Message-Id: <164E7707-758C-44AA-BB75-B6560725C8CD@joelfernandes.org>
Date: Sun, 28 Dec 2025 21:49:45 -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 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.

thanks,

- Joel

> 
>                            Thanx, Paul
> 

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