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 EACE7C4332F for ; Wed, 13 Dec 2023 00:16:13 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235236AbjLMAQF (ORCPT ); Tue, 12 Dec 2023 19:16:05 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:46614 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1378052AbjLMAPl (ORCPT ); Tue, 12 Dec 2023 19:15:41 -0500 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 62002D51 for ; Tue, 12 Dec 2023 16:14:58 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1702426497; 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=BrOsn2y3KPHnbMuEkFCu/8T/VhjzG1Tr6VnplESBRyU=; b=HxUVQT9b+yD/DKOlc27mWZbFflKhBi2LHrky1W6t0YYsh8NrPSKOJZquaCTPI3Co1b1wdw Ct+6EwseRWN6lAxSV25DPtLt/RSoBB1HtTNXW9orx0HD256raVQ7zalRkID72G16W64/jI IjBgIFqmrteypylIBPMAa23zVX+UO+Q= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-688-20Upbt2qPKekpDbquNwUuw-1; Tue, 12 Dec 2023 19:14:54 -0500 X-MC-Unique: 20Upbt2qPKekpDbquNwUuw-1 Received: from smtp.corp.redhat.com (int-mx06.intmail.prod.int.rdu2.redhat.com [10.11.54.6]) (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 mimecast-mx02.redhat.com (Postfix) with ESMTPS id D965B83DDE5; Wed, 13 Dec 2023 00:14:53 +0000 (UTC) Received: from fedora (unknown [10.72.120.3]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 861082166B32; Wed, 13 Dec 2023 00:14:49 +0000 (UTC) Date: Wed, 13 Dec 2023 08:14:45 +0800 From: Ming Lei To: Yury Norov Cc: Andrew Morton , Thomas Gleixner , linux-kernel@vger.kernel.org, Andy Shevchenko , Rasmus Villemoes Subject: Re: [PATCH v3 3/7] lib/group_cpus: relax atomicity requirement in grp_spread_init_one() Message-ID: References: <20231212042108.682072-1-yury.norov@gmail.com> <20231212042108.682072-4-yury.norov@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Scanned-By: MIMEDefang 3.4.1 on 10.11.54.6 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Dec 12, 2023 at 08:52:14AM -0800, Yury Norov wrote: > On Tue, Dec 12, 2023 at 05:50:04PM +0800, Ming Lei wrote: > > On Mon, Dec 11, 2023 at 08:21:03PM -0800, Yury Norov wrote: > > > Because nmsk and irqmsk are stable, extra atomicity is not required. > > > > > > Signed-off-by: Yury Norov > > > --- > > > lib/group_cpus.c | 8 ++++---- > > > 1 file changed, 4 insertions(+), 4 deletions(-) > > > > > > diff --git a/lib/group_cpus.c b/lib/group_cpus.c > > > index 10dead3ab0e0..7ac94664230f 100644 > > > --- a/lib/group_cpus.c > > > +++ b/lib/group_cpus.c > > > @@ -24,8 +24,8 @@ static void grp_spread_init_one(struct cpumask *irqmsk, struct cpumask *nmsk, > > > if (cpu >= nr_cpu_ids) > > > return; > > > > > > - cpumask_clear_cpu(cpu, nmsk); > > > - cpumask_set_cpu(cpu, irqmsk); > > > + __cpumask_clear_cpu(cpu, nmsk); > > > + __cpumask_set_cpu(cpu, irqmsk); > > > cpus_per_grp--; > > > > > > /* If the cpu has siblings, use them first */ > > > @@ -33,8 +33,8 @@ static void grp_spread_init_one(struct cpumask *irqmsk, struct cpumask *nmsk, > > > sibl = cpu + 1; > > > > > > for_each_cpu_and_from(sibl, siblmsk, nmsk) { > > > - cpumask_clear_cpu(sibl, nmsk); > > > - cpumask_set_cpu(sibl, irqmsk); > > > + __cpumask_clear_cpu(sibl, nmsk); > > > + __cpumask_set_cpu(sibl, irqmsk); > > > > I think this kind of change should be avoided, here the code is > > absolutely in slow path, and we care code cleanness and readability > > much more than the saved cycle from non atomicity. > > Atomic ops have special meaning and special function. This 'atomic' way > of moving a bit from one bitmap to another looks completely non-trivial > and puzzling to me. > > A sequence of atomic ops is not atomic itself. Normally it's a sing of > a bug. But in this case, both masks are stable, and we don't need > atomicity at all. Here we don't care the atomicity. > > It's not about performance, it's about readability. __cpumask_clear_cpu() and __cpumask_set_cpu() are more like private helper, and more hard to follow. [@linux]$ git grep -n -w -E "cpumask_clear_cpu|cpumask_set_cpu" ./ | wc 674 2055 53954 [@linux]$ git grep -n -w -E "__cpumask_clear_cpu|__cpumask_set_cpu" ./ | wc 21 74 1580 I don't object to comment the current usage, but NAK for this change. Thanks, Ming