From: "Paul E. McKenney" <paulmck@xxxxxxxxxx> The RCU Tasks Trace grace periods are too slow, as in 40x slower than those of RCU Tasks. This is due to my having assumed a one-second grace period was OK, and thus not having optimized any further. This commit provides the first step in this optimization process, namely by allowing the task_list scan backoff interval to be specified on a per-flavor basis, and then speeding up the scans for RCU Tasks Trace. However, kernels built with CONFIG_TASKS_TRACE_RCU_READ_MB=y continue to use the old slower backoff, consistent with that Kconfig option's goal of reducing IPIs. Link: https://lore.kernel.org/bpf/CAADnVQK_AiX+S_L_A4CQWT11XyveppBbQSQgH_qWGyzu_E8Yeg@xxxxxxxxxxxxxx/ Reported-by: Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx> Cc: Daniel Borkmann <daniel@xxxxxxxxxxxxx> Cc: Jiri Olsa <jolsa@xxxxxxxxxx> Cc: <bpf@xxxxxxxxxxxxxxx> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx> --- kernel/rcu/tasks.h | 16 ++++++++++++++-- 1 file changed, 14 insertions(+), 2 deletions(-) diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h index 978508e..ad8c4f3 100644 --- a/kernel/rcu/tasks.h +++ b/kernel/rcu/tasks.h @@ -28,6 +28,7 @@ typedef void (*postgp_func_t)(struct rcu_tasks *rtp); * @kthread_ptr: This flavor's grace-period/callback-invocation kthread. * @gp_func: This flavor's grace-period-wait function. * @gp_state: Grace period's most recent state transition (debugging). + * @init_fract: Initial backoff sleep interval. * @gp_jiffies: Time of last @gp_state transition. * @gp_start: Most recent grace-period start in jiffies. * @n_gps: Number of grace periods completed since boot. @@ -48,6 +49,7 @@ struct rcu_tasks { struct wait_queue_head cbs_wq; raw_spinlock_t cbs_lock; int gp_state; + int init_fract; unsigned long gp_jiffies; unsigned long gp_start; unsigned long n_gps; @@ -329,8 +331,10 @@ static void rcu_tasks_wait_gp(struct rcu_tasks *rtp) */ lastreport = jiffies; - /* Start off with HZ/10 wait and slowly back off to 1 HZ wait. */ - fract = 10; + // Start off with initial wait and slowly back off to 1 HZ wait. + fract = rtp->init_fract; + if (fract > HZ) + fract = HZ; for (;;) { bool firstreport; @@ -553,6 +557,7 @@ EXPORT_SYMBOL_GPL(rcu_barrier_tasks); static int __init rcu_spawn_tasks_kthread(void) { + rcu_tasks.init_fract = 10; rcu_tasks.pregp_func = rcu_tasks_pregp_step; rcu_tasks.pertask_func = rcu_tasks_pertask; rcu_tasks.postscan_func = rcu_tasks_postscan; @@ -1163,6 +1168,13 @@ EXPORT_SYMBOL_GPL(rcu_barrier_tasks_trace); static int __init rcu_spawn_tasks_trace_kthread(void) { + if (IS_ENABLED(CONFIG_TASKS_TRACE_RCU_READ_MB)) { + rcu_tasks_trace.init_fract = 10; + } else { + rcu_tasks_trace.init_fract = HZ / 5; + if (rcu_tasks_trace.init_fract <= 0) + rcu_tasks_trace.init_fract = 1; + } rcu_tasks_trace.pregp_func = rcu_tasks_trace_pregp_step; rcu_tasks_trace.pertask_func = rcu_tasks_trace_pertask; rcu_tasks_trace.postscan_func = rcu_tasks_trace_postscan; -- 2.9.5
