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=-9.9 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH, MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS autolearn=ham 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 19E98C4363A for ; Tue, 27 Oct 2020 19:13:54 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 9602121527 for ; Tue, 27 Oct 2020 19:13:53 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="wRl0WMjV" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1829485AbgJ0TMI (ORCPT ); Tue, 27 Oct 2020 15:12:08 -0400 Received: from merlin.infradead.org ([205.233.59.134]:51986 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1762525AbgJ0TJe (ORCPT ); Tue, 27 Oct 2020 15:09:34 -0400 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=merlin.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=CvGwbxdQWU0yUwxUSuiiHN1xqzMsQ3tR0c+NFhztTjs=; b=wRl0WMjVqZ536/QENSM5F95D2Z zKmyl13OUdU21RVCTpvd88havNzLv2qYPgEiKg0f67eXJ0Bv9tbF6S+gay5raVLsbJAwI+/syUoPV jmsj+R+tfp73VHXr9Lgm8XiypR9YDePazRl47Yjp49d9IYcf7Hy2XR7aIqVKBbq5/V8+/XIG9NF/L mfDFQ6W94Fzmb+FL7Wh6qtXCESNZcpWSk5tZ7k5pJGUvh3dF4yChOsUZu9nD09bHLZvjJllXy6ZU3 g9bvuR7ry5c7LvO4mhzJ/Opoz4ZwldrKehbSzR8AXXpmvfVlAsqQe/0bajz/LnBeCPEB3ybykNNB+ w726Fd2Q==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=noisy.programming.kicks-ass.net) by merlin.infradead.org with esmtpsa (Exim 4.92.3 #3 (Red Hat Linux)) id 1kXULS-0002ex-Rq; Tue, 27 Oct 2020 19:09:23 +0000 Received: from hirez.programming.kicks-ass.net (hirez.programming.kicks-ass.net [192.168.1.225]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (Client did not present a certificate) by noisy.programming.kicks-ass.net (Postfix) with ESMTPS id EC2F5307A7C; Tue, 27 Oct 2020 20:09:19 +0100 (CET) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id D109F203C2679; Tue, 27 Oct 2020 20:09:19 +0100 (CET) Date: Tue, 27 Oct 2020 20:09:19 +0100 From: Peter Zijlstra To: David Woodhouse Cc: linux-kernel@vger.kernel.org, Ingo Molnar , Juri Lelli , Vincent Guittot , Dietmar Eggemann , Steven Rostedt , Ben Segall , Mel Gorman , Daniel Bristot de Oliveira , Paolo Bonzini , kvm@vger.kernel.org, Oleg Nesterov Subject: Re: [PATCH v2 1/2] sched/wait: Add add_wait_queue_priority() Message-ID: <20201027190919.GO2628@hirez.programming.kicks-ass.net> References: <20201026175325.585623-1-dwmw2@infradead.org> <20201027143944.648769-1-dwmw2@infradead.org> <20201027143944.648769-2-dwmw2@infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201027143944.648769-2-dwmw2@infradead.org> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Oct 27, 2020 at 02:39:43PM +0000, David Woodhouse wrote: > From: David Woodhouse > > This allows an exclusive wait_queue_entry to be added at the head of the > queue, instead of the tail as normal. Thus, it gets to consume events > first without allowing non-exclusive waiters to be woken at all. > > The (first) intended use is for KVM IRQFD, which currently has Do you have more? You could easily special case this inside the KVM code. I don't _think_ the other users of __add_wait_queue() will mind the extra branch, but what do I know. > inconsistent behaviour depending on whether posted interrupts are > available or not. If they are, KVM will bypass the eventfd completely > and deliver interrupts directly to the appropriate vCPU. If not, events > are delivered through the eventfd and userspace will receive them when > polling on the eventfd. > > By using add_wait_queue_priority(), KVM will be able to consistently > consume events within the kernel without accidentally exposing them > to userspace when they're supposed to be bypassed. This, in turn, means > that userspace