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Mon, 19 Jan 2026 13:45:13 GMT Received: from smtpav02.fra02v.mail.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id BB2DC2004B; Mon, 19 Jan 2026 13:45:13 +0000 (GMT) Received: from smtpav02.fra02v.mail.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id C116420040; Mon, 19 Jan 2026 13:45:10 +0000 (GMT) Received: from [9.124.220.45] (unknown [9.124.220.45]) by smtpav02.fra02v.mail.ibm.com (Postfix) with ESMTP; Mon, 19 Jan 2026 13:45:10 +0000 (GMT) Message-ID: <88f03a33-133f-44dd-bd49-395d4fa754d6@linux.ibm.com> Date: Mon, 19 Jan 2026 19:15:09 +0530 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] cpuhp: Expedite synchronize_rcu during SMT switch To: Peter Zijlstra , Vishal Chourasia Cc: boqun.feng@gmail.com, frederic@kernel.org, joelagnelf@nvidia.com, josh@joshtriplett.org, linux-kernel@vger.kernel.org, neeraj.upadhyay@kernel.org, paulmck@kernel.org, rcu@vger.kernel.org, rostedt@goodmis.org, srikar@linux.ibm.com, tglx@linutronix.de, urezki@gmail.com, samir@linux.ibm.com References: <20260113090153.GS830755@noisy.programming.kicks-ass.net> <20260119104739.439799-2-vishalc@linux.ibm.com> <20260119114333.GI1890602@noisy.programming.kicks-ass.net> Content-Language: en-US From: Shrikanth Hegde In-Reply-To: <20260119114333.GI1890602@noisy.programming.kicks-ass.net> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-TM-AS-GCONF: 00 X-Proofpoint-GUID: USaFucdIheBdrkI-CDqjmCy403pRIr_V X-Proofpoint-ORIG-GUID: XvX8fL3ciX-H4mb_cihSCS0QF6Ey0MMs X-Proofpoint-Spam-Details-Enc: AW1haW4tMjYwMTE5MDExMCBTYWx0ZWRfX+ZvqkGUTkmIe RmnqSbEHFDA2rYGSQgGn90pEUV64tTcIzWtAxyu5hG10KKr56OFGDn0GqYFuMIRptRpDd2fvTvG cKrbY4sTgAEqUM8UH9H3LwMilNCt9qHDn8mYq+2QKWidpSHs7Q40KIzrtaRrV26kz8RVwtSbJqm 1oJt03SFNMR3KZf7RHn4g/n1MKpYoZKQxqIErQuwMixUoyMRaob1txGelJT4LHs6WPGpqNeQaFw +mt8Ic1pmFe2L3KO7V5lpvs71QnBm1z/1y6QthnL6UApLMjxiL03jCEbtkXYlE52Il1J1CVVzHf Qtvta7WzTdLG0cLUYS3bAIMcs91IcTZm9guC55m6uQ8UhFZd/+AQfQ5H6BvPmSpxfn99+1lZ6s/ StMMDiFpjsn9/i5m4alxqQS3v24OxqtVfdmZflPJVPrp5cPgc7Xr91dOGC1Qlb3X2CmrFa5holz 7di5uy/nMlPLlYA2GvA== X-Authority-Analysis: v=2.4 cv=bopBxUai c=1 sm=1 tr=0 ts=696e356f cx=c_pps a=AfN7/Ok6k8XGzOShvHwTGQ==:117 a=AfN7/Ok6k8XGzOShvHwTGQ==:17 a=IkcTkHD0fZMA:10 a=vUbySO9Y5rIA:10 a=VkNPw1HP01LnGYTKEx00:22 a=Siv2N4SOSaTVEqBjBL0A:9 a=3ZKOabzyN94A:10 a=QEXdDO2ut3YA:10 X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1121,Hydra:6.1.9,FMLib:17.12.100.49 definitions=2026-01-19_03,2026-01-19_02,2025-10-01_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 spamscore=0 bulkscore=0 adultscore=0 suspectscore=0 impostorscore=0 phishscore=0 malwarescore=0 lowpriorityscore=0 priorityscore=1501 clxscore=1015 classifier=typeunknown authscore=0 authtc= authcc= route=outbound adjust=0 reason=mlx scancount=1 engine=8.19.0-2601150000 definitions=main-2601190110 Hi Peter. On 1/19/26 5:13 PM, Peter Zijlstra wrote: > On Mon, Jan 19, 2026 at 04:17:40PM +0530, Vishal Chourasia wrote: >> Expedite synchronize_rcu() during the cpuhp_smt_[enable|disable] path to >> accelerate the operation. >> >> Bulk CPU hotplug operations—such as switching SMT modes across all >> cores—require hotplugging multiple CPUs in rapid succession. On large >> systems, this process takes significant time, increasing as the number >> of CPUs to hotplug during SMT switch grows, leading to substantial >> delays on high-core-count machines. Analysis [1] reveals that the >> majority of this time is spent waiting for synchronize_rcu(). >> > > You seem to have left out all the useful bits from your changelog again > :/ > > Anyway, ISTR Joel posted a patch hoisting a lock; it was a icky, but not > something we can't live with either. > > Also, memory got jogged and I think something like the below will remove > 2/3 of your rcu woes as well. > > diff --git a/kernel/cpu.c b/kernel/cpu.c > index 8df2d773fe3b..1365c19444b2 100644 > --- a/kernel/cpu.c > +++ b/kernel/cpu.c > @@ -2669,6 +2669,7 @@ int cpuhp_smt_disable(enum cpuhp_smt_control ctrlval) > int cpu, ret = 0; > > cpu_maps_update_begin(); > + rcu_sync_enter(&cpu_hotplug_lock.rss); > for_each_online_cpu(cpu) { > if (topology_is_primary_thread(cpu)) > continue; > @@ -2698,6 +2699,7 @@ int cpuhp_smt_disable(enum cpuhp_smt_control ctrlval) > } > if (!ret) > cpu_smt_control = ctrlval; > + rcu_sync_exit(&cpu_hotplug_lock.rss); > cpu_maps_update_done(); > return ret; > } > @@ -2715,6 +2717,7 @@ int cpuhp_smt_enable(void) > int cpu, ret = 0; > > cpu_maps_update_begin(); > + rcu_sync_enter(&cpu_hotplug_lock.rss); > cpu_smt_control = CPU_SMT_ENABLED; > for_each_present_cpu(cpu) { > /* Skip online CPUs and CPUs on offline nodes */ > @@ -2728,6 +2731,7 @@ int cpuhp_smt_enable(void) > /* See comment in cpuhp_smt_disable() */ > cpuhp_online_cpu_device(cpu); > } > + rcu_sync_exit(&cpu_hotplug_lock.rss); > cpu_maps_update_done(); > return ret; > } Currently, cpuhp_smt_[enable/disable] calls _cpu_up/_cpu_down which does the same in cpus_write_lock/unlock. though it is per cpu enable/disable one after another. How hoisting this up will help?