From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756718Ab2IKS2D (ORCPT ); Tue, 11 Sep 2012 14:28:03 -0400 Received: from e7.ny.us.ibm.com ([32.97.182.137]:50913 "EHLO e7.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751489Ab2IKS17 (ORCPT ); Tue, 11 Sep 2012 14:27:59 -0400 Subject: Re: [RFC][PATCH] Improving directed yield scalability for PLE handler From: Andrew Theurer Reply-To: habanero@linux.vnet.ibm.com To: Raghavendra K T Cc: Peter Zijlstra , Srikar Dronamraju , Avi Kivity , Marcelo Tosatti , Ingo Molnar , Rik van Riel , KVM , chegu vinod , LKML , X86 , Gleb Natapov , Srivatsa Vaddagiri In-Reply-To: <504ED54E.6040608@linux.vnet.ibm.com> References: <20120718133717.5321.71347.sendpatchset@codeblue.in.ibm.com> <500D2162.8010209@redhat.com> <1347023509.10325.53.camel@oc6622382223.ibm.com> <504A37B0.7020605@linux.vnet.ibm.com> <1347046931.7332.51.camel@oc2024037011.ibm.com> <20120908084345.GU30238@linux.vnet.ibm.com> <1347283005.10325.55.camel@oc6622382223.ibm.com> <1347293035.2124.22.camel@twins> <20120910165653.GA28033@linux.vnet.ibm.com> <1347297124.2124.42.camel@twins> <1347307972.7332.78.camel@oc2024037011.ibm.com> <504ED54E.6040608@linux.vnet.ibm.com> Content-Type: text/plain; charset="UTF-8" Date: Tue, 11 Sep 2012 13:27:41 -0500 Message-ID: <1347388061.19098.20.camel@oc2024037011.ibm.com> Mime-Version: 1.0 X-Mailer: Evolution 2.28.3 (2.28.3-24.el6) Content-Transfer-Encoding: 7bit X-Content-Scanned: Fidelis XPS MAILER x-cbid: 12091118-5806-0000-0000-0000197C7E5F Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 2012-09-11 at 11:38 +0530, Raghavendra K T wrote: > On 09/11/2012 01:42 AM, Andrew Theurer wrote: > > On Mon, 2012-09-10 at 19:12 +0200, Peter Zijlstra wrote: > >> On Mon, 2012-09-10 at 22:26 +0530, Srikar Dronamraju wrote: > >>>> +static bool __yield_to_candidate(struct task_struct *curr, struct task_struct *p) > >>>> +{ > >>>> + if (!curr->sched_class->yield_to_task) > >>>> + return false; > >>>> + > >>>> + if (curr->sched_class != p->sched_class) > >>>> + return false; > >>> > >>> > >>> Peter, > >>> > >>> Should we also add a check if the runq has a skip buddy (as pointed out > >>> by Raghu) and return if the skip buddy is already set. > >> > >> Oh right, I missed that suggestion.. the performance improvement went > >> from 81% to 139% using this, right? > >> > >> It might make more sense to keep that separate, outside of this > >> function, since its not a strict prerequisite. > >> > >>>> > >>>> + if (task_running(p_rq, p) || p->state) > >>>> + return false; > >>>> + > >>>> + return true; > >>>> +} > >> > >> > >>>> @@ -4323,6 +4340,10 @@ bool __sched yield_to(struct task_struct *p, > >>> bool preempt) > >>>> rq = this_rq(); > >>>> > >>>> again: > >>>> + /* optimistic test to avoid taking locks */ > >>>> + if (!