Commit 4fe192df authored by Paul E. McKenney's avatar Paul E. McKenney
Browse files

rcu-tasks: Shorten per-grace-period sleep for RCU Tasks Trace 

The various RCU tasks flavors currently wait 100 milliseconds between each
grace period in order to prevent CPU-bound loops and to favor efficiency
over latency.  However, RCU Tasks Trace needs to have a grace-period
latency of roughly 25 milliseconds, which is completely infeasible given
the 100-millisecond per-grace-period sleep.  This commit therefore reduces
this sleep duration to 5 milliseconds (or one jiffy, whichever is longer)
in kernels built with CONFIG_TASKS_TRACE_RCU_READ_MB=y.

Link: https://lore.kernel.org/bpf/CAADnVQK_AiX+S_L_A4CQWT11XyveppBbQSQgH_qWGyzu_E8Yeg@mail.gmail.com/


Reported-by: default avatarAlexei Starovoitov <alexei.starovoitov@gmail.com>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: <bpf@vger.kernel.org>
Signed-off-by: default avatarPaul E. McKenney <paulmck@kernel.org>
parent 574de876

kernel/rcu/tasks.h

+9 −1
Original line number Diff line number Diff line
@@ -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).

 * @gp_sleep: Per-grace-period sleep to prevent CPU-bound looping.

 * @init_fract: Initial backoff sleep interval.

 * @gp_jiffies: Time of last @gp_state transition.

 * @gp_start: Most recent grace-period start in jiffies.
@@ -49,6 +50,7 @@ struct rcu_tasks { 
	struct wait_queue_head cbs_wq;

	raw_spinlock_t cbs_lock;

	int gp_state;

	int gp_sleep;

	int init_fract;

	unsigned long gp_jiffies;

	unsigned long gp_start;
@@ -233,7 +235,7 @@ static int __noreturn rcu_tasks_kthread(void *arg) 
			cond_resched();

		}

		/* Paranoid sleep to keep this from entering a tight loop */

		schedule_timeout_idle(HZ/10);

		schedule_timeout_idle(rtp->gp_sleep);



		set_tasks_gp_state(rtp, RTGS_WAIT_CBS);

	}
@@ -557,6 +559,7 @@ EXPORT_SYMBOL_GPL(rcu_barrier_tasks); 


static int __init rcu_spawn_tasks_kthread(void)

{

	rcu_tasks.gp_sleep = HZ / 10;

	rcu_tasks.init_fract = 10;

	rcu_tasks.pregp_func = rcu_tasks_pregp_step;

	rcu_tasks.pertask_func = rcu_tasks_pertask;
@@ -690,6 +693,7 @@ EXPORT_SYMBOL_GPL(rcu_barrier_tasks_rude); 


static int __init rcu_spawn_tasks_rude_kthread(void)

{

	rcu_tasks_rude.gp_sleep = HZ / 10;

	rcu_spawn_tasks_kthread_generic(&rcu_tasks_rude);

	return 0;

}
@@ -1170,8 +1174,12 @@ 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.gp_sleep = HZ / 10;

		rcu_tasks_trace.init_fract = 10;

	} else {

		rcu_tasks_trace.gp_sleep = HZ / 200;

		if (rcu_tasks_trace.gp_sleep <= 0)

			rcu_tasks_trace.gp_sleep = 1;

		rcu_tasks_trace.init_fract = HZ / 5;

		if (rcu_tasks_trace.init_fract <= 0)

			rcu_tasks_trace.init_fract = 1;

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