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

[Uladzislau Rezki <urezki@gmail.com>]

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.

--
Uladzislau Rezki