From nobody Mon Feb 9 18:44:16 2026 Delivered-To: importer@patchew.org Received-SPF: pass (zohomail.com: domain of lists.xenproject.org designates 192.237.175.120 as permitted sender) client-ip=192.237.175.120; envelope-from=xen-devel-bounces@lists.xenproject.org; helo=lists.xenproject.org; Authentication-Results: mx.zohomail.com; spf=pass (zohomail.com: domain of lists.xenproject.org designates 192.237.175.120 as permitted sender) smtp.mailfrom=xen-devel-bounces@lists.xenproject.org ARC-Seal: i=1; a=rsa-sha256; t=1585214411; cv=none; d=zohomail.com; s=zohoarc; b=N17xItEmzGQhxZNDSKmwY3g80jlsknqu+KMqJfcoyB/498+yGxUVqnzKU46eqqbG83pgGzNIC9VsC+icFdu8XKeEmGQBOZ+T96GxJW+iEfQV+iYiGKUeTkg+/kHy4DxnvfM/byMtTDtS+jPBestXzShr98UauNxBevXlCglKyFs= ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=zohomail.com; s=zohoarc; t=1585214411; h=Cc:Date:From:In-Reply-To:List-Subscribe:List-Post:List-Id:List-Help:List-Unsubscribe:Message-ID:References:Sender:Subject:To; bh=mXS4NdVS5UJO3Qq+HsCK66E8PHbg1Zhtcd2i8315WN8=; b=ec+t4V/mcOoRsu9XhOcD+Y9oXqss20XojAAWZ+9VbaYsX08YaEwJxhFWXlyqc41e8ETuiDgwmj1f14/BSQ/DndEKVfVd0cVwpBHL1ZS7CyXgioGqNxA11zwJeVvD2o5AWRIlyTiIbFirXJf6RPma7+vXxKJbGBNFusTVOassLoE= ARC-Authentication-Results: i=1; mx.zohomail.com; spf=pass (zohomail.com: domain of lists.xenproject.org designates 192.237.175.120 as permitted sender) smtp.mailfrom=xen-devel-bounces@lists.xenproject.org Return-Path: Received: from lists.xenproject.org (lists.xenproject.org [192.237.175.120]) by mx.zohomail.com with SMTPS id 1585214411616293.41887339442997; Thu, 26 Mar 2020 02:20:11 -0700 (PDT) Received: from localhost ([127.0.0.1] helo=lists.xenproject.org) by lists.xenproject.org with esmtp (Exim 4.89) (envelope-from ) id 1jHOfo-0005dA-Ge; Thu, 26 Mar 2020 09:19:36 +0000 Received: from all-amaz-eas1.inumbo.com ([34.197.232.57] helo=us1-amaz-eas2.inumbo.com) by lists.xenproject.org with esmtp (Exim 4.89) (envelope-from ) id 1jHOfn-0005ci-2U for xen-devel@lists.xenproject.org; Thu, 26 Mar 2020 09:19:35 +0000 Received: from mx2.suse.de (unknown [195.135.220.15]) by us1-amaz-eas2.inumbo.com (Halon) with ESMTPS id e1ddaf34-6f42-11ea-877f-12813bfff9fa; Thu, 26 Mar 2020 09:19:24 +0000 (UTC) Received: from relay2.suse.de (unknown [195.135.220.254]) by mx2.suse.de (Postfix) with ESMTP id CD23BAC0C; Thu, 26 Mar 2020 09:19:22 +0000 (UTC) X-Inumbo-ID: e1ddaf34-6f42-11ea-877f-12813bfff9fa X-Virus-Scanned: by amavisd-new at test-mx.suse.de From: Juergen Gross To: xen-devel@lists.xenproject.org Date: Thu, 26 Mar 2020 10:19:15 +0100 Message-Id: <20200326091918.12388-3-jgross@suse.com> X-Mailer: git-send-email 2.16.4 In-Reply-To: <20200326091918.12388-1-jgross@suse.com> References: <20200326091918.12388-1-jgross@suse.com> Subject: [Xen-devel] [PATCH v8 2/5] xen/rcu: don't use stop_machine_run() for rcu_barrier() X-BeenThere: xen-devel@lists.xenproject.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: Xen developer discussion List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Cc: Juergen Gross , Stefano Stabellini , Julien Grall , Wei Liu , Andrew Cooper , Ian Jackson , George Dunlap , Jan Beulich Errors-To: xen-devel-bounces@lists.xenproject.org Sender: "Xen-devel" Content-Transfer-Encoding: quoted-printable MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Today rcu_barrier() is calling stop_machine_run() to synchronize all physical cpus in order to ensure