From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 19F4DC77B73 for ; Mon, 17 Apr 2023 02:10:49 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229636AbjDQCKr (ORCPT ); Sun, 16 Apr 2023 22:10:47 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:42608 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229458AbjDQCKo (ORCPT ); Sun, 16 Apr 2023 22:10:44 -0400 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 3386611D for ; Sun, 16 Apr 2023 19:09:57 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1681697396; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=CLjKnf5uknZ4P+fwmv5E2U11bhOC2FIN/rpzg2Ro8Ds=; b=LMVgDDWV2LMfWzo9JSKQAdf/BtC8WChyZLcIxN7QlkCNruJVaPl0zIxEFcq2xi4NtPHMO1 VFftvbr9KjIbRik3MJSkqMwFOH10H1CUFUHoRsfEVYHX8vZjCW1jL/LET3dZcW8fymmyk0 I8z0EAgI9mVA8vYcKL110bVDe6Ktrh4= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-3-fpeBPTchMlmZpa5tuOjigw-1; Sun, 16 Apr 2023 22:09:44 -0400 X-MC-Unique: fpeBPTchMlmZpa5tuOjigw-1 Received: from smtp.corp.redhat.com (int-mx10.intmail.prod.int.rdu2.redhat.com [10.11.54.10]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 54381101A551; Mon, 17 Apr 2023 02:09:44 +0000 (UTC) Received: from rh (vpn2-52-17.bne.redhat.com [10.64.52.17]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 3821C492B03; Mon, 17 Apr 2023 02:09:43 +0000 (UTC) Received: from localhost ([::1] helo=rh) by rh with esmtps (TLS1.3) tls TLS_AES_256_GCM_SHA384 (Exim 4.96) (envelope-from ) id 1poEJH-0027rH-2H; Mon, 17 Apr 2023 12:09:39 +1000 Date: Mon, 17 Apr 2023 12:09:37 +1000 From: Dave Chinner To: Thomas Gleixner Cc: LKML , Peter Zijlstra , Valentin Schneider , Dennis Zhou , Tejun Heo , Christoph Lameter , Yury Norov , Andy Shevchenko , Rasmus Villemoes , Ye Bin , linux-mm@kvack.org Subject: Re: [patch 1/3] lib/percpu_counter: Fix CPU hotplug handling Message-ID: References: <20230414162755.281993820@linutronix.de> <20230414162841.166896739@linutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20230414162841.166896739@linutronix.de> X-Scanned-By: MIMEDefang 3.1 on 10.11.54.10 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Apr 14, 2023 at 06:30:43PM +0200, Thomas Gleixner wrote: > Commit 8b57b11cca88 ("pcpcntrs: fix dying cpu summation race") tried to > address a race condition between percpu_counter_sum() and a concurrent CPU > hotplug operation. > > The race window is between the point where an un-plugged CPU removed itself > from the online_cpu_mask and the hotplug state callback which folds the per > CPU counters of the now dead CPU into the global count. > > percpu_counter_sum() used for_each_online_cpu() to accumulate the per CPU > local counts, so during the race window it missed to account for the not > yet folded back local count of the offlined CPU. > > The attempt to address this used the admittedly undocumented and > pointlessly public cpu_dying_mask by changing the loop iterator to take > both the cpu_online_mask and the cpu_dying_mask into account. > > That works to some extent, but it is incorrect. > > The cpu_dying_mask bits are sticky even after cpu_up()/cpu_down() > completes. That means that all offlined CPUs are always taken into > account. In the case of disabling SMT at boottime or runtime this results > in evaluating _all_ offlined SMT siblings counters forever. Depending on > system size, that's a massive amount of cache-lines to be touched forever. > > It might be argued, that the cpu_dying_mask bit could be cleared when > cpu_down() completes, but that's not possible under all circumstances. > > Especially with partial hotplug the bit must be sticky in order to keep the > initial user, i.e. the scheduler correct. Partial hotplug which allows > explicit state transitions also can create a situation where the race > window gets recreated: > > cpu_down(target = CPUHP_PERCPU_CNT_DEAD + 1) > > brings a CPU down to one state before the per CPU counter folding > callback. As this did not reach CPUHP_OFFLINE state the bit would stay set. > Now the next partial operation: > > cpu_up(target = CPUHP_PERCPU_CNT_DEAD + 2) > > has to clear the bit and the race window is open again. > > There are two ways to solve this: > > 1) Maintain a local CPU mask in the per CPU counter code which > gets the bit set when a CPU comes online and removed in the > the CPUHP_PERCPU_CNT_DEAD state after folding. > > This adds more code and complexity. > > 2) Move the folding hotplug state into the DYING callback section, which > runs on the outgoing CPU immediatedly after it cleared its online bit. > > There is no concurrency vs. percpu_counter_sum() on another CPU > because all still online CPUs are waiting in stop_machine() for the > outgoing CPU to complete its shutdown. The raw spinlock held around > the CPU mask iteration prevents that an online CPU reaches the stop > machine thread while iterating, which implicitely prevents the > outgoing CPU from clearing its online bit. > > This is way simpler than #1 and makes the hotplug