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 X-Spam-Level: X-Spam-Status: No, score=-6.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id CFD4EC4363A for ; Tue, 27 Oct 2020 20:58:12 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 6EA5520706 for ; Tue, 27 Oct 2020 20:58:12 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=linutronix.de header.i=@linutronix.de header.b="OCXS40Fv"; dkim=permerror (0-bit key) header.d=linutronix.de header.i=@linutronix.de header.b="GHAeAruU" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S373481AbgJ0U6L (ORCPT ); Tue, 27 Oct 2020 16:58:11 -0400 Received: from Galois.linutronix.de ([193.142.43.55]:49188 "EHLO galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S373468AbgJ0U6K (ORCPT ); Tue, 27 Oct 2020 16:58:10 -0400 Date: Tue, 27 Oct 2020 21:58:06 +0100 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020; t=1603832287; 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=F0fJAkaJOg8MghLZ3WG/sCJuQU/mVqUutTyE3oHxMzk=; b=OCXS40Fvu9bQ6xCZaPug1KgaEmYKhXN/ZJ0k4P8VVRI2Tn6uMCLWJWkjnf8gIxEngul2+X mXTIHdNex5s7rq9bE9hgbcOB73QRTEDlIjtYfQDWyP8SvmKxKyHXGwCt5loVW0lOUXk+gs F7c3CaXAedTVsLkrnGdg7SJxnkpvTEF3v+8tXTCL5ErFaz2KMq/Shhqr7xu3EjgpEt4+68 IXHwkYjnj1GfmhodJFblNq8QlHtIvnATLxy/q2ZEz9b1m1YC4q+3okhAK9iiB2J9QR/zTd Y+Te+uDDyJ7yW2qi6cdra41TXaphnfHmfogxYU1cp2F07uREdQ4iq0J1XkZIuQ== DKIM-Signature: v=1; a=ed25519-sha256; c=relaxed/relaxed; d=linutronix.de; s=2020e; t=1603832287; 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=F0fJAkaJOg8MghLZ3WG/sCJuQU/mVqUutTyE3oHxMzk=; b=GHAeAruUaSSLW9G3JOZouivv79VFgzdGEu5igh8KPJTlMXQq2AlG8Sy380jTg6FjuJJEod Rr7B6BYfwg2clIBw== From: Sebastian Andrzej Siewior To: Christoph Hellwig Cc: Thomas Gleixner , David Runge , linux-rt-users@vger.kernel.org, Jens Axboe , linux-block@vger.kernel.org, linux-kernel@vger.kernel.org, Peter Zijlstra , Daniel Wagner Subject: Re: [PATCH RFC] blk-mq: Don't IPI requests on PREEMPT_RT Message-ID: <20201027205806.s7qau5xf4lzuslcu@linutronix.de> References: <20201021175059.GA4989@hmbx> <20201023110400.bx3uzsb7xy5jtsea@linutronix.de> <20201023112130.GA23790@infradead.org> <20201023135219.mzzl76eqqy6tqwhe@linutronix.de> <20201027092606.GA20805@infradead.org> <20201027101102.cvczdb3mkvtoguo5@linutronix.de> <20201027160742.GA19073@infradead.org> <87eelj1tx0.fsf@nanos.tec.linutronix.de> <20201027172309.GA15004@infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20201027172309.GA15004@infradead.org> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2020-10-27 17:23:09 [+0000], Christoph Hellwig wrote: > Ok. I was hoping we could hide this in core code somehow, especially > a peterz didn't like the use of smp_call_function_single_async in the > blk-mq completion code very much. No idea how you could efficiently avoid smp_call_function_single_async(): - workqueue (okay) - a timer_list timer which expires now. More code plus it may delay up to HZ. > Sebastian, would this solve your preempt-rt and lockdep issues? the problem with that is that on RT/force-threaded it will always wake ksoftirqd thread and complete there. That are extra steps which should be probably avoided. Now. The hunk in blk_mq_complete_need_ipi() will avoid the waking a thread with force-threaded enabled. The remaining part is a switch to llist which avoids locking (IRQ off/on) and it allows invoke the IPI/raise softirq only if something was added. The entries are now processed in the reverse order but this shouldn't matter right? I would split this into two patches (the blk_mq_complete_need_ipi() hunk and the llist part) unless there are objections. diff --git a/block/blk-mq.c b/block/blk-mq.c index 55bcee5dc0320..d2452ee9b0e2c 100644 --- a/block/blk-mq.c +++ b/block/blk-mq.c @@ -41,7 +41,7 @@ #include "blk-mq-sched.h" #include "blk-rq-qos.h" -static DEFINE_PER_CPU(struct list_head, blk_cpu_done); +static DEFINE_PER_CPU(struct llist_head, blk_cpu_done); static void blk_mq_poll_stats_start(struct request_queue *q); static void blk_mq_poll_stats_fn(struct blk_stat_callback *cb); @@ -565,80 +565,31 @@ void blk_mq_end_request(struct request *rq, blk_status_t error) } EXPORT_SYMBOL(blk_mq_end_request); -/* - * Softirq action handler - move entries to local list and loop over them - * while passing them to the queue registered handler. - */ -static __latent_entropy void blk_done_softirq(struct softirq_action *h) +static void