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 lists.ozlabs.org (lists.ozlabs.org [112.213.38.117]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 6F9BFC7618E for ; Fri, 21 Apr 2023 05:21:05 +0000 (UTC) Received: from boromir.ozlabs.org (localhost [IPv6:::1]) by lists.ozlabs.org (Postfix) with ESMTP id 4Q2jYC5bKnz3fWB for ; Fri, 21 Apr 2023 15:21:03 +1000 (AEST) Authentication-Results: lists.ozlabs.org; dkim=fail reason="signature verification failed" (2048-bit key; unprotected) header.d=ellerman.id.au header.i=@ellerman.id.au header.a=rsa-sha256 header.s=201909 header.b=ZcrR2/bg; dkim-atps=neutral Received: from gandalf.ozlabs.org (gandalf.ozlabs.org [150.107.74.76]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 4Q2jXF017gz3cj1 for ; Fri, 21 Apr 2023 15:20:12 +1000 (AEST) Authentication-Results: lists.ozlabs.org; dkim=pass (2048-bit key; unprotected) header.d=ellerman.id.au header.i=@ellerman.id.au header.a=rsa-sha256 header.s=201909 header.b=ZcrR2/bg; dkim-atps=neutral Received: from authenticated.ozlabs.org (localhost [127.0.0.1]) (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) (No client certificate requested) by mail.ozlabs.org (Postfix) with ESMTPSA id 4Q2jX84k4Gz4x5Q; Fri, 21 Apr 2023 15:20:08 +1000 (AEST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=ellerman.id.au; s=201909; t=1682054408; bh=qMMIN3ov7JZAgmH7R1umRdp7sW0RTy/xY8uoQpaTt5g=; h=From:To:Cc:Subject:In-Reply-To:References:Date:From; b=ZcrR2/bg6xc0PZPWxmB0oL8x18DaTjaSedqE3OgsFxXtbyuV6rkH8HyjumygWoFVe D4ZDqqgbhuHyKA85EeXwCf9ymLR2PaFYUrecpBkZHuh/jD88xp85lE4+m2QgiultnF 9go7eHgOl76wNnRmgw04nW8GzkV0cu3GOTFOQ/iJy0bnDb1ZIUI7fmLl2FYXs+ZSHO YuOvw/apxBOxfI7c93qPdswA3/AAQOoOdC6+3hU7Sx2ve+Zml9iQwdFVNhFWgRL74D JCuaIb4V6UMFGRGWyjYGqdCzClDgoxDba4AoqDYsewbm1+WJbE0tKnHnH6ge/JsFzw PpiK5Eu+/tLOA== From: Michael Ellerman To: Tejun Heo , jiangshanlai@gmail.com Subject: Re: [PATCH 01/22] powerpc, workqueue: Use alloc_ordered_workqueue() to create ordered workqueues In-Reply-To: <20230421025046.4008499-2-tj@kernel.org> References: <20230421025046.4008499-1-tj@kernel.org> <20230421025046.4008499-2-tj@kernel.org> Date: Fri, 21 Apr 2023 15:20:08 +1000 Message-ID: <87ildpg6bb.fsf@mail.concordia> MIME-Version: 1.0 Content-Type: text/plain X-BeenThere: linuxppc-dev@lists.ozlabs.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Nathan Lynch , kernel-team@meta.com, linux-kernel@vger.kernel.org, Nicholas Piggin , Tejun Heo , linuxppc-dev@lists.ozlabs.org Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" Tejun Heo writes: > BACKGROUND > ========== > > When multiple work items are queued to a workqueue, their execution order > doesn't match the queueing order. They may get executed in any order and > simultaneously. When fully serialized execution - one by one in the queueing > order - is needed, an ordered workqueue should be used which can be created > with alloc_ordered_workqueue(). > > However, alloc_ordered_workqueue() was a later addition. Before it, an > ordered workqueue could be obtained by creating an UNBOUND workqueue with > @max_active==1. This originally was an implementation side-effect which was > broken by 4c16bd327c74 ("workqueue: restore WQ_UNBOUND/max_active==1 