rcuscale: Permit blocking delays between writers

Some workloads do isolated RCU work, and this can affect operation
latencies.  This commit therefore adds a writer_holdoff_jiffies module
parameter that causes writers to block for the specified number of
jiffies between each pair of consecutive write-side operations.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index a145799..5ba231b 100644 (file)
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -4983,6 +4983,11 @@
                        in microseconds.  The default of zero says
                        no holdoff.
 
+       rcuscale.writer_holdoff_jiffies= [KNL]
+                       Additional write-side holdoff between grace
+                       periods, but in jiffies.  The default of zero
+                       says no holdoff.
+
        rcutorture.fqs_duration= [KNL]
                        Set duration of force_quiescent_state bursts
                        in microseconds.

diff --git a/kernel/rcu/rcuscale.c b/kernel/rcu/rcuscale.c
index d122173..4277037 100644 (file)
--- a/kernel/rcu/rcuscale.c
+++ b/kernel/rcu/rcuscale.c
@@ -93,6 +93,7 @@ torture_param(bool, shutdown, RCUSCALE_SHUTDOWN,
              "Shutdown at end of scalability tests.");
 torture_param(int, verbose, 1, "Enable verbose debugging printk()s");
 torture_param(int, writer_holdoff, 0, "Holdoff (us) between GPs, zero to disable");
+torture_param(int, writer_holdoff_jiffies, 0, "Holdoff (jiffies) between GPs, zero to disable");
 torture_param(int, kfree_rcu_test, 0, "Do we run a kfree_rcu() scale test?");
 torture_param(int, kfree_mult, 1, "Multiple of kfree_obj size to allocate.");
 torture_param(int, kfree_by_call_rcu, 0, "Use call_rcu() to emulate kfree_rcu()?");
@@ -414,6 +415,7 @@ rcu_scale_writer(void *arg)
        struct rcu_head *rhp = NULL;
        bool started = false, done = false, alldone = false;
        u64 t;
+       DEFINE_TORTURE_RANDOM(tr);
        u64 *wdp;
        u64 *wdpp = writer_durations[me];
 
@@ -448,6 +450,8 @@ rcu_scale_writer(void *arg)
        do {
                if (writer_holdoff)
                        udelay(writer_holdoff);
+               if (writer_holdoff_jiffies)
+                       schedule_timeout_idle(torture_random(&tr) % writer_holdoff_jiffies + 1);
                wdp = &wdpp[i];
                *wdp = ktime_get_mono_fast_ns();
                if (gp_async) {