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Thu, 28 Mar 2019 16:50:11 +0000 Received: from aserv0122.oracle.com (aserv0122.oracle.com [141.146.126.236]) by userv0022.oracle.com (8.14.4/8.14.4) with ESMTP id x2SGo9FE013350 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Thu, 28 Mar 2019 16:50:10 GMT Received: from abhmp0016.oracle.com (abhmp0016.oracle.com [141.146.116.22]) by aserv0122.oracle.com (8.14.4/8.14.4) with ESMTP id x2SGo7TG015478; Thu, 28 Mar 2019 16:50:07 GMT Received: from dhcp-burlington7-2nd-B-east-10-152-55-162.usdhcp.oraclecorp.com (/10.152.32.65) by default (Oracle Beehive Gateway v4.0) with ESMTP ; Thu, 28 Mar 2019 09:50:07 -0700 Subject: Re: [PATCH] x86/Xen: streamline (and fix) PV CPU enumeration To: Jan Beulich Cc: Stefano Stabellini , xen-devel , Juergen Gross , linux-kernel@vger.kernel.org References: <5C9B92EA020000780022227B@prv1-mh.provo.novell.com> <2f027b4b-dce2-3e90-dc1b-c824bc8eb355@oracle.com> <5C9C8DDC0200007800222606@prv1-mh.provo.novell.com> From: Boris Ostrovsky Openpgp: preference=signencrypt Autocrypt: addr=boris.ostrovsky@oracle.com; 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Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.4.0 MIME-Version: 1.0 In-Reply-To: <5C9C8DDC0200007800222606@prv1-mh.provo.novell.com> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Content-Language: en-US X-Proofpoint-Virus-Version: vendor=nai engine=5900 definitions=9209 signatures=668685 X-Proofpoint-Spam-Details: rule=notspam policy=default score=0 priorityscore=1501 malwarescore=0 suspectscore=0 phishscore=0 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 mlxscore=0 impostorscore=0 mlxlogscore=999 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1810050000 definitions=main-1903280112 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 3/28/19 5:03 AM, Jan Beulich wrote: >>>> On 27.03.19 at 18:07, wrote: >> On 3/27/19 11:12 AM, Jan Beulich wrote: >>> - >>> -static void __init xen_filter_cpu_maps(void) >>> +static void __init _get_smp_config(unsigned int early) >>> { >>> int i, rc; >>> unsigned int subtract = 0; >>> >>> - if (!xen_initial_domain()) >>> + if (early) >>> return; >>> >>> num_processors = 0; >> >> Is there a reason to set_cpu_possible() here (not in the diff, but in >> this routine)? This will be called from setup_arch() before >> prefill_possible_map(), which will clear and then re-initialize >> __cpu_possible_mask. > I don't think it's needed before my change either, so I'd call > removing this an orthogonal change. As said in the commit > message, the goal was to leave this function alone as far as > possible. > > Before my patch, xen_filter_cpu_maps() gets called long after > prefill_possible_map(), and by the purpose of the latter function > the possible map shouldn't be altered anymore once that has > run. Adding bits to it is surely not going to have the intended > effect (setup_per_cpu_areas() has already run), while removing > bits may have some effect, but comes too late at least for > setup_per_cpu_areas(). OK. Then it looks like even though your patch changes behavior slightly (so technically I guess it's not purely a cleanup) this shouldn't makes any difference at least as far as possible cpu mask is concerned: if we don't have percpu areas set up we can't do much for that vcpu since it seems to me xen_vcpu_setup(), for example, won't do well. > > And if we were to remove this, I think the CONFIG_HOTPLUG_CPU > section should go away as well. After all prefill_possible_map() > also sets nr_cpu_ids. To be honest, it was largely this code > fragment which made me want not touch the function more than > necessary: The comment there makes not clear to me at all why > all of this needs to be in an #ifdef in the first place. This was introduced by cf405ae612b0 ("xen/smp: Fix crash when booting with ACPI hotplug CPUs."). I am not sure this is still relevant. The ACPI hotplug code had changed, not significantly but sufficiently enough to alter behavior. acpi_processor_hotadd_init() now fails before it gets a chance to call arch_register_cpu() for vcpu>dom0_max_vcpus. > > Let me know whether you really want me to fold this extra > cleanup into this patch. If so, I'd then wonder whether the > set_cpu_present() from xen_pv_smp_prepare_cpus() shouldn't > be moved here, too. And the fiddling with the possible map > there looks bogus as well: Bring-up of CPUs past the command > line option should be avoided at boot time, but they shouldn't > be excluded from getting brought up later on. Note how > native_smp_prepare_cpus() ignores its parameter altogether. Yes, that does look strange. Given especially xen_pv_smp_prepare_cpus(), I think re-working proper setting of present/possible masks is well beyond the scope of your original patch. -boris