Re: [PATCH] cpuhp: Expedite synchronize_rcu during SMT switch

[Samir M <samir@linux.ibm.com>]

On 27/01/26 11:18 pm, Samir M wrote:
>
> On 19/01/26 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;
>>   }
>
>
> Hi,
>
> I verified this patch using the configuration described below.
> Configuration:
>     •    Kernel version: 6.19.0-rc6
>     •    Number of CPUs: 1536
>
> Earlier verification of an older version of this patch was performed 
> on a system with *2048 CPUs*. Due to system unavailability, the 
> current verification was carried out on a *different system.*
>
>
> Using this setup, I evaluated the patch with both SMT enabled and SMT 
> disabled. patch shows a significant improvement in the SMT=off case 
> and a measurable improvement in the SMT=on case.
> The results indicate that when SMT is enabled, the system time is 
> noticeably higher. In contrast, with SMT disabled, no significant 
> increase in system time is observed.
>
> SMT=ON  -> sys 50m42.805s
> SMT=OFF -> sys 0m0.064s
>
>
> SMT Mode    | Without Patch    | With Patch   | % Improvement   |
> ------------------------------------------------------------------
> SMT=off     | 20m 32.210s      |  5m 30.898s  | +73.15%         |
> SMT=on      | 62m 46.549s      | 55m 45.671s  | +11.18%         |
>
>
> Please add below tag: 
Tested-by: Samir M <samir@linux.ibm.com>
>
> Regards,
> Samir
>
Hi All,

For reference, I am updating the results for the Vishal and Rezki patches.

Configuration:
Kernel version: 6.19.0-rc6
Number of CPUs: 1536

Earlier verification of an older revision of this patch was performed on 
a system with 2048 CPUs*.* Due to system unavailability, the current 
verification was carried out on a different**system. For reference, I am 
updating the results corresponding to the patches verified using the 
above configuration.


Patch: 
https://lore.kernel.org/all/20260112094332.66006-2-vishalc@linux.ibm.com/
SMT Mode    | Without Patch    | With Patch  | % Improvement   |
------------------------------------------------------------------
SMT=off     | 20m 32.210s      |  5m 31.807s | +73.09%        |
SMT=on      | 62m 46.549s      | 55m 48.801s  | +11.08%        |

SMT=ON  -> sys 50m44.105s
SMT=OFF -> sys 0m0.035s


patch: https://lore.kernel.org/all/20260114183415.286489-1-urezki@gmail.com/
SMT Mode    | Without Patch    | With Patch  | % Improvement   |
------------------------------------------------------------------
SMT=off     | 20m 32.210s      | 18m 2.012s  | +12.19%         |
SMT=on      | 62m 46.549s      | 62m 35.076s | +0.30%         |

SMT=ON  -> sys 50m43.806s
SMT=OFF -> sys 0m0.109s


Regards,
Samir