rcutorture: Give the scheduler a chance on PREEMPT && NO_HZ_FULL kernels

In !PREEMPT kernels, cond_resched() is a no-op.  In NO_HZ_FULL kernels,
in-kernel execution (such as that of rcutorture's kthreads) might extend
indefinitely without the scheduler gaining the aid of a scheduling-clock
interrupt.  This combination can make the interaction of an rcutorture
forward-progress test and a CPU-hotplug stop_machine operation make less
forward progress than one might like.  Additionally, Sebastian Siewior
notes that NO_HZ_FULL kernels have a scheduler check upon return to
userspace execution, which suggests that in-kernel emulation of tight
userspace loops containing system calls doing call_rcu() might also need
explicit checks in the PREEMPT && NO_HZ_FULL case.

This commit therefore introduces a rcu_torture_fwd_prog_cond_resched()
function that explicitly invokes schedule() in such kernels whenever
need_resched() returns true, while retaining use of cond_resched()
for kernels that are either !PREEMPT or !NO_HZ_FULL.

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

diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
index 6a45585..ef6f6de 100644 (file)
--- a/kernel/rcu/rcutorture.c
+++ b/kernel/rcu/rcutorture.c
@@ -1667,6 +1667,17 @@ static void rcu_torture_fwd_cb_cr(struct rcu_head *rhp)
        spin_unlock_irqrestore(&rcu_fwd_lock, flags);
 }
 
+// Give the scheduler a chance, even on nohz_full CPUs.
+static void rcu_torture_fwd_prog_cond_resched(void)
+{
+       if (IS_ENABLED(CONFIG_PREEMPT) && IS_ENABLED(CONFIG_NO_HZ_FULL)) {
+               if (need_resched())
+                       schedule();
+       } else {
+               cond_resched();
+       }
+}
+
 /*
  * Free all callbacks on the rcu_fwd_cb_head list, either because the
  * test is over or because we hit an OOM event.
@@ -1690,7 +1701,7 @@ static unsigned long rcu_torture_fwd_prog_cbfree(void)
                spin_unlock_irqrestore(&rcu_fwd_lock, flags);
                kfree(rfcp);
                freed++;
-               cond_resched();
+               rcu_torture_fwd_prog_cond_resched();
        }
        return freed;
 }
@@ -1734,7 +1745,7 @@ static void rcu_torture_fwd_prog_nr(int *tested, int *tested_tries)
                udelay(10);
                cur_ops->readunlock(idx);
                if (!fwd_progress_need_resched || need_resched())
-                       cond_resched();
+                       rcu_torture_fwd_prog_cond_resched();
        }
        (*tested_tries)++;
        if (!time_before(jiffies, stopat) &&
@@ -1817,7 +1828,7 @@ static void rcu_torture_fwd_prog_cr(void)
                        rfcp->rfc_gps = 0;
                }
                cur_ops->call(&rfcp->rh, rcu_torture_fwd_cb_cr);
-               cond_resched();
+               rcu_torture_fwd_prog_cond_resched();
        }
        stoppedat = jiffies;
        n_launders_cb_snap = READ_ONCE(n_launders_cb);