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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: Neeraj Upadhyay <quic_neeraju@quicinc.com>
Signed-off-by: Frederic 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: Juri Lelli <juri.lelli@redhat.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
This commit is contained in:
Frederic Weisbecker 2021-11-23 01:37:05 +01:00 committed by Paul E. McKenney
parent 8d97039646
commit a81aeaf7a1
1 changed files with 17 additions and 7 deletions
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24
kernel/rcu/tree_nocb.h
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@ -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 (!zalloc_cpumask_var(&rcu_nocb_mask, GFP_KERNEL)) {
pr_info("rcu_nocb_mask allocation failed, callback offloading disabled.\n");
return;
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;
}
}
rcu_nocb_is_setup = true;
}
if (!cpumask_available(rcu_nocb_mask))
if (!rcu_nocb_is_setup)
return;
#if defined(CONFIG_NO_HZ_FULL)
@ -1275,8 +1283,10 @@ static void __init rcu_spawn_nocb_kthreads(void)
{
int cpu;
for_each_online_cpu(cpu)
rcu_spawn_cpu_nocb_kthread(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). */