rcutorture: Decorate failing reader segments with CPU ID

This commit adds CPU number to the "Failure/close-call rcutorture reader
segments" list printed at the end of an rcutorture run that had too-short
grace periods.  This information can help debugging interactions with
migration and CPU hotplug.

However, experience indicates that sampling the CPU number in rcutorture's
read-side code can reduce the probability of too-short bugs by a small
integer factor.  And small integer factors are crucial to RCU bug hunting,
so this commit also introduces a default-off RCU_TORTURE_TEST_LOG_CPU
Kconfig option to enable this CPU-number-logging functionality at
build time.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>

Author: paulmckrcu

kernel/rcu/Kconfig.debug

@@ -53,6 +53,21 @@ config RCU_TORTURE_TEST
 	  Say M if you want the RCU torture tests to build as a module.
 	  Say N if you are unsure.
 
+config RCU_TORTURE_TEST_LOG_CPU
+	tristate "Log CPU for rcutorture failures"
+	depends on RCU_TORTURE_TEST
+	default n
+	help
+	  This option causes rcutorture to decorate each entry of its
+	  log of failure/close-call rcutorture reader segments with the
+	  number of the CPU that the reader was running on at the time.
+	  This information can be useful, but it does incur additional
+	  overhead, overhead that can make both failures and close calls
+	  less probable.
+
+	  Say Y here if you want CPU IDs logged.
+	  Say N if you are unsure.
+
 config RCU_REF_SCALE_TEST
 	tristate "Scalability tests for read-side synchronization (RCU and others)"
 	depends on DEBUG_KERNEL

kernel/rcu/rcutorture.c

@@ -262,6 +262,7 @@ struct rt_read_seg {
 	unsigned long rt_delay_ms;
 	unsigned long rt_delay_us;
 	bool rt_preempted;
+	int rt_cpu;
 };
 static int err_segs_recorded;
 static struct rt_read_seg err_segs[RCUTORTURE_RDR_MAX_SEGS];
@@ -1862,6 +1863,8 @@ static void rcutorture_one_extend(int *readstate, int newstate,
 	WARN_ON_ONCE(idxold2 < 0);
 	WARN_ON_ONCE(idxold2 & ~RCUTORTURE_RDR_ALLBITS);
 	rtrsp->rt_readstate = newstate;
+	if (IS_ENABLED(CONFIG_RCU_TORTURE_TEST_LOG_CPU))
+		rtrsp->rt_cpu = raw_smp_processor_id();
 
 	/* First, put new protection in place to avoid critical-section gap. */
 	if (statesnew & RCUTORTURE_RDR_BH)
@@ -3559,8 +3562,10 @@ rcu_torture_cleanup(void)
 					err_segs[i].rt_delay_us);
 				firsttime = 0;
 			}
-			pr_cont("%s\n",
-				err_segs[i].rt_preempted ? "preempted" : "");
+			pr_cont("%s", err_segs[i].rt_preempted ? "preempted" : "");
+			if (IS_ENABLED(CONFIG_RCU_TORTURE_TEST_LOG_CPU))
+				pr_cont(" CPU %d", err_segs[i].rt_cpu);
+			pr_cont("\n");
 
 		}
 	}