diff options
| author | Kalesh Singh <kaleshsingh@google.com> | 2022-04-08 17:35:27 -0700 |
|---|---|---|
| committer | Paul E. McKenney <paulmck@kernel.org> | 2022-05-11 11:47:10 -0700 |
| commit | 9621fbee44df940e2e1b94b0676460a538dffefa (patch) | |
| tree | 4ae910165cef0927d5c0e6ad1555427537ed20b6 /kernel/rcu/Kconfig | |
| parent | 28b3ae426598e722cf5d5ab9cc7038791b955a56 (diff) | |
rcu: Move expedited grace period (GP) work to RT kthread_worker
Enabling CONFIG_RCU_BOOST did not reduce RCU expedited grace-period
latency because its workqueues run at SCHED_OTHER, and thus can be
delayed by normal processes. This commit avoids these delays by moving
the expedited GP work items to a real-time-priority kthread_worker.
This option is controlled by CONFIG_RCU_EXP_KTHREAD and disabled by
default on PREEMPT_RT=y kernels which disable expedited grace periods
after boot by unconditionally setting rcupdate.rcu_normal_after_boot=1.
The results were evaluated on arm64 Android devices (6GB ram) running
5.10 kernel, and capturing trace data in critical user-level code.
The table below shows the resulting order-of-magnitude improvements
in synchronize_rcu_expedited() latency:
------------------------------------------------------------------------
| | workqueues | kthread_worker | Diff |
------------------------------------------------------------------------
| Count | 725 | 688 | |
------------------------------------------------------------------------
| Min Duration (ns) | 326 | 447 | 37.12% |
------------------------------------------------------------------------
| Q1 (ns) | 39,428 | 38,971 | -1.16% |
------------------------------------------------------------------------
| Q2 - Median (ns) | 98,225 | 69,743 | -29.00% |
------------------------------------------------------------------------
| Q3 (ns) | 342,122 | 126,638 | -62.98% |
------------------------------------------------------------------------
| Max Duration (ns) | 372,766,967 | 2,329,671 | -99.38% |
------------------------------------------------------------------------
| Avg Duration (ns) | 2,746,353 | 151,242 | -94.49% |
------------------------------------------------------------------------
| Standard Deviation (ns) | 19,327,765 | 294,408 | |
------------------------------------------------------------------------
The below table show the range of maximums/minimums for
synchronize_rcu_expedited() latency from all experiments:
------------------------------------------------------------------------
| | workqueues | kthread_worker | Diff |
------------------------------------------------------------------------
| Total No. of Experiments | 25 | 23 | |
------------------------------------------------------------------------
| Largest Maximum (ns) | 372,766,967 | 2,329,671 | -99.38% |
------------------------------------------------------------------------
| Smallest Maximum (ns) | 38,819 | 86,954 | 124.00% |
------------------------------------------------------------------------
| Range of Maximums (ns) | 372,728,148 | 2,242,717 | |
------------------------------------------------------------------------
| Largest Minimum (ns) | 88,623 | 27,588 | -68.87% |
------------------------------------------------------------------------
| Smallest Minimum (ns) | 326 | 447 | 37.12% |
------------------------------------------------------------------------
| Range of Minimums (ns) | 88,297 | 27,141 | |
------------------------------------------------------------------------
Cc: "Paul E. McKenney" <paulmck@kernel.org>
Cc: Tejun Heo <tj@kernel.org>
Reported-by: Tim Murray <timmurray@google.com>
Reported-by: Wei Wang <wvw@google.com>
Tested-by: Kyle Lin <kylelin@google.com>
Tested-by: Chunwei Lu <chunweilu@google.com>
Tested-by: Lulu Wang <luluw@google.com>
Signed-off-by: Kalesh Singh <kaleshsingh@google.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Diffstat (limited to 'kernel/rcu/Kconfig')
| -rw-r--r-- | kernel/rcu/Kconfig | 14 |
1 files changed, 14 insertions, 0 deletions
diff --git a/kernel/rcu/Kconfig b/kernel/rcu/Kconfig index bf8e341e75b4..fd64a75823cb 100644 --- a/kernel/rcu/Kconfig +++ b/kernel/rcu/Kconfig @@ -195,6 +195,20 @@ config RCU_BOOST_DELAY Accept the default if unsure. +config RCU_EXP_KTHREAD + bool "Perform RCU expedited work in a real-time kthread" + depends on RCU_BOOST && RCU_EXPERT + default !PREEMPT_RT && NR_CPUS <= 32 + help + Use this option to further reduce the latencies of expedited + grace periods at the expense of being more disruptive. + + This option is disabled by default on PREEMPT_RT=y kernels which + disable expedited grace periods after boot by unconditionally + setting rcupdate.rcu_normal_after_boot=1. + + Accept the default if unsure. + config RCU_NOCB_CPU bool "Offload RCU callback processing from boot-selected CPUs" depends on TREE_RCU |