Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752040AbdGEPe4 (ORCPT ); Wed, 5 Jul 2017 11:34:56 -0400 Received: from merlin.infradead.org ([205.233.59.134]:32872 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751756AbdGEPez (ORCPT ); Wed, 5 Jul 2017 11:34:55 -0400 Date: Wed, 5 Jul 2017 17:34:39 +0200 From: Peter Zijlstra To: Thomas Gleixner Cc: Vikas Shivappa , x86@kernel.org, linux-kernel@vger.kernel.org, hpa@zytor.com, ravi.v.shankar@intel.com, vikas.shivappa@intel.com, tony.luck@intel.com, fenghua.yu@intel.com, andi.kleen@intel.com Subject: Re: [PATCH 08/21] x86/intel_rdt/cqm: Add RMID(Resource monitoring ID) management Message-ID: <20170705153439.xudhew5wpq3liivf@hirez.programming.kicks-ass.net> References: <1498503368-20173-1-git-send-email-vikas.shivappa@linux.intel.com> <1498503368-20173-9-git-send-email-vikas.shivappa@linux.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: NeoMutt/20170113 (1.7.2) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3541 Lines: 116 On Mon, Jul 03, 2017 at 11:55:37AM +0200, Thomas Gleixner wrote: > > if (static_branch_likely(&rdt_mon_enable_key)) { > if (unlikely(current->rmid)) { > newstate.rmid = current->rmid; > __set_bit(newstate.rmid, this_cpu_ptr(rmid_bitmap)); Non atomic op > } > } > > Now in rmid_free() we can collect that information: > > cpumask_clear(&tmpmask); > cpumask_clear(rmid_entry->mask); > > cpus_read_lock(); > for_each_online_cpu(cpu) { > if (test_and_clear_bit(rmid, per_cpu_ptr(cpu, rmid_bitmap))) atomic op > cpumask_set(cpu, tmpmask); > } > > for_each_domain(d, resource) { > cpu = cpumask_any_and(d->cpu_mask, tmpmask); > if (cpu < nr_cpu_ids) > cpumask_set(cpu, rmid_entry->mask); > } > > list_add(&rmid_entry->list, &limbo_list); > > for_each_cpu(cpu, rmid_entry->mask) > schedule_delayed_work_on(cpu, rmid_work); > cpus_read_unlock(); > > The work function: > > boot resched = false; > > list_for_each_entry(rme, limbo_list,...) { > if (!cpumask_test_cpu(cpu, rme->mask)) > continue; > > if (!rmid_is_reusable(rme)) { > resched = true; > continue; > } > > cpumask_clear_cpu(cpu, rme->mask); > if (!cpumask_empty(rme->mask)) > continue; > > /* Ready for reuse */ > list_del(rme->list); > list_add(&rme->list, &free_list); > } > > The alloc function then becomes: > > if (list_empty(&free_list)) > return list_empty(&limbo_list) ? -ENOSPC : -EBUSY; > > The switch_to() covers the task rmids. The per cpu default rmids can be > marked at the point where they are installed on a CPU in the per cpu > rmid_bitmap. The free path is the same for per task and per cpu. > > Another thing which needs some thought it the CPU hotplug code. We need to > make sure that pending work which is scheduled on an outgoing CPU is moved > in the offline callback to a still online CPU of the same domain and not > moved to some random CPU by the workqueue hotplug code. just flush the workqueue for that CPU? That's what the workqueue core _should_ do in any case. And that also covers the case where @cpu is the last in the set of CPUs we could run on. > There is another subtle issue. Assume a RMID is freed. The limbo stuff is > scheduled on all domains which have online CPUs. > > Now the last CPU of a domain goes offline before the threshold for clearing > the domain CPU bit in the rme->mask is reached. > > So we have two options here: > > 1) Clear the bit unconditionally when the last CPU of a domain goes > offline. Arguably this. This is cache level stuff, that means this is the last CPU of a cache, so just explicitly kill the _entire_ cache and insta mark everything good again; WBINVD ftw. > 2) Arm a timer which clears the bit after a grace period > > #1 The RMID might become available for reuse right away because all other > domains have not used it or have cleared their bits already. > > If one of the CPUs of that domain comes online again and is associated > to that reused RMID again, then the counter content might still contain > leftovers from the previous usage. Not if we kill the cache on offline -- also, if all CPUs have been offline, its not too weird to expect something like a package idle state to have happened and shot down the caches anyway. > #2 Prevents #1 but has it's own issues vs. serialization and coordination > with CPU hotplug. > > I'd say we go for #1 as the simplest solution, document it and if really > the need arises revisit it later. > > Thanks, > > tglx