From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from canpmsgout01.his.huawei.com (canpmsgout01.his.huawei.com [113.46.200.216]) (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 DE1023B8BCC for ; Fri, 11 Sep 2026 07:16:35 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=113.46.200.216 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789111004; cv=none; b=dOpZ7ZRpbBeJCRFGNeZG8t7QZ3Yd8syNNXcKLorjRkkKfHJpMIvqnGOcKXD2Mai2+r3NvVi9Y1YJ6Y8kuqO+6YTFlgqHQLUPQJgSgZBZDRu9L3/JG7/upHTYS/3VjCOwywCEQ61v9YTzL67uCb1lXrrr8NlLM5Db8q1EG7LCjE8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789111004; c=relaxed/simple; bh=BuiR4oTs+8gMqO0czVoF2Kp9Fi+Lc6NImRbf0axOnA4=; h=Message-ID:Date:MIME-Version:Subject:To:CC:References:From: In-Reply-To:Content-Type; b=czIkhw8bp96eFM236zkc0joRDlVs75AZFCyjb4Mbjog3qj0vZwpQOcQlVfZY+VaZnwrQ1vAiWW0nWdkB1PQ6gqRvN67eJo+0ih+l5QOI9cnIBv/PzDo6nRXAjnhWLEkbZlQt1u7gK9GKUvosORjLDBtsZlIwWZPTJhU4OeafDtQ= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=huawei.com; spf=pass smtp.mailfrom=huawei.com; dkim=pass (1024-bit key) header.d=huawei.com header.i=@huawei.com header.b=JlGuL2YW; arc=none smtp.client-ip=113.46.200.216 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=huawei.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=huawei.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=huawei.com header.i=@huawei.com header.b="JlGuL2YW" dkim-signature: v=1; a=rsa-sha256; d=huawei.com; s=dkim; c=relaxed/relaxed; q=dns/txt; h=From; bh=8uJfGzvZ4UfZNRvl+QJ3IIITqizbFys+QsmzU2Q6ZmY=; b=JlGuL2YWOcWUOgp2ty0N8wzraY7AGN7t7d6h/tHCR/RQZEWkzNA0qSkNbgBJ+1Hi4IRBlugVX e0Erb4XRFiE+XDQSVuQ8Hw7cgxqgs5Y2CPsCKOaCLx9r9jmtVKfZCcaWQJ5pOTCRagAqKJmbcyA GOykGflqYUkIvdo6mu2CCo0= Received: from mail.maildlp.com (unknown [172.19.162.140]) by canpmsgout01.his.huawei.com (SkyGuard) with ESMTPS id 4hh5C76CZZz1T4KL; Fri, 11 Sep 2026 15:04:51 +0800 (CST) Received: from kwepemk200008.china.huawei.com (unknown [7.202.194.74]) by mail.maildlp.com (Postfix) with ESMTPS id 597AD203C1; Fri, 11 Sep 2026 15:16:25 +0800 (CST) Received: from [10.67.109.254] (10.67.109.254) by kwepemk200008.china.huawei.com (7.202.194.74) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.2.2562.45; Fri, 11 Sep 2026 15:16:24 +0800 Message-ID: <0fa7d64c-9ccb-4a25-b40a-6ffbf5e33406@huawei.com> Date: Fri, 11 Sep 2026 15:16:23 +0800 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 03/19] cpu/hotplug: Avoid busy-polling on archs where cpu_relax() is a no-op To: Will Deacon , CC: , Thomas Gleixner , Catalin Marinas , Borislav Petkov , Lorenzo Pieralisi , Mark Rutland , David Woodhouse , Peter Zijlstra , Marc Zyngier References: <20260907164024.17164-1-will@kernel.org> <20260907164024.17164-4-will@kernel.org> From: Jinjie Ruan In-Reply-To: <20260907164024.17164-4-will@kernel.org> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 8bit X-ClientProxiedBy: kwepems100002.china.huawei.com (7.221.188.206) To kwepemk200008.china.huawei.com (7.202.194.74) 在 2026/9/8 0:40, Will Deacon 写道: > 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); > +} As sashiko pointed out, it seems that the else branch unreachable for the call of cpuhp_wait_for_sync_state(cpu, SYNC_STATE_ALIVE, SYNC_STATE_SHOULD_ONLINE), the old state can only be "SYNC_STATE_KICKED" or "SYNC_STATE_SHOULD_DIE", which go the if branch. Link: https://sashiko.dev/#/patchset/20260907164024.17164-1-will%40kernel.org > > 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)