Message-Id: <501F3B9B-C5A2-4537-A8E2-50D33164EFD5@joelfernandes.org>
Date: Mon, 29 Dec 2025 15:36:39 -0500
From: Joel Fernandes <joel@...lfernandes.org>
To: paulmck@...nel.org
Cc: Uladzislau Rezki <urezki@...il.com>,
 Joel Fernandes <joelagnelf@...dia.com>, linux-kernel@...r.kernel.org,
 Frederic Weisbecker <frederic@...nel.org>,
 Neeraj Upadhyay <neeraj.upadhyay@...nel.org>,
 Josh Triplett <josh@...htriplett.org>, Boqun Feng <boqun.feng@...il.com>,
 Steven Rostedt <rostedt@...dmis.org>,
 Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
 Lai Jiangshan <jiangshanlai@...il.com>, Zqiang <qiang.zhang@...ux.dev>,
 rcu@...r.kernel.org
Subject: Re: [PATCH v2] rcu: Reduce synchronize_rcu() latency by reporting GP kthread's CPU QS early



> On Dec 29, 2025, at 12:02 PM, Paul E. McKenney <paulmck@...nel.org> wrote:
> 
> On Mon, Dec 29, 2025 at 05:25:24PM +0100, Uladzislau Rezki wrote:
>>> On Mon, Dec 29, 2025 at 07:53:59AM -0800, Paul E. McKenney wrote:
>>> On Mon, Dec 29, 2025 at 02:28:43PM +0100, Uladzislau Rezki wrote:
>>>> On Sun, Dec 28, 2025 at 09:49:45PM -0500, Joel Fernandes wrote:
>>>>> 
>>>>> 
>>>>>> On Dec 28, 2025, at 7:04 PM, Paul E. McKenney <paulmck@...nel.org> wrote:
>>>>>> 
>>>>>> On Sun, Dec 28, 2025 at 06:57:58PM +0100, Uladzislau Rezki wrote:
>>>>>>>> On Thu, Dec 25, 2025 at 09:33:39PM -0500, Joel Fernandes wrote:
>>>>>>>> On Thu, Dec 25, 2025 at 10:35:44AM -0800, Paul E. McKenney wrote:
>>>>>>>>> On Mon, Dec 22, 2025 at 10:46:29PM -0500, Joel Fernandes wrote:
>>>>>>>>>> The RCU grace period mechanism uses a two-phase FQS (Force Quiescent
>>>>>>>>>> State) design where the first FQS saves dyntick-idle snapshots and
>>>>>>>>>> the second FQS compares them. This results in long and unnecessary latency
>>>>>>>>>> for synchronize_rcu() on idle systems (two FQS waits of ~3ms each with
>>>>>>>>>> 1000HZ) whenever one FQS wait sufficed.
>>>>>>>>>> 
>>>>>>>>>> Some investigations showed that the GP kthread's CPU is the holdout CPU
>>>>>>>>>> a lot of times after the first FQS as - it cannot be detected as "idle"
>>>>>>>>>> because it's actively running the FQS scan in the GP kthread.
>>>>>>>>>> 
>>>>>>>>>> Therefore, at the end of rcu_gp_init(), immediately report a quiescent
>>>>>>>>>> state for the GP kthread's CPU using rcu_qs() + rcu_report_qs_rdp(). The
>>>>>>>>>> GP kthread cannot be in an RCU read-side critical section while running
>>>>>>>>>> GP initialization, so this is safe and results in significant latency
>>>>>>>>>> improvements.
>>>>>>>>>> 
>>>>>>>>>> I benchmarked 100 synchronize_rcu() calls with 32 CPUs, 10 runs each
>>>>>>>>>> showing significant latency improvements (default settings for fqs jiffies):
>>>>>>>>>> 
>>>>>>>>>> Baseline (without fix):
>>>>>>>>>> | Run | Mean      | Min      | Max       |
>>>>>>>>>> |-----|-----------|----------|-----------|
>>>>>>>>>> | 1   | 10.088 ms | 9.989 ms | 18.848 ms |
>>>>>>>>>> | 2   | 10.064 ms | 9.982 ms | 16.470 ms |
>>>>>>>>>> | 3   | 10.051 ms | 9.988 ms | 15.113 ms |
>>>>>>>>>> | 4   | 10.125 ms | 9.929 ms | 22.411 ms |
>>>>>>>>>> | 5   |  8.695 ms | 5.996 ms | 15.471 ms |
>>>>>>>>>> | 6   | 10.157 ms | 9.977 ms | 25.723 ms |
>>>>>>>>>> | 7   | 10.102 ms | 9.990 ms | 20.224 ms |
>>>>>>>>>> | 8   |  8.050 ms | 5.985 ms | 10.007 ms |
>>>>>>>>>> | 9   | 10.059 ms | 9.978 ms | 15.934 ms |
>>>>>>>>>> | 10  | 10.077 ms | 9.984 ms | 17.703 ms |
