SYNC_STATE_SHOULD_ONLINE); } static bool cpuhp_can_boot_ap(unsigned int cpu) -- 2.55.0.979.g7e5102b832-goog