rcu: Optionally run grace-period kthreads at real-time priority

Recent testing has shown that under heavy load, running RCU's grace-period
kthreads at real-time priority can improve performance (according to 0day
test robot) and reduce the incidence of RCU CPU stall warnings.  However,
most systems do just fine with the default non-realtime priorities for
these kthreads, and it does not make sense to expose the entire user
base to any risk stemming from this change, given that this change is
of use only to a few users running extremely heavy workloads.

Therefore, this commit allows users to specify realtime priorities
for the grace-period kthreads, but leaves them running SCHED_OTHER
by default.  The realtime priority may be specified at build time
via the RCU_KTHREAD_PRIO Kconfig parameter, or at boot time via the
rcutree.kthread_prio parameter.  Either way, 0 says to continue the
default SCHED_OTHER behavior and values from 1-99 specify that priority
of SCHED_FIFO behavior.  Note that a value of 0 is not permitted when
the RCU_BOOST Kconfig parameter is specified.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

diff --git a/init/Kconfig b/init/Kconfig
index 9afb971497f4c9972b0dcf14071edf9ef81320be..d3ee66a6990f7f7b974deb8238bb6ce779e84cac 100644 (file)
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -668,9 +668,10 @@ config RCU_BOOST
 
 config RCU_KTHREAD_PRIO
        int "Real-time priority to use for RCU worker threads"
-       range 1 99
-       depends on RCU_BOOST
-       default 1
+       range 1 99 if RCU_BOOST
+       range 0 99 if !RCU_BOOST
+       default 1 if RCU_BOOST
+       default 0 if !RCU_BOOST
        help
          This option specifies the SCHED_FIFO priority value that will be
          assigned to the rcuc/n and rcub/n threads and is also the value

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 5987fdc85fc4c5d29eb3b4f8c4329f93259764d3..75ce12316b4c5455f9d36f69cf1364ba0407df7d 100644 (file)
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -156,6 +156,10 @@ static void rcu_boost_kthread_setaffinity(struct rcu_node *rnp, int outgoingcpu)
 static void invoke_rcu_core(void);
 static void invoke_rcu_callbacks(struct rcu_state *rsp, struct rcu_data *rdp);
 
+/* rcuc/rcub kthread realtime priority */
+static int kthread_prio = CONFIG_RCU_KTHREAD_PRIO;
+module_param(kthread_prio, int, 0644);
+
 /*
  * Track the rcutorture test sequence number and the update version
  * number within a given test.  The rcutorture_testseq is incremented
@@ -3597,17 +3601,35 @@ static int rcu_pm_notify(struct notifier_block *self,
 static int __init rcu_spawn_gp_kthread(void)
 {
        unsigned long flags;
+       int kthread_prio_in = kthread_prio;
        struct rcu_node *rnp;
        struct rcu_state *rsp;
+       struct sched_param sp;
        struct task_struct *t;
 
+       /* Force priority into range. */
+       if (IS_ENABLED(CONFIG_RCU_BOOST) && kthread_prio < 1)
+               kthread_prio = 1;
+       else if (kthread_prio < 0)
+               kthread_prio = 0;
+       else if (kthread_prio > 99)
+               kthread_prio = 99;
+       if (kthread_prio != kthread_prio_in)
+               pr_alert("rcu_spawn_gp_kthread(): Limited prio to %d from %d\n",
+                        kthread_prio, kthread_prio_in);
+
        rcu_scheduler_fully_active = 1;
        for_each_rcu_flavor(rsp) {
-               t = kthread_run(rcu_gp_kthread, rsp, "%s", rsp->name);
+               t = kthread_create(rcu_gp_kthread, rsp, "%s", rsp->name);
                BUG_ON(IS_ERR(t));
                rnp = rcu_get_root(rsp);
                raw_spin_lock_irqsave(&rnp->lock, flags);
                rsp->gp_kthread = t;
+               if (kthread_prio) {
+                       sp.sched_priority = kthread_prio;
+                       sched_setscheduler_nocheck(t, SCHED_FIFO, &sp);
+               }
+               wake_up_process(t);
                raw_spin_unlock_irqrestore(&rnp->lock, flags);
        }
        rcu_spawn_nocb_kthreads();

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 81ff8b9a5a39138befde38d16ec3f04b03e4a02e..dcb32638f88bfb83da15f26ae281a519bf39e88d 100644 (file)
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -34,10 +34,6 @@
 
 #include "../locking/rtmutex_common.h"
 
-/* rcuc/rcub kthread realtime priority */
-static int kthread_prio = CONFIG_RCU_KTHREAD_PRIO;
-module_param(kthread_prio, int, 0644);
-
 /*
  * Control variables for per-CPU and per-rcu_node kthreads.  These
  * handle all flavors of RCU.