>>>>>>>>>> 
>>>>>>>>>> With fix:
>>>>>>>>>> | Run | Mean     | Min      | Max       |
>>>>>>>>>> |-----|----------|----------|-----------|
>>>>>>>>>> | 1   | 6.027 ms | 5.915 ms |  8.589 ms |
>>>>>>>>>> | 2   | 6.032 ms | 5.984 ms |  9.241 ms |
>>>>>>>>>> | 3   | 6.010 ms | 5.986 ms |  7.004 ms |
>>>>>>>>>> | 4   | 6.076 ms | 5.993 ms | 10.001 ms |
>>>>>>>>>> | 5   | 6.084 ms | 5.893 ms | 10.250 ms |
>>>>>>>>>> | 6   | 6.034 ms | 5.908 ms |  9.456 ms |
>>>>>>>>>> | 7   | 6.051 ms | 5.993 ms | 10.000 ms |
>>>>>>>>>> | 8   | 6.057 ms | 5.941 ms | 10.001 ms |
>>>>>>>>>> | 9   | 6.016 ms | 5.927 ms |  7.540 ms |
>>>>>>>>>> | 10  | 6.036 ms | 5.993 ms |  9.579 ms |
>>>>>>>>>> 
>>>>>>>>>> Summary:
>>>>>>>>>> - Mean latency: 9.75 ms -> 6.04 ms (38% improvement)
>>>>>>>>>> - Max latency:  25.72 ms -> 10.25 ms (60% improvement)
>>>>>>>>>> 
>>>>>>>>>> Tested rcutorture TREE and SRCU configurations.
>>>>>>>>>> 
>>>>>>>>>> [apply paulmck feedack on moving logic to rcu_gp_init()]
>>>>>>>>> 
>>>>>>>>> If anything, these numbers look better, so good show!!!
>>>>>>>> 
>>>>>>>> Thanks, I ended up collecting more samples in the v2 to further confirm the
>>>>>>>> improvements.
>>>>>>>> 
>>>>>>>>> Are there workloads that might be hurt by some side effect such
>>>>>>>>> as increased CPU utilization by the RCU grace-period kthread?  One
>>>>>>>>> non-mainstream hypothetical situation that comes to mind is a kernel
>>>>>>>>> built with SMP=y but running on a single-CPU system with a high-frequence
>>>>>>>>> periodic interrupt that does call_rcu().  Might that result in the RCU
>>>>>>>>> grace-period kthread chewing up the entire CPU?
>>>>>>>> 
>>>>>>>> There are still GP delays due to FQS, even with this change, so it could not
>>>>>>>> chew up the entire CPU I believe. The GP cycle should still insert delays
>>>>>>>> into the GP kthread. I did not notice in my testing that synchronize_rcu()
>>>>>>>> latency dropping to sub millisecond, it was still limited by the timer wheel
>>>>>>>> delays and the FQS delays.
>>>>>>>> 
>>>>>>>>> For a non-hypothetical case, could you please see if one of the
>>>>>>>>> battery-powered embedded guys would be willing to test this?
>>>>>>>> 
>>>>>>>> My suspicion is the battery-powered folks are already running RCU_LAZY to
>>>>>>>> reduce RCU activity, so they wouldn't be effected. call_rcu() during idleness
>>>>>>>> will be going to the bypass. Last I checked, Android and ChromeOS were both
>>>>>>>> enabling RCU_LAZY everywhere (back when I was at Google).
>>>>>>>> 
>>>>>>>> Uladzislau works on embedded (or at least till recently) and had recently
>>>>>>>> checked this area for improvements so I think he can help quantify too
>>>>>>>> perhaps. He is on CC. I personally don't directly work on embedded at the
>>>>>>>> moment, just big compute hungry machines. ;-) Uladzislau, would you have some
>>>>>>>> time to test on your Android devices?
>>>>>>>> 
>>>>>>> I will check the patch on my home based systems, big machines also :)
>>>>>>> I do not work with mobile area any more thus do not have access to our
>>>>>>> mobile devices. In fact i am glad that i have switched to something new.
>>>>>>> I was a bit tired by the applied Google restrictions when it comes to
>>>>>>> changes to the kernel and other Android layers.
>>>>>> 
>>>>>> How quickly I forget!  ;-)
>>>>>> 