doesn't have to jump through hoops to avoid listening > on the erroneously noisy eventfd and injecting duplicate interrupts. > > Signed-off-by: David Woodhouse > --- > include/linux/wait.h | 12 +++++++++++- > kernel/sched/wait.c | 17 ++++++++++++++++- > 2 files changed, 27 insertions(+), 2 deletions(-) > > diff --git a/include/linux/wait.h b/include/linux/wait.h > index 27fb99cfeb02..fe10e8570a52 100644 > --- a/include/linux/wait.h > +++ b/include/linux/wait.h > @@ -22,6 +22,7 @@ int default_wake_function(struct wait_queue_entry *wq_entry, unsigned mode, int > #define WQ_FLAG_BOOKMARK 0x04 > #define WQ_FLAG_CUSTOM 0x08 > #define WQ_FLAG_DONE 0x10 > +#define WQ_FLAG_PRIORITY 0x20 > > /* > * A single wait-queue entry structure: > @@ -164,11 +165,20 @@ static inline bool wq_has_sleeper(struct wait_queue_head *wq_head) > > extern void add_wait_queue(struct wait_queue_head *wq_head, struct wait_queue_entry *wq_entry); > extern void add_wait_queue_exclusive(struct wait_queue_head *wq_head, struct wait_queue_entry *wq_entry); > +extern void add_wait_queue_priority(struct wait_queue_head *wq_head, struct wait_queue_entry *wq_entry); > extern void remove_wait_queue(struct wait_queue_head *wq_head, struct wait_queue_entry *wq_entry); > > static inline void __add_wait_queue(struct wait_queue_head *wq_head, struct wait_queue_entry *wq_entry) > { > - list_add(&wq_entry->entry, &wq_head->head); > + struct list_head *head = &wq_head->head; > + struct wait_queue_entry *wq; > + > + list_for_each_entry(wq, &wq_head->head, entry) { > + if (!(wq->flags & WQ_FLAG_PRIORITY)) > + break; > + head = &wq->entry; > + } > + list_add(&wq_entry->entry, head); > } So you're adding the PRIORITY things to the head of the list and need the PRIORITY flag to keep them in FIFO order there, right? While looking at this I found that weird __add_wait_queue_exclusive() which is used by fs/eventpoll.c and does something similar, except it doesn't keep the FIFO order. The Changelog doesn't state how important this property is to you. > /* > diff --git a/kernel/sched/wait.c b/kernel/sched/wait.c > index 01f5d3020589..183cc6ae68a6 100644 > --- a/kernel/sched/wait.c > +++ b/kernel/sched/wait.c > @@ -37,6 +37,17 @@ void add_wait_queue_exclusive(struct wait_queue_head *wq_head, struct wait_queue > } > EXPORT_SYMBOL(add_wait_queue_exclusive); > > +void add_wait_queue_priority(struct wait_queue_head *wq_head, struct wait_queue_entry *wq_entry) > +{ > + unsigned long flags; > + > + wq_entry->flags |= WQ_FLAG_EXCLUSIVE | WQ_FLAG_PRIORITY; > + spin_lock_irqsave(&wq_head->lock, flags); > + __add_wait_queue(wq_head, wq_entry); > + spin_unlock_irqrestore(&wq_head->lock, flags); > +} > +EXPORT_SYMBOL_GPL(add_wait_queue_priority); > + > void remove_wait_queue(struct wait_queue_head *wq_head, struct wait_queue_entry *wq_entry) > { > unsigned long flags; > @@ -57,7 +68,11 @@ EXPORT_SYMBOL(remove_wait_queue); > /* > * The core wakeup function. Non-exclusive wakeups (nr_exclusive == 0) just > * wake everything up. If it's an exclusive wakeup (nr_exclusive == small +ve > - * number) then we wake all the non-exclusive tasks and one exclusive task. > + * number) then we wake that number of exclusive tasks, and potentially all > + * the non-exclusive tasks. Normally, exclusive tasks will be at the end of > + * the list and any non-exclusive tasks will be woken first. A priority task > + * may be at the head of the list, and can consume the event without any other > + * tasks being woken. > * > * There are circumstances in which we can try to wake a task which has already > * started to run but is not in state TASK_RUNNING. try_to_wake_up() returns > -- > 2.26.2 >