rcutorture: Allow negative values of nreaders to oversubscribe

By default, with rcutorture.nreaders equal to -1, rcutorture provisions
N-1 reader kthreads, where N is the number of CPUs.  This avoids
rcutorture-induced stalls, but also avoids heavier levels of torture.
This commit therefore allows negative values of rcutorture.nreaders
to specify larger numbers of reader kthreads, so that for example
rcutorture.nreaders=-2 provisions N kthreads and rcutorture.nreaders=-5
provisions N+3 kthreads.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
[ paulmck: Update documentation, as suggested by Josh Triplett. ]

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index 61ab162..04b8110 100644 (file)
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -3101,7 +3101,11 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
                        test, hence the "fake".
 
        rcutorture.nreaders= [KNL]
-                       Set number of RCU readers.
+                       Set number of RCU readers.  The value -1 selects
+                       N-1, where N is the number of CPUs.  A value
+                       "n" less than -1 selects N-n-2, where N is again
+                       the number of CPUs.  For example, -2 selects N
+                       (the number of CPUs), -3 selects N+1, and so on.
 
        rcutorture.object_debug= [KNL]
                        Enable debug-object double-call_rcu() testing.

diff --git a/kernel/rcu/rcutorture.c b/kernel/rcu/rcutorture.c
index a67ef6f..7294d60 100644 (file)
--- a/kernel/rcu/rcutorture.c
+++ b/kernel/rcu/rcutorture.c
@@ -1701,7 +1701,7 @@ rcu_torture_init(void)
        if (nreaders >= 0) {
                nrealreaders = nreaders;
        } else {
-               nrealreaders = num_online_cpus() - 1;
+               nrealreaders = num_online_cpus() - 2 - nreaders;
                if (nrealreaders <= 0)
                        nrealreaders = 1;
        }