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