calls symmetric for > the price of a slightly longer wait time in stop_machine(), which is > not the end of the world as CPU un-plug is already slow. The overall > time for a cpu_down() operation stays exactly the same. > > Implement #2 and plug the race completely. > > percpu_counter_sum() is still inherently racy against a concurrent > percpu_counter_add_batch() fastpath unless externally serialized. That's > completely independent of CPU hotplug though. > > Fixes: 8b57b11cca88 ("pcpcntrs: fix dying cpu summation race") > Signed-off-by: Thomas Gleixner > Cc: Dennis Zhou > Cc: Tejun Heo > Cc: Christoph Lameter > Cc: Dave Chinner > Cc: Yury Norov > Cc: Andy Shevchenko > Cc: Rasmus Villemoes > Cc: Ye Bin > Cc: linux-mm@kvack.org > --- > include/linux/cpuhotplug.h | 2 - > lib/percpu_counter.c | 57 +++++++++++++++++++-------------------------- > 2 files changed, 26 insertions(+), 33 deletions(-) > > --- a/include/linux/cpuhotplug.h > +++ b/include/linux/cpuhotplug.h > @@ -91,7 +91,6 @@ enum cpuhp_state { > CPUHP_PRINTK_DEAD, > CPUHP_MM_MEMCQ_DEAD, > CPUHP_XFS_DEAD, > - CPUHP_PERCPU_CNT_DEAD, > CPUHP_RADIX_DEAD, > CPUHP_PAGE_ALLOC, > CPUHP_NET_DEV_DEAD, > @@ -196,6 +195,7 @@ enum cpuhp_state { > CPUHP_AP_SMPCFD_DYING, > CPUHP_AP_X86_TBOOT_DYING, > CPUHP_AP_ARM_CACHE_B15_RAC_DYING, > + CPUHP_AP_PERCPU_COUNTER_STARTING, > CPUHP_AP_ONLINE, > CPUHP_TEARDOWN_CPU, > > --- a/lib/percpu_counter.c > +++ b/lib/percpu_counter.c > @@ -12,7 +12,7 @@ > > #ifdef CONFIG_HOTPLUG_CPU > static LIST_HEAD(percpu_counters); > -static DEFINE_SPINLOCK(percpu_counters_lock); > +static DEFINE_RAW_SPINLOCK(percpu_counters_lock); > #endif > > #ifdef CONFIG_DEBUG_OBJECTS_PERCPU_COUNTER > @@ -126,13 +126,8 @@ EXPORT_SYMBOL(percpu_counter_sync); > * Add up all the per-cpu counts, return the result. This is a more accurate > * but much slower version of percpu_counter_read_positive(). > * > - * We use the cpu mask of (cpu_online_mask | cpu_dying_mask) to capture sums > - * from CPUs that are in the process of being taken offline. Dying cpus have > - * been removed from the online mask, but may not have had the hotplug dead > - * notifier called to fold the percpu count back into the global counter sum. > - * By including dying CPUs in the iteration mask, we avoid this race condition > - * so __percpu_counter_sum() just does the right thing when CPUs are being taken > - * offline. > + * Note: This function is inherently racy against the lockless fastpath of > + * percpu_counter_add_batch() unless externaly serialized. > */ > s64 __percpu_counter_sum(struct percpu_counter *fbc) > { > @@ -142,10 +137,8 @@ s64 __percpu_counter_sum(struct percpu_c > > raw_spin_lock_irqsave(&fbc->lock, flags); > ret = fbc->count; > - for_each_cpu_or(cpu, cpu_online_mask, cpu_dying_mask) { > - s32 *pcount = per_cpu_ptr(fbc->counters, cpu); > - ret += *pcount; > - } > + for_each_online_cpu(cpu) > + ret += *per_cpu_ptr(fbc->counters, cpu); > raw_spin_unlock_irqrestore(&fbc->lock, flags); > return ret; > } > @@ -167,9 +160,9 @@ int __percpu_counter_init(struct percpu_ > > #ifdef CONFIG_HOTPLUG_CPU > INIT_LIST_HEAD(&fbc->list); > - spin_lock_irqsave(&percpu_counters_lock, flags); > + raw_spin_lock_irqsave(&percpu_counters_lock, flags); > list_add(&fbc->list, &percpu_counters); > - spin_unlock_irqrestore(&percpu_counters_lock, flags); > + raw_spin_unlock_irqrestore(&percpu_counters_lock, flags); > #endif > return 0; > } > @@ -185,9 +178,9 @@ void percpu_counter_destroy(struct percp > debug_percpu_counter_deactivate(fbc); > > #ifdef CONFIG_HOTPLUG_CPU > - spin_lock_irqsave(&percpu_counters_lock, flags); > + raw_spin_lock_irqsave(&percpu_counters_lock, flags); > list_del(&fbc->list); > - spin_unlock_irqrestore(&percpu_counters_lock, flags); > + raw_spin_unlock_irqrestore(&percpu_counters_lock, flags); > #endif > free_percpu(fbc->counters); > fbc->counters = NULL; > @@ -197,22 +190,29 @@ EXPORT_SYMBOL(percpu_counter_destroy); > int percpu_counter_batch __read_mostly = 32; > EXPORT_SYMBOL(percpu_counter_batch); > > -static int compute_batch_value(unsigned int cpu) > +static void compute_batch_value(int offs) > { > - int nr = num_online_cpus(); > + int nr = num_online_cpus() + offs; > + > + percpu_counter_batch = max(32, nr * 2); > +} > > - percpu_counter_batch = max(32, nr*2); > +static int percpu_counter_cpu_starting(unsigned int cpu) > +{ > + /* If invoked during hotplug @cpu is not yet marked online. */ > + compute_batch_value(cpu_online(cpu) ? 0 : 1); > return 0; > } So this changes the batch size based on whether the CPU is starting or dying to try to get _compare() to fall into the slow path correctly? How is this supposed to work with counters that have caller supplied custom batch sizes? i.e. use percpu_counter_add_batch() and __percpu_counter_compare() with their own batch sizes directly? Do they now need to add their own cpu hotplug hooks to screw around with their batch sizes as well? -Dave. -- Dave Chinner dchinner@redhat.com