Re: [PATCH v2] rcu: Reduce synchronize_rcu() latency by reporting GP kthread's CPU QS early

[Uladzislau Rezki <urezki@gmail.com>]

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@kernel.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@pc638:~$ 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 ${BRIDGE}
urezki@pc638:~$
<snip>

1)
# /tmp/default.txt
urezki@pc638:~$ time for i in $(seq 1 100); do ./bridge.sh; done
real    0m24.221s
user    0m1.875s
sys     0m2.013s
urezki@pc638:~$

2)
# echo 1 > /sys/module/rcutree/parameters/enable_joel_patch
# /tmp/enable_joel_patch.txt
urezki@pc638:~$ time for i in $(seq 1 100); do ./bridge.sh; done
real    0m20.754s
user    0m1.950s
sys     0m1.888s
urezki@pc638:~$

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@pc638:~$ time for i in $(seq 1 100); do ./bridge.sh; done
real    0m15.895s
user    0m2.023s
sys     0m1.935s
urezki@pc638:~$

4)
# echo 1 > /sys/module/rcutree/parameters/rcu_normal_wake_from_gp
# /tmp/enable_rcu_normal_wake_from_gp.txt
urezki@pc638:~$ time for i in $(seq 1 100); do ./bridge.sh; done
real    0m18.947s
user    0m2.145s
sys     0m1.735s
urezki@pc638:~$ 

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.

--
Uladzislau Rezki