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 65EC0C83003 for ; Thu, 24 Aug 2023 16:59:35 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S242850AbjHXQ7M (ORCPT ); Thu, 24 Aug 2023 12:59:12 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:34696 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S242853AbjHXQ6j (ORCPT ); Thu, 24 Aug 2023 12:58:39 -0400 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by lindbergh.monkeyblade.net (Postfix) with ESMTP id E95B4199D for ; Thu, 24 Aug 2023 09:58:15 -0700 (PDT) Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 0142B165C; Thu, 24 Aug 2023 09:58:48 -0700 (PDT) Received: from [10.1.197.60] (eglon.cambridge.arm.com [10.1.197.60]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 00DDB3F64C; Thu, 24 Aug 2023 09:58:04 -0700 (PDT) Message-ID: Date: Thu, 24 Aug 2023 17:57:58 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux aarch64; rv:102.0) Gecko/20100101 Thunderbird/102.11.0 Subject: Re: [PATCH v5 24/24] x86/resctrl: Separate arch and fs resctrl locks Content-Language: en-GB To: Reinette Chatre , x86@kernel.org, linux-kernel@vger.kernel.org Cc: Fenghua Yu , Thomas Gleixner , Ingo Molnar , Borislav Petkov , H Peter Anvin , Babu Moger , shameerali.kolothum.thodi@huawei.com, D Scott Phillips OS , carl@os.amperecomputing.com, lcherian@marvell.com, bobo.shaobowang@huawei.com, tan.shaopeng@fujitsu.com, xingxin.hx@openanolis.org, baolin.wang@linux.alibaba.com, Jamie Iles , Xin Hao , peternewman@google.com, dfustini@baylibre.com References: <20230728164254.27562-1-james.morse@arm.com> <20230728164254.27562-25-james.morse@arm.com> <4085f4f3-7a3e-8af8-1ae3-1040ca78f59f@intel.com> From: James Morse In-Reply-To: <4085f4f3-7a3e-8af8-1ae3-1040ca78f59f@intel.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi Reinette, On 09/08/2023 23:41, Reinette Chatre wrote: > On 7/28/2023 9:42 AM, James Morse wrote: >> resctrl has one mutex that is taken by the architecture specific code, >> and the filesystem parts. The two interact via cpuhp, where the >> architecture code updates the domain list. Filesystem handlers that >> walk the domains list should not run concurrently with the cpuhp >> callback modifying the list. >> >> Exposing a lock from the filesystem code means the interface is not >> cleanly defined, and creates the possibility of cross-architecture >> lock ordering headaches. The interaction only exists so that certain >> filesystem paths are serialised against cpu hotplug. The cpu hotplug > > cpu hotplug -> CPU hotplug > >> code already has a mechanism to do this using cpus_read_lock(). >> >> MPAM's monitors have an overflow interrupt, so it needs to be possible >> to walk the domains list in irq context. RCU is ideal for this, >> but some paths need to be able to sleep to allocate memory. >> >> Because resctrl_{on,off}line_cpu() take the rdtgroup_mutex as part >> of a cpuhp callback, cpus_read_lock() must always be taken first. >> rdtgroup_schemata_write() already does this. >> >> Most of the filesystem code's domain list walkers are currently >> protected by the rdtgroup_mutex taken in rdtgroup_kn_lock_live(). >> The exceptions are rdt_bit_usage_show() and the mon_config helpers >> which take the lock directly. >> >> Make the domain list protected by RCU. An architecture-specific >> lock prevents concurrent writers. rdt_bit_usage_show() can >> walk the domain list under rcu_read_lock(). The mon_config helpers >> send multiple IPIs, take the cpus_read_lock() in these cases. >> >> The other filesystem list walkers need to be able to sleep. >> Add cpus_read_lock() to rdtgroup_kn_lock_live() so that the >> cpuhp callbacks can't be invoked when file system operations are >> occurring. >> >> Add lockdep_assert_cpus_held() in the cases where the >> rdtgroup_kn_lock_live() call isn't obvious. >> >> Resctrl's domain online/offline calls now need to take the >> rdtgroup_mutex themselves. >> diff --git a/arch/x86/kernel/cpu/resctrl/rdtgroup.c b/arch/x86/kernel/cpu/resctrl/rdtgroup.c >> index a256a96df487..47dcf2cb76ca 100644 >> --- a/arch/x86/kernel/cpu/resctrl/rdtgroup.c >> +++ b/arch/x86/kernel/cpu/resctrl/rdtgroup.c >> @@ -954,7 +958,8 @@ static int rdt_bit_usage_show(struct kernfs_open_file *of, >> >> mutex_lock(&rdtgroup_mutex); >> hw_shareable = r->cache.shareable_bits; >> - list_for_each_entry(dom, &r->domains, list) { >> + rcu_read_lock(); >> + list_for_each_entry_rcu(dom, &r->domains, list) { >> if (sep) >> seq_putc(seq, ';'); >> sw_shareable = 0; > > Does rdt_bit_usage_show() really need RCU? It is another filesystem callback and I > do not see a reason why it should access the domain list in a different way. It > can follow the same pattern as all the other resctrl filesystem ops and use > cpus_read_lock(). It doesn't today, and it was useful to have an example where RCU was used. I'll make this call cpus_read_lock() instead. Thanks, James