blk_complete_reqs(struct llist_head *cpu_list) { - struct list_head *cpu_list, local_list; + struct llist_node *entry; + struct request *rq, *rq_next; - local_irq_disable(); - cpu_list = this_cpu_ptr(&blk_cpu_done); - list_replace_init(cpu_list, &local_list); - local_irq_enable(); + entry = llist_del_all(cpu_list); - while (!list_empty(&local_list)) { - struct request *rq; - - rq = list_entry(local_list.next, struct request, ipi_list); - list_del_init(&rq->ipi_list); + llist_for_each_entry_safe(rq, rq_next, entry, ipi_list) rq->q->mq_ops->complete(rq); - } } -static void blk_mq_trigger_softirq(struct request *rq) +static __latent_entropy void blk_done_softirq(struct softirq_action *h) { - struct list_head *list; - unsigned long flags; - - local_irq_save(flags); - list = this_cpu_ptr(&blk_cpu_done); - list_add_tail(&rq->ipi_list, list); - - /* - * If the list only contains our just added request, signal a raise of - * the softirq. If there are already entries there, someone already - * raised the irq but it hasn't run yet. - */ - if (list->next == &rq->ipi_list) - raise_softirq_irqoff(BLOCK_SOFTIRQ); - local_irq_restore(flags); + blk_complete_reqs(this_cpu_ptr(&blk_cpu_done)); } static int blk_softirq_cpu_dead(unsigned int cpu) { - /* - * If a CPU goes away, splice its entries to the current CPU - * and trigger a run of the softirq - */ - local_irq_disable(); - list_splice_init(&per_cpu(blk_cpu_done, cpu), - this_cpu_ptr(&blk_cpu_done)); - raise_softirq_irqoff(BLOCK_SOFTIRQ); - local_irq_enable(); - + blk_complete_reqs(&per_cpu(blk_cpu_done, cpu)); return 0; } - static void __blk_mq_complete_request_remote(void *data) { - struct request *rq = data; - - /* - * For most of single queue controllers, there is only one irq vector - * for handling I/O completion, and the only irq's affinity is set - * to all possible CPUs. On most of ARCHs, this affinity means the irq - * is handled on one specific CPU. - * - * So complete I/O requests in softirq context in case of single queue - * devices to avoid degrading I/O performance due to irqsoff latency. - */ - if (rq->q->nr_hw_queues == 1) - blk_mq_trigger_softirq(rq); - else - rq->q->mq_ops->complete(rq); + __raise_softirq_irqoff(BLOCK_SOFTIRQ); } static inline bool blk_mq_complete_need_ipi(struct request *rq) @@ -648,6 +599,14 @@ static inline bool blk_mq_complete_need_ipi(struct request *rq) if (!IS_ENABLED(CONFIG_SMP) || !test_bit(QUEUE_FLAG_SAME_COMP, &rq->q->queue_flags)) return false; + /* + * With force threaded interrupts enabled, raising softirq from an SMP + * function call will always result in waking the ksoftirqd thread. + * This is probably worse than completing the request on a different + * cache domain. + */ + if (force_irqthreads) + return false; /* same CPU or cache domain? Complete locally */ if (cpu == rq->mq_ctx->cpu || @@ -661,6 +620,7 @@ static inline bool blk_mq_complete_need_ipi(struct request *rq) bool blk_mq_complete_request_remote(struct request *rq) { + struct llist_head *cpu_list; WRITE_ONCE(rq->state, MQ_RQ_COMPLETE); /* @@ -671,14 +631,21 @@ bool blk_mq_complete_request_remote(struct request *rq) return false; if (blk_mq_complete_need_ipi(rq)) { - rq->csd.func = __blk_mq_complete_request_remote; - rq->csd.info = rq; - rq->csd.flags = 0; - smp_call_function_single_async(rq->mq_ctx->cpu, &rq->csd); + unsigned int cpu; + + cpu = rq->mq_ctx->cpu; + cpu_list = &per_cpu(blk_cpu_done, cpu); + if (llist_add(&rq->ipi_list, cpu_list)) { + rq->csd.func = __blk_mq_complete_request_remote; + rq->csd.flags = 0; + smp_call_function_single_async(cpu, &rq->csd); + } } else { if (rq->q->nr_hw_queues > 1) return false; - blk_mq_trigger_softirq(rq); + cpu_list = this_cpu_ptr(&blk_cpu_done); + if (llist_add(&rq->ipi_list, cpu_list)) + raise_softirq(BLOCK_SOFTIRQ); } return true; @@ -3909,7 +3876,7 @@ static int __init blk_mq_init(void) int i; for_each_possible_cpu(i) - INIT_LIST_HEAD(&per_cpu(blk_cpu_done, i)); + init_llist_head(&per_cpu(blk_cpu_done, i)); open_softirq(BLOCK_SOFTIRQ, blk_done_softirq); cpuhp_setup_state_nocalls(CPUHP_BLOCK_SOFTIRQ_DEAD, diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h index 639cae2c158b5..331b2b675b417 100644 --- a/include/linux/blkdev.h +++ b/include/linux/blkdev.h @@ -156,7 +156,7 @@ struct request { */ union { struct hlist_node hash; /* merge hash */ - struct list_head ipi_list; + struct llist_node ipi_list; }; /*