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 5E30DEB64D7 for ; Wed, 21 Jun 2023 14:20:51 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232834AbjFUOUu (ORCPT ); Wed, 21 Jun 2023 10:20:50 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:37006 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232814AbjFUOUp (ORCPT ); Wed, 21 Jun 2023 10:20:45 -0400 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 9867C94 for ; Wed, 21 Jun 2023 07:20:43 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=2jA/tM4AlkQTFj+2INBFFGNIdh2rLm+mNnKcsM50PIs=; b=CEIamnuW80b9glwGdAltGZZ7A4 gdZC9jTxbR8oPaKOCXoFaQiYIUwRttHcbeXqKfWk+vPTJof0xU2xXPTkJmTxbT58z7uoykso+boee +5HIDnYkZMU3vGQZOvpRwbtfGdqbc636r1D8ZGc7DReWSO0xrY7aRksr/lc0396Du8AzuyPnMlE57 8ksIlSwoylVym2525KxvZLDwjseDQBhu+7PdiGp8TuhFm3AgWveQ6F9atOBoh6abHOM3uVPb8muTf 67m/OZQ0u5KUjIva2/uOZjy2M1SHkOIpMXzJe/vKdUbXR8y3O3V3c4Wfohx9BRJ5kRdZyPA97F44R w14iy7Ow==; Received: from j130084.upc-j.chello.nl ([24.132.130.84] helo=noisy.programming.kicks-ass.net) by casper.infradead.org with esmtpsa (Exim 4.94.2 #2 (Red Hat Linux)) id 1qByh4-00EPKo-U7; Wed, 21 Jun 2023 14:20:23 +0000 Received: from hirez.programming.kicks-ass.net (hirez.programming.kicks-ass.net [192.168.1.225]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (4096 bits) server-digest SHA256) (Client did not present a certificate) by noisy.programming.kicks-ass.net (Postfix) with ESMTPS id CF0B8300274; Wed, 21 Jun 2023 16:20:20 +0200 (CEST) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 88E6F2130B3BB; Wed, 21 Jun 2023 16:20:20 +0200 (CEST) Date: Wed, 21 Jun 2023 16:20:20 +0200 From: Peter Zijlstra To: David Vernet Cc: linux-kernel@vger.kernel.org, mingo@redhat.com, juri.lelli@redhat.com, vincent.guittot@linaro.org, rostedt@goodmis.org, dietmar.eggemann@arm.com, bsegall@google.com, mgorman@suse.de, bristot@redhat.com, vschneid@redhat.com, joshdon@google.com, roman.gushchin@linux.dev, tj@kernel.org, kernel-team@meta.com Subject: Re: [RFC PATCH 3/3] sched: Implement shared wakequeue in CFS Message-ID: <20230621142020.GG2053369@hirez.programming.kicks-ass.net> References: <20230613052004.2836135-1-void@manifault.com> <20230613052004.2836135-4-void@manifault.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20230613052004.2836135-4-void@manifault.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jun 13, 2023 at 12:20:04AM -0500, David Vernet wrote: > +struct swqueue { > + struct list_head list; > + spinlock_t lock; > +} ____cacheline_aligned; I'm thinking you can shard this just fine, it makes that pop() needs to iterate all shards, but that shouldn't be a problem, and it would still only need to take a single lock. I'm thinking 4 or 8 shards should be plenty, even for Intel LLC. > #ifdef CONFIG_SMP > +static struct task_struct *swqueue_pull_task(struct swqueue *swqueue) > +{ > + unsigned long flags; > + > + struct task_struct *p; > + > + spin_lock_irqsave(&swqueue->lock, flags); > + p = list_first_entry_or_null(&swqueue->list, struct task_struct, > + swqueue_node); > + if (p) > + list_del_init(&p->swqueue_node); > + spin_unlock_irqrestore(&swqueue->lock, flags); > + > + return p; > +} Would this not normally be called pop() or somesuch? > +static void swqueue_enqueue(struct rq *rq, struct task_struct *p, int enq_flags) > +{ > + unsigned long flags; > + struct swqueue *swqueue; > + bool task_migrated = enq_flags & ENQUEUE_MIGRATED; > + bool task_wakeup = enq_flags & ENQUEUE_WAKEUP; > + > + /* > + * Only enqueue the task in the shared wakequeue if: > + * > + * - SWQUEUE is enabled > + * - The task is on the wakeup path > + * - The task wasn't purposefully migrated to the current rq by > + * select_task_rq() > + * - The task isn't pinned to a specific CPU > + */ > + if (!task_wakeup || task_migrated || p->nr_cpus_allowed == 1) > + return; Elsewhere you mentioned heuristics, this smells like them. This and the is_cpus_allowed() thing makes you loose plenty of opportunities. > + swqueue = rq_swqueue(rq); > + spin_lock_irqsave(&swqueue->lock, flags); > + list_add_tail(&p->swqueue_node, &swqueue->list); > + spin_unlock_irqrestore(&swqueue->lock, flags); > +} > + > static int swqueue_pick_next_task(struct rq *rq, struct rq_flags *rf) > { > - return 0; > + struct swqueue *swqueue; > + struct task_struct *p = NULL; > + struct rq *src_rq; > + struct rq_flags src_rf; > + int ret; > + > + swqueue = rq_swqueue(rq); > + if (!list_empty(&swqueue->list)) > + p = swqueue_pull_task(swqueue); > + > + if (!p) > + return 0; At this point you can do the whole is_cpu_allowed() and avoid the whole lock dance if not. > + > + rq_unpin_lock(rq, rf); > + raw_spin_rq_unlock(rq); > + > + src_rq = task_rq_lock(p, &src_rf); > + > + if (task_on_rq_queued(p) && !task_on_cpu(rq, p)) > + src_rq = migrate_task_to(src_rq, &src_rf, p, cpu_of(rq)); And then this becomes move_queued_task(). > + if (src_rq->cpu != rq->cpu) > + ret = 1; > + else > + ret = -1; > + > + task_rq_unlock(src_rq, p, &src_rf); > + > + raw_spin_rq_lock(rq); > + rq_repin_lock(rq, rf); > + > + return ret; > } > > static void swqueue_remove_task(struct task_struct *p) > +{ > + unsigned long flags; > + struct swqueue *swqueue; > + > + if (!list_empty(&p->swqueue_node)) { > + swqueue = rq_swqueue(task_rq(p)); > + spin_lock_irqsave(&swqueue->lock, flags); > + list_del_init(&p->swqueue_node); > + spin_unlock_irqrestore(&swqueue->lock, flags); > + } > +} dequeue()