to be > ordered"). Because there were users that depended on the ordered execution, > 5c0338c68706 ("workqueue: restore WQ_UNBOUND/max_active==1 to be ordered") > made workqueue allocation path to implicitly promote UNBOUND workqueues w/ > @max_active==1 to ordered workqueues. > > While this has worked okay, overloading the UNBOUND allocation interface > this way creates other issues. It's difficult to tell whether a given > workqueue actually needs to be ordered and users that legitimately want a > min concurrency level wq unexpectedly gets an ordered one instead. With > planned UNBOUND workqueue updates to improve execution locality and more > prevalence of chiplet designs which can benefit from such improvements, this > isn't a state we wanna be in forever. > > This patch series audits all callsites that create an UNBOUND workqueue w/ > @max_active==1 and converts them to alloc_ordered_workqueue() as necessary. > > WHAT TO LOOK FOR > ================ > > The conversions are from > > alloc_workqueue(WQ_UNBOUND | flags, 1, args..) > > to > > alloc_ordered_workqueue(flags, args...) > > which don't cause any functional changes. If you know that fully ordered > execution is not ncessary, please let me know. I'll drop the conversion and > instead add a comment noting the fact to reduce confusion while conversion > is in progress. > > If you aren't fully sure, it's completely fine to let the conversion > through. The behavior will stay exactly the same and we can always > reconsider later. > > As there are follow-up workqueue core changes, I'd really appreciate if the > patch can be routed through the workqueue tree w/ your acks. Thanks. > > Signed-off-by: Tejun Heo > Cc: Michael Ellerman > Cc: Nicholas Piggin > Cc: Christophe Leroy > Cc: Nathan Lynch > Cc: linuxppc-dev@lists.ozlabs.org > --- > arch/powerpc/kernel/tau_6xx.c | 2 +- > arch/powerpc/platforms/pseries/dlpar.c | 3 +-- > 2 files changed, 2 insertions(+), 3 deletions(-) Acked-by: Michael Ellerman (powerpc) > diff --git a/arch/powerpc/kernel/tau_6xx.c b/arch/powerpc/kernel/tau_6xx.c > index 828d0f4106d2..cba6dd15de3b 100644 > --- a/arch/powerpc/kernel/tau_6xx.c > +++ b/arch/powerpc/kernel/tau_6xx.c > @@ -200,7 +200,7 @@ static int __init TAU_init(void) > tau_int_enable = IS_ENABLED(CONFIG_TAU_INT) && > !strcmp(cur_cpu_spec->platform, "ppc750"); > > - tau_workq = alloc_workqueue("tau", WQ_UNBOUND, 1); > + tau_workq = alloc_ordered_workqueue("tau", 0); > if (!tau_workq) > return -ENOMEM; > > diff --git a/arch/powerpc/platforms/pseries/dlpar.c b/arch/powerpc/platforms/pseries/dlpar.c > index 75ffdbcd2865..e9117b03807e 100644 > --- a/arch/powerpc/platforms/pseries/dlpar.c > +++ b/arch/powerpc/platforms/pseries/dlpar.c > @@ -564,8 +564,7 @@ int __init dlpar_workqueue_init(void) > if (pseries_hp_wq) > return 0; > > - pseries_hp_wq = alloc_workqueue("pseries hotplug workqueue", > - WQ_UNBOUND, 1); > + pseries_hp_wq = alloc_ordered_workqueue("pseries hotplug workqueue", 0); > > return pseries_hp_wq ? 0 : -ENOMEM; > } The change log of commit 9054619ef54a ("powerpc/pseries: Add pseries hotplug workqueue") makes it fairly clear that this code does explicitly want an ordered queue. cheers