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=-1.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,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 66112C004C9 for ; Fri, 3 May 2019 17:15:26 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 418C52075E for ; Fri, 3 May 2019 17:15:26 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728597AbfECRPZ (ORCPT ); Fri, 3 May 2019 13:15:25 -0400 Received: from mx1.redhat.com ([209.132.183.28]:50532 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726633AbfECRPY (ORCPT ); Fri, 3 May 2019 13:15:24 -0400 Received: from smtp.corp.redhat.com (int-mx07.intmail.prod.int.phx2.redhat.com [10.5.11.22]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 481CD3081252; Fri, 3 May 2019 17:15:24 +0000 (UTC) Received: from llong.remote.csb (dhcp-17-85.bos.redhat.com [10.18.17.85]) by smtp.corp.redhat.com (Postfix) with ESMTP id D45E31001944; Fri, 3 May 2019 17:15:22 +0000 (UTC) Subject: Re: [PATCH-tip v7 11/20] locking/rwsem: Wake up almost all readers in wait queue To: Peter Zijlstra Cc: Ingo Molnar , Will Deacon , Thomas Gleixner , Borislav Petkov , "H. Peter Anvin" , linux-kernel@vger.kernel.org, x86@kernel.org, Davidlohr Bueso , Linus Torvalds , Tim Chen , huang ying References: <20190428212557.13482-1-longman@redhat.com> <20190428212557.13482-12-longman@redhat.com> <20190503165141.GI2623@hirez.programming.kicks-ass.net> From: Waiman Long Organization: Red Hat Message-ID: <1e5e939f-3725-14d9-feec-aabb63998406@redhat.com> Date: Fri, 3 May 2019 13:15:22 -0400 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.6.1 MIME-Version: 1.0 In-Reply-To: <20190503165141.GI2623@hirez.programming.kicks-ass.net> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Content-Language: en-US X-Scanned-By: MIMEDefang 2.84 on 10.5.11.22 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.49]); Fri, 03 May 2019 17:15:24 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 5/3/19 12:51 PM, Peter Zijlstra wrote: > On Sun, Apr 28, 2019 at 05:25:48PM -0400, Waiman Long wrote: >> When the front of the wait queue is a reader, other readers >> immediately following the first reader will also be woken up at the >> same time. However, if there is a writer in between. Those readers >> behind the writer will not be woken up. >> @@ -345,13 +359,20 @@ static void __rwsem_mark_wake(struct rw_semaphore *sem, >> * 2) For each waiters in the new list, clear waiter->task and >> * put them into wake_q to be woken up later. >> */ >> - list_for_each_entry(waiter, &sem->wait_list, list) { >> + INIT_LIST_HEAD(&wlist); >> + list_for_each_entry_safe(waiter, tmp, &sem->wait_list, list) { >> if (waiter->type == RWSEM_WAITING_FOR_WRITE) >> - break; >> + continue; >> >> woken++; >> + list_move_tail(&waiter->list, &wlist); >> + >> + /* >> + * Limit # of readers that can be woken up per wakeup call. >> + */ >> + if (woken >= MAX_READERS_WAKEUP) >> + break; >> } >> - list_cut_before(&wlist, &sem->wait_list, &waiter->list); >> >> adjustment = woken * RWSEM_READER_BIAS - adjustment; >> lockevent_cond_inc(rwsem_wake_reader, woken); > An idea for later; maybe we can simplify this by playing silly games > with the queueing. > > Writers: always list_add_tail() > Readers: keep a pointer to first_reader in the queue; > when NULL; list_add_tail() and set > otherwise: list_add_tail(, first_reader); > > Possily also keep a count of first_reader list size, and if 'big' reset > first_reader. > > That way we never have to skip over writers. > Yes, that can work. However, that will require adding one more pointer to the rw_semaphore structure. The performance gain with this optimization may not justify increasing the size of the structure by 4/8 bytes. Cheers, Longman