sched,rcu: Make cond_resched() provide RCU quiescent state

There is some confusion as to which of cond_resched() or
cond_resched_rcu_qs() should be added to long in-kernel loops.
This commit therefore eliminates the decision by adding RCU quiescent
states to cond_resched().  This commit also simplifies the code that
used to interact with cond_resched_rcu_qs(), and that now interacts with
cond_resched(), to reduce its overhead.  This reduction is necessary to
allow the heavier-weight cond_resched_rcu_qs() mechanism to be invoked
everywhere that cond_resched() is invoked.

Part of that reduction in overhead converts the jiffies_till_sched_qs
kernel parameter to read-only at runtime, thus eliminating the need for
bounds checking.

Reported-by: Michal Hocko <mhocko@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
[ paulmck: Keep PREEMPT=n cond_resched a no-op, per Peter Zijlstra. ]

2 files changed, 6 insertions, 20 deletions

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index b0ad62b0e7b8..0dda57a28276 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -534,8 +534,8 @@ module_param(rcu_kick_kthreads, bool, 0644);
  * How long the grace period must be before we start recruiting
  * quiescent-state help from rcu_note_context_switch().
  */
-static ulong jiffies_till_sched_qs = HZ / 20;
-module_param(jiffies_till_sched_qs, ulong, 0644);
+static ulong jiffies_till_sched_qs = HZ / 10;
+module_param(jiffies_till_sched_qs, ulong, 0444);
 
 static bool rcu_start_gp_advanced(struct rcu_state *rsp, struct rcu_node *rnp,
                                   struct rcu_data *rdp);
@@ -1235,7 +1235,6 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp)
         unsigned long jtsq;
         bool *rnhqp;
         bool *ruqp;
-        unsigned long rjtsc;
         struct rcu_node *rnp;
 
         /*
@@ -1252,23 +1251,13 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp)
                 return 1;
         }
 
-        /* Compute and saturate jiffies_till_sched_qs. */
-        jtsq = jiffies_till_sched_qs;
-        rjtsc = rcu_jiffies_till_stall_check();
-        if (jtsq > rjtsc / 2) {
-                WRITE_ONCE(jiffies_till_sched_qs, rjtsc);
-                jtsq = rjtsc / 2;
-        } else if (jtsq < 1) {
-                WRITE_ONCE(jiffies_till_sched_qs, 1);
-                jtsq = 1;
-        }
-
         /*
          * Has this CPU encountered a cond_resched_rcu_qs() since the
          * beginning of the grace period?  For this to be the case,
          * the CPU has to have noticed the current grace period.  This
          * might not be the case for nohz_full CPUs looping in the kernel.
          */
+        jtsq = jiffies_till_sched_qs;
         rnp = rdp->mynode;
         ruqp = per_cpu_ptr(&rcu_dynticks.rcu_urgent_qs, rdp->cpu);
         if (time_after(jiffies, rdp->rsp->gp_start + jtsq) &&
@@ -1276,7 +1265,7 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp)
             READ_ONCE(rdp->gpnum) == rnp->gpnum && !rdp->gpwrap) {
                 trace_rcu_fqs(rdp->rsp->name, rdp->gpnum, rdp->cpu, TPS("rqc"));
                 return 1;
-        } else {
+        } else if (time_after(jiffies, rdp->rsp->gp_start + jtsq)) {
                 /* Load rcu_qs_ctr before store to rcu_urgent_qs. */
                 smp_store_release(ruqp, true);
         }
@@ -1304,10 +1293,6 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp)
          * updates are only once every few jiffies, the probability of
          * lossage (and thus of slight grace-period extension) is
          * quite low.
-         *
-         * Note that if the jiffies_till_sched_qs boot/sysfs parameter
-         * is set too high, we override with half of the RCU CPU stall
-         * warning delay.
          */
         rnhqp = &per_cpu(rcu_dynticks.rcu_need_heavy_qs, rdp->cpu);
         if (!READ_ONCE(*rnhqp) &&
@@ -1316,7 +1301,7 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp)
                 WRITE_ONCE(*rnhqp, true);
                 /* Store rcu_need_heavy_qs before rcu_urgent_qs. */
                 smp_store_release(ruqp, true);
-                rdp->rsp->jiffies_resched += 5; /* Re-enable beating. */
+                rdp->rsp->jiffies_resched += jtsq; /* Re-enable beating. */
         }
 
         /*
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index d17c5da523a0..904d3ab35c83 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4842,6 +4842,7 @@ int __sched _cond_resched(void)
                 preempt_schedule_common();
                 return 1;
         }
+        rcu_all_qs();
         return 0;
 }
 EXPORT_SYMBOL(_cond_resched);