__yield_to_candidate(curr, p)) > >>>> + goto out_irq; > >>>> + > >> > >> So add something like: > >> > >> /* Optimistic, if we 'raced' with another yield_to(), don't bother */ > >> if (p_rq->cfs_rq->skip) > >> goto out_irq; > >>> > >>> > >>>> p_rq = task_rq(p); > >>>> double_rq_lock(rq, p_rq); > >>> > >>> > >> But I do have a question on this optimization though,.. Why do we check > >> p_rq->cfs_rq->skip and not rq->cfs_rq->skip ? > >> > >> That is, I'd like to see this thing explained a little better. > >> > >> Does it go something like: p_rq is the runqueue of the task we'd like to > >> yield to, rq is our own, they might be the same. If we have a ->skip, > >> there's nothing we can do about it, OTOH p_rq having a ->skip and > >> failing the yield_to() simply means us picking the next VCPU thread, > >> which might be running on an entirely different cpu (rq) and could > >> succeed? > > > > Here's two new versions, both include a __yield_to_candidate(): "v3" > > uses the check for p_rq->curr in guest mode, and "v4" uses the cfs_rq > > skip check. Raghu, I am not sure if this is exactly what you want > > implemented in v4. > > > > Andrew, Yes that is what I had. I think there was a mis-understanding. > My intention was to if there is a directed_yield happened in runqueue > (say rqA), do not bother to directed yield to that. But unfortunately as > PeterZ pointed that would have resulted in setting next buddy of a > different run queue than rqA. > So we can drop this "skip" idea. Pondering more over what to do? can we > use next buddy itself ... thinking.. As I mentioned earlier today, I did not have your changes from kvm.git tree when I tested my changes. Here are your changes and my changes compared: throughput in MB/sec kvm_vcpu_on_spin changes: 4636 +/- 15.74% yield_to changes: 4515 +/- 12.73% I would be inclined to stick with your changes which are kept in kvm code. I did try both combined, and did not get good results: both changes: 4074 +/- 19.12% So, having both is probably not a good idea. However, I feel like there's more work to be done. With no over-commit (10 VMs), total throughput is 23427 +/- 2.76%. A 2x over-commit will no doubt have some overhead, but a reduction to ~4500 is still terrible. By contrast, 8-way VMs with 2x over-commit have a total throughput roughly 10% less than 8-way VMs with no overcommit (20 vs 10 8-way VMs on 80 cpu-thread host). We still have what appears to be scalability problems, but now it's not so much in runqueue locks for yield_to(), but now get_pid_task(): perf on host: 32.10% 320131 qemu-system-x86 [kernel.kallsyms] [k] get_pid_task 11.60% 115686 qemu-system-x86 [kernel.kallsyms] [k] _raw_spin_lock 10.28% 102522 qemu-system-x86 [kernel.kallsyms] [k] yield_to 9.17% 91507 qemu-system-x86 [kvm] [k] kvm_vcpu_on_spin 7.74% 77257 qemu-system-x86 [kvm] [k] kvm_vcpu_yield_to 3.56% 35476 qemu-system-x86 [kernel.kallsyms] [k] __srcu_read_lock 3.00% 29951 qemu-system-x86 [kvm] [k] __vcpu_run 2.93% 29268 qemu-system-x86 [kvm_intel] [k] vmx_vcpu_run 2.88% 28783 qemu-system-x86 [kvm] [k] vcpu_enter_guest 2.59% 25827 qemu-system-x86 [kernel.kallsyms] [k] __schedule 1.40% 13976 qemu-system-x86 [kernel.kallsyms] [k] _raw_spin_lock_irq 1.28% 12823 qemu-system-x86 [kernel.kallsyms] [k] resched_task 1.14% 11376 qemu-system-x86 [kvm_intel] [k] vmcs_writel 0.85% 8502 qemu-system-x86 [kernel.kallsyms] [k] pick_next_task_fair 0.53% 5315 qemu-system-x86 [kernel.kallsyms] [k] native_write_msr_safe 0.46% 4553 qemu-system-x86 [kernel.kallsyms] [k] native_load_tr_desc get_pid_task() uses some rcu fucntions, wondering how scalable this is.... I tend to think of rcu as -not- having issues like this... is there a rcu stat/tracing tool which would help identify potential problems? -Andrew