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Mon, 12 Jan 2026 12:46:21 GMT Received: from smtpav05.fra02v.mail.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id C6B6820043; Mon, 12 Jan 2026 12:46:21 +0000 (GMT) Received: from smtpav05.fra02v.mail.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id B1FB020040; Mon, 12 Jan 2026 12:46:17 +0000 (GMT) Received: from [9.39.17.9] (unknown [9.39.17.9]) by smtpav05.fra02v.mail.ibm.com (Postfix) with ESMTP; Mon, 12 Jan 2026 12:46:17 +0000 (GMT) Message-ID: <00c9096d-20cc-4584-a413-74e60995c3d8@linux.ibm.com> Date: Mon, 12 Jan 2026 18:16:16 +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 CPU hotplug operations To: Shrikanth Hegde , rcu@vger.kernel.org, linux-kernel@vger.kernel.org Cc: paulmck@kernel.org, frederic@kernel.org, neeraj.upadhyay@kernel.org, joelagnelf@nvidia.com, josh@joshtriplett.org, boqun.feng@gmail.com, urezki@gmail.com, rostedt@goodmis.org, tglx@linutronix.de, peterz@infradead.org, srikar@linux.ibm.com References: <20260112094332.66006-2-vishalc@linux.ibm.com> <595f2ecd-020c-44f2-ba51-39a54c0e656f@linux.ibm.com> Content-Language: en-US, en-IN From: Vishal Chourasia In-Reply-To: <595f2ecd-020c-44f2-ba51-39a54c0e656f@linux.ibm.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-TM-AS-GCONF: 00 X-Proofpoint-GUID: i8fCodDk0Q-zBAUmRv1R3SrAGSVw0nwk X-Authority-Analysis: v=2.4 cv=LLxrgZW9 c=1 sm=1 tr=0 ts=6964ed23 cx=c_pps a=aDMHemPKRhS1OARIsFnwRA==:117 a=aDMHemPKRhS1OARIsFnwRA==:17 a=IkcTkHD0fZMA:10 a=vUbySO9Y5rIA:10 a=VkNPw1HP01LnGYTKEx00:22 a=VwQbUJbxAAAA:8 a=VnNF1IyMAAAA:8 a=NEAV23lmAAAA:8 a=Vfsp1XFiFrZfeqIENTQA:9 a=3ZKOabzyN94A:10 a=QEXdDO2ut3YA:10 X-Proofpoint-Spam-Details-Enc: AW1haW4tMjYwMTEyMDA5OCBTYWx0ZWRfX7NSNns84qoyg dfAdV5rj7OEEMgl+qanPH9c/09uZUFtbcrddqnsB5KLVESgp4JNQ1qikiLhbU57/dDy0Q4JHgx5 ZOcHGiM4lH7/nZNmookjVpqoAOVadMCgpODDslNVbg6RJp7wUmYItZLjL5WKzujy3uFQwgYNoIg dusJrKo/HvOJZp58j7DQxeCcAms1NpQTMmWWcVm/HjhNTsAZkym58el/BfElhfLTWxeeZNkhum/ kskfnicTGzFtncUFTHqUoLAEtJq0/YpuNbJTUsfHik9JjO4q+H9eskEMazEUdB41q2OyILi9qQ5 LkxcWihFa+lJwblHQ8OovYu9s6sCkYUxbyaec+HVdjB7W6+/OP8RRBKcxIvuqeC80gFv6QN5UtZ aAS5ndkwDpH7B5NSE1M6wK9Y31uCChLnsiWCqWx3MvpIf0reEZc0vFQUaAVbLrTcxJudJf+wnyt uD8V+KVHKv58JSFvqtA== X-Proofpoint-ORIG-GUID: Y4nqisoM6wqGXkbOsnQVkTqwT12Jxju1 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=2026-01-12_03,2026-01-09_02,2025-10-01_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 suspectscore=0 clxscore=1015 spamscore=0 impostorscore=0 malwarescore=0 phishscore=0 adultscore=0 lowpriorityscore=0 bulkscore=0 classifier=typeunknown authscore=0 authtc= authcc= route=outbound adjust=0 reason=mlx scancount=1 engine=8.19.0-2512120000 definitions=main-2601120098 On 12/01/26 17:51, Shrikanth Hegde wrote: > > > On 1/12/26 3:13 PM, Vishal Chourasia wrote: >> 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 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(). >> >> Expedite synchronize_rcu() during the hotplug path to accelerate the >> operation. Since CPU hotplug is a user-initiated administrative task, >> it should complete as quickly as possible. >> >> Performance data on a PPC64 system with 400 CPUs: >> >> + ppc64_cpu --smt=1 (SMT8 to SMT1) >> Before: real 1m14.792s >> After:  real 0m03.205s  # ~23x improvement >> >> + ppc64_cpu --smt=8 (SMT1 to SMT8) >> Before: real 2m27.695s >> After:  real 0m02.510s  # ~58x improvement >> >> Above numbers were collected on Linux 6.19.0-rc4-00310-g755bc1335e3b >> > > Hi Vishal, > > I tried on tip/master at 315f416d3e26. > It fails to apply. is rcu tree updated? I'm currently working off the GitHub mirror (|github.com/torvalds/linux)| > > >> [1] >> https://lore.kernel.org/all/5f2ab8a44d685701fe36cdaa8042a1aef215d10d.camel@linux.vnet.ibm.com >> >> Signed-off-by: Vishal Chourasia >> --- >>   include/linux/rcupdate.h | 3 +++ >>   kernel/cpu.c             | 2 ++ >>   2 files changed, 5 insertions(+) >> >> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h >> index c5b30054cd01..03c06cfb2b6d 100644 >> --- a/include/linux/rcupdate.h >> +++ b/include/linux/rcupdate.h >> @@ -1192,6 +1192,9 @@ rcu_head_after_call_rcu(struct rcu_head *rhp, >> rcu_callback_t f) >>   extern int rcu_expedited; >>   extern int rcu_normal; >>   +extern void rcu_expedite_gp(void); >> +extern void rcu_unexpedite_gp(void); >> + > > Why extern is needed? All it needs is declarations no? Already declared in kernel/rcu/rcu.h kernel/cpu.c already includes linux/rcupdate.h, therefore added an extern. > > >>   DEFINE_LOCK_GUARD_0(rcu, >>       do { >>           rcu_read_lock(); >> diff --git a/kernel/cpu.c b/kernel/cpu.c >> index 8df2d773fe3b..6b0d491d73f4 100644 >> --- a/kernel/cpu.c >> +++ b/kernel/cpu.c >> @@ -506,12 +506,14 @@ EXPORT_SYMBOL_GPL(cpus_read_unlock); >>     void cpus_write_lock(void) >>   { >> +    rcu_expedite_gp(); >>       percpu_down_write(&cpu_hotplug_lock); >>   } >>     void cpus_write_unlock(void) >>   { >>       percpu_up_write(&cpu_hotplug_lock); >> +    rcu_unexpedite_gp(); >>   } >>     void lockdep_assert_cpus_held(void) > > Have you tested kexec path or suspend/resume path? I did test kexec patch by booting into another kernel via kexec path. But, I didn't test suspend/resume. > Seems like the counter can nest, but would be good to verify.