all pending rcu calls have finished when returning. As stop_machine_run() is using tasklets this requires scheduling of idle vcpus on all cpus imposing the need to call rcu_barrier() on idle cpus only in case of core scheduling being active, as otherwise a scheduling deadlock would occur. There is no need at all to do the syncing of the cpus in tasklets, as rcu activity is started in __do_softirq() called whenever softirq activity is allowed. So rcu_barrier() can easily be modified to use softirq for synchronization of the cpus no longer requiring any scheduling activity. As there already is a rcu softirq reuse that for the synchronization. Remove the barrier element from struct rcu_data as it isn't used. Finally switch rcu_barrier() to return void as it now can never fail. Partially-based-on-patch-by: Igor Druzhinin Signed-off-by: Juergen Gross Reviewed-by: Jan Beulich --- V2: - add recursion detection V3: - fix races (Igor Druzhinin) V5: - rename done_count to pending_count (Jan Beulich) - fix race (Jan Beulich) V6: - add barrier (Julien Grall) - add ASSERT() (Julien Grall) - hold cpu_map lock until end of rcu_barrier() (Julien Grall) V7: - update comment (Jan Beulich) - add barriers (Jan Beulich) V8: - adjust barrier and comment (Julien Grall, Jan Beulich) --- xen/common/rcupdate.c | 108 +++++++++++++++++++++++++++++++++--------= ---- xen/include/xen/rcupdate.h | 2 +- 2 files changed, 82 insertions(+), 28 deletions(-) diff --git a/xen/common/rcupdate.c b/xen/common/rcupdate.c index 03d84764d2..f3159e470f 100644 --- a/xen/common/rcupdate.c +++ b/xen/common/rcupdate.c @@ -83,7 +83,6 @@ struct rcu_data { struct rcu_head **donetail; long blimit; /* Upper limit on a processed batch = */ int cpu; - struct rcu_head barrier; long last_rs_qlen; /* qlen during the last resched */ =20 /* 3) idle CPUs handling */ @@ -91,6 +90,7 @@ struct rcu_data { bool idle_timer_active; =20 bool process_callbacks; + bool barrier_active; }; =20 /* @@ -143,51 +143,98 @@ static int qhimark =3D 10000; static int qlowmark =3D 100; static int rsinterval =3D 1000; =20 -struct rcu_barrier_data { - struct rcu_head head; - atomic_t *cpu_count; -}; +/* + * rcu_barrier() handling: + * Two counters are used to synchronize rcu_barrier() work: + * - cpu_count holds the number of cpus required to finish barrier handlin= g. + * It is decremented by each cpu when it has performed all pending rcu c= alls. + * - pending_count shows whether any rcu_barrier() activity is running and + * it is used to synchronize leaving rcu_barrier() only after all cpus + * have finished their processing. pending_count is initialized to nr_cp= us + 1 + * and it is decremented by each cpu when it has seen that cpu_count has + * reached 0. The cpu where rcu_barrier() has been called will wait until + * pending_count has been decremented to 1 (so all cpus have seen cpu_co= unt + * reaching 0) and will then set pending_count to 0 indicating there is = no + * rcu_barrier() running. + * Cpus are synchronized via softirq mechanism. rcu_barrier() is regarded = to + * be active if pending_count is not zero. In case rcu_barrier() is called= on + * multiple cpus it is enough to check for pending_count being not zero on= entry + * and to call process_pending_softirqs() in a loop until pending_count dr= ops to + * zero, before starting the new rcu_barrier() processing. + */ +static atomic_t cpu_count =3D ATOMIC_INIT(0); +static atomic_t pending_count =3D ATOMIC_INIT(0); =20 static void rcu_barrier_callback(struct rcu_head *head) { - struct rcu_barrier_data *data =3D container_of( - head, struct rcu_barrier_data, head); - atomic_inc(data->cpu_count); + /* + * We need a barrier making all previous writes visible to other cpus + * before doing the atomic_dec(). This would be something like + * smp_mb__before_atomic() limited to writes, which isn't existing. + * So we choose the best alternative available which is smp_wmb() + * (correct on Arm and only a minor impact on x86, while + * smp_mb__before_atomic() would be correct on x86, but with a larger + * impact on Arm). + */ + smp_wmb(); + atomic_dec(&cpu_count); } =20 -static int rcu_barrier_action(void *_cpu_count) +static void rcu_barrier_action(void) { - struct rcu_barrier_data data =3D { .cpu_count =3D _cpu_count }; - - ASSERT(!local_irq_is_enabled()); - local_irq_enable(); + struct rcu_head head; =20 /* * When callback is executed, all previously-queued RCU work on this C= PU - * is completed. When all CPUs have executed their callback, data.cpu_= count - * will have been incremented to include every online CPU. + * is completed. When all CPUs have executed their callback, cpu_count + * will have been decremented to 0. */ - call_rcu(&data.head, rcu_barrier_callback); + call_rcu(&head, rcu_barrier_callback); =20 - while ( atomic_read(data.cpu_count) !=3D num_online_cpus() ) + while ( atomic_read(&cpu_count) ) { process_pending_softirqs(); cpu_relax(); } =20 - local_irq_disable(); - - return 0; + smp_mb__before_atomic(); + atomic_dec(&pending_count); } =20 -/* - * As rcu_barrier() is using stop_machine_run() it is allowed to be used in - * idle context only (see comment for stop_machine_run()). - */ -int rcu_barrier(void) +void rcu_barrier(void) { - atomic_t cpu_count =3D ATOMIC_INIT(0); - return stop_machine_run(rcu_barrier_action, &cpu_count, NR_CPUS); + unsigned int n_cpus; + + ASSERT(!in_irq() && local_irq_is_enabled()); + + for ( ; ; ) + { + if ( !atomic_read(&pending_count) && get_cpu_maps() ) + { + n_cpus =3D num_online_cpus(); + + if ( atomic_cmpxchg(&pending_count, 0, n_cpus + 1) =3D=3D 0 ) + break; + + put_cpu_maps(); + } + + process_pending_softirqs(); + cpu_relax(); + } + + atomic_set(&cpu_count, n_cpus); + cpumask_raise_softirq(&cpu_online_map, RCU_SOFTIRQ); + + while ( atomic_read(&pending_count) !=3D 1 ) + { + process_pending_softirqs(); + cpu_relax(); + } + + atomic_set(&pending_count, 0); + + put_cpu_maps(); } =20 /* Is batch a before batch b ? */ @@ -426,6 +473,13 @@ static void rcu_process_callbacks(void) rdp->process_callbacks =3D false; __rcu_process_callbacks(&rcu_ctrlblk, rdp); } + + if ( atomic_read(&cpu_count) && !rdp->barrier_active ) + { + rdp->barrier_active =3D true; + rcu_barrier_action(); + rdp->barrier_active =3D false; + } } =20 static int __rcu_pending(struct rcu_ctrlblk *rcp, struct rcu_data *rdp) diff --git a/xen/include/xen/rcupdate.h b/xen/include/xen/rcupdate.h index eb9b60df07..31c8b86d13 100644 --- a/xen/include/xen/rcupdate.h +++ b/xen/include/xen/rcupdate.h @@ -144,7 +144,7 @@ void rcu_check_callbacks(int cpu); void call_rcu(struct rcu_head *head,=20 void (*func)(struct rcu_head *head)); =20 -int rcu_barrier(void); +void rcu_barrier(void); =20 void rcu_idle_enter(unsigned int cpu); void rcu_idle_exit(unsigned int cpu); --=20 2.16.4