>>>>>> Any thoughts on who would be a good person to ask about testing Joel's
>>>>>> patch on mobile platforms?
>>>>> 
>>>>> Maybe Suren? As precedent and fwiw, When rcu_normal_wake_from_gp optimization happened, it only improved things for Android.
>>>>> 
>>>>> Also Android already uses RCU_LAZY so this should not affect power for non-hurry usages.
>>>>> 
>>>>> Also networking bridge removal depends on synchronize_rcu() latency. When I forced rcu_normal_wake_from_gp on large machines, it improved bridge removal speed by about 5% per my notes. I would expect similar improvements with this.
>>>>> 
>>>> Here we go with some results. I tested bridge setup test case(100 loops):
>>>> 
>>>> <snip>
>>>> urezki@...38:~$ cat bridge.sh
>>>> #!/bin/sh
>>>> 
>>>> BRIDGE="virbr0"
>>>> NETWORK="192.0.0.1"
>>>> 
>>>> # setup bridge
>>>> sudo brctl addbr ${BRIDGE}
>>>> sudo ifconfig ${BRIDGE} ${NETWORK} up
>>>> sudo ifconfig ${BRIDGE} ${NETWORK} down
>>>> 
>>>> sudo brctl delbr ${BRIDGE}
>>>> urezki@...38:~$
>>>> <snip>
>>>> 
>>>> 1)
>>>> # /tmp/default.txt
>>>> urezki@...38:~$ time for i in $(seq 1 100); do ./bridge.sh; done
>>>> real    0m24.221s
>>>> user    0m1.875s
>>>> sys     0m2.013s
>>>> urezki@...38:~$
>>>> 
>>>> 2)
>>>> # echo 1 > /sys/module/rcutree/parameters/enable_joel_patch
>>>> # /tmp/enable_joel_patch.txt
>>>> urezki@...38:~$ time for i in $(seq 1 100); do ./bridge.sh; done
>>>> real    0m20.754s
>>>> user    0m1.950s
>>>> sys     0m1.888s
>>>> urezki@...38:~$
>>>> 
>>>> 3)
>>>> # echo 1 > /sys/module/rcutree/parameters/enable_joel_patch
>>>> # echo 1 > /sys/module/rcutree/parameters/rcu_normal_wake_from_gp
>>>> # /tmp/enable_joel_patch_enable_rcu_normal_wake_from_gp.txt
>>>> urezki@...38:~$ time for i in $(seq 1 100); do ./bridge.sh; done
>>>> real    0m15.895s
>>>> user    0m2.023s
>>>> sys     0m1.935s
>>>> urezki@...38:~$
>>>> 
>>>> 4)
>>>> # echo 1 > /sys/module/rcutree/parameters/rcu_normal_wake_from_gp
>>>> # /tmp/enable_rcu_normal_wake_from_gp.txt
>>>> urezki@...38:~$ time for i in $(seq 1 100); do ./bridge.sh; done
>>>> real    0m18.947s
>>>> user    0m2.145s
>>>> sys     0m1.735s
>>>> urezki@...38:~$
>>>> 
>>>> x86_64/64CPUs(in usec)
>>>>          1         2         3       4
>>>> median: 37249.5   31540.5   15765   22480
>>>> min:    7881      7918      9803    7857
>>>> max:    63651     55639     31861   32040
>>>> 
>>>> 1 - default;
>>>> 2 - Joel patch
>>>> 3 - Joel patch + enable_rcu_normal_wake_from_gp
>>>> 4 - enable_rcu_normal_wake_from_gp
>>>> 
>>>> Joel patch + enable_rcu_normal_wake_from_gp is a winner.
>>>> Time dropped from 24 seconds to 15 seconds to complete the test.
>>> 
>>> There was also an increase in system time from 1.735s to 1.935s with
>>> Joel's patch, correct?  Or is that in the noise?
>>> 
>> 
>> See below 5 run with just posted "sys" time:
>> 
>> #default
>> sys     0m1.936s
>> sys     0m1.894s
>> sys     0m1.937s
>> sys     0m1.698s
>> sys     0m1.740s
>> 
>> # Joel patch
>> sys     0m1.753s
>> sys     0m1.667s
>> sys     0m1.861s
>> sys     0m1.930s
>> sys     0m1.896s
>> 
>> i do not see increase, IMO it is a noise.
> 
> Even better, thank you!

Thanks a lot Vlad and Paul, I will include these numbers in the respin as well (with Tested-by from Vlad).

- Joel

> 
>                            Thanx, Paul
> 

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