Commit a81aeaf7 authored by Frederic Weisbecker's avatar Frederic Weisbecker Committed by Paul E. McKenney
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rcu/nocb: Optimize kthreads and rdp initialization 



Currently cpumask_available() is used to prevent from unwanted NOCB
initialization.  However if neither "rcu_nocbs=" nor "nohz_full="
parameters are passed to a kernel built with CONFIG_CPUMASK_OFFSTACK=n,
the initialization path is still taken, running through all sorts of
needless operations and iterations on an empty cpumask.

Fix this by relying on a real initialization state instead.  This also
optimizes kthread creation, preventing needless iteration over all online
CPUs when the kernel is booted without any offloaded CPUs.

Reviewed-by: default avatarNeeraj Upadhyay <quic_neeraju@quicinc.com>
Signed-off-by: default avatarFrederic Weisbecker <frederic@kernel.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Uladzislau Rezki <urezki@gmail.com>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Joel Fernandes <joel@joelfernandes.org>
Tested-by: default avatarJuri Lelli <juri.lelli@redhat.com>
Signed-off-by: default avatarPaul E. McKenney <paulmck@kernel.org>
parent 8d970396

kernel/rcu/tree_nocb.h

+17 −7
Original line number Diff line number Diff line
@@ -60,6 +60,9 @@ static inline bool rcu_current_is_nocb_kthread(struct rcu_data *rdp) 
 * Parse the boot-time rcu_nocb_mask CPU list from the kernel parameters.

 * If the list is invalid, a warning is emitted and all CPUs are offloaded.

 */



static bool rcu_nocb_is_setup;



static int __init rcu_nocb_setup(char *str)

{

	alloc_bootmem_cpumask_var(&rcu_nocb_mask);
@@ -67,6 +70,7 @@ static int __init rcu_nocb_setup(char *str) 
		pr_warn("rcu_nocbs= bad CPU range, all CPUs set\n");

		cpumask_setall(rcu_nocb_mask);

	}

	rcu_nocb_is_setup = true;

	return 1;

}

__setup("rcu_nocbs=", rcu_nocb_setup);
@@ -1167,13 +1171,17 @@ void __init rcu_init_nohz(void) 
		need_rcu_nocb_mask = true;

#endif /* #if defined(CONFIG_NO_HZ_FULL) */



	if (!cpumask_available(rcu_nocb_mask) && need_rcu_nocb_mask) {

	if (need_rcu_nocb_mask) {

		if (!cpumask_available(rcu_nocb_mask)) {

			if (!zalloc_cpumask_var(&rcu_nocb_mask, GFP_KERNEL)) {

				pr_info("rcu_nocb_mask allocation failed, callback offloading disabled.\n");

				return;

			}

		}

	if (!cpumask_available(rcu_nocb_mask))

		rcu_nocb_is_setup = true;

	}



	if (!rcu_nocb_is_setup)

		return;



#if defined(CONFIG_NO_HZ_FULL)
@@ -1275,9 +1283,11 @@ static void __init rcu_spawn_nocb_kthreads(void) 
{

	int cpu;



	if (rcu_nocb_is_setup) {

		for_each_online_cpu(cpu)

			rcu_spawn_cpu_nocb_kthread(cpu);

	}

}



/* How many CB CPU IDs per GP kthread?  Default of -1 for sqrt(nr_cpu_ids). */

static int rcu_nocb_gp_stride = -1;