[PATCH] Use round_jiffies() for slow work thread pool's cull and OOM timers

[David Howells <dhowells@redhat.com>]

From: Chris Peterson <cpeterso@cpeterso.com>

Round the slow work queue's cull and OOM timeouts to whole second boundary with
round_jiffies().  The slow work queue uses a pair of timers to cull idle
threads and, after OOM, to delay new thread creation.

This patch also extracts the mod_timer() logic for the cull timer into a
separate helper function.

By rounding non-time-critical timers such as these to whole seconds, they will
be batched up to fire at the same time rather than being spread out.  This
allows the CPU wake up less, which saves power.

Signed-off-by: Chris Peterson <cpeterso@cpeterso.com>
Signed-off-by: David Howells <dhowells@redhat.com>
---

 kernel/slow-work.c |   23 ++++++++++++++---------
 1 files changed, 14 insertions(+), 9 deletions(-)


diff --git a/kernel/slow-work.c b/kernel/slow-work.c
index 96e418d..a3ce6cf 100644
--- a/kernel/slow-work.c
+++ b/kernel/slow-work.c
@@ -319,6 +319,15 @@ cant_get_ref:
 EXPORT_SYMBOL(slow_work_enqueue);
 
 /*
+ * Schedule a cull of the thread pool at some time in the near future
+ */
+static void slow_work_schedule_cull(void)
+{
+	mod_timer(&slow_work_cull_timer,
+		  round_jiffies(jiffies + SLOW_WORK_CULL_TIMEOUT));
+}
+
+/*
  * Worker thread culling algorithm
  */
 static bool slow_work_cull_thread(void)
@@ -335,8 +344,7 @@ static bool slow_work_cull_thread(void)
 		    list_empty(&vslow_work_queue) &&
 		    atomic_read(&slow_work_thread_count) >
 		    slow_work_min_threads) {
-			mod_timer(&slow_work_cull_timer,
-				  jiffies + SLOW_WORK_CULL_TIMEOUT);
+			slow_work_schedule_cull();
 			do_cull = true;
 		}
 	}
@@ -394,8 +402,7 @@ static int slow_work_thread(void *_data)
 			    list_empty(&vslow_work_queue) &&
 			    atomic_read(&slow_work_thread_count) >
 			    slow_work_min_threads)
-				mod_timer(&slow_work_cull_timer,
-					  jiffies + SLOW_WORK_CULL_TIMEOUT);
+				slow_work_schedule_cull();
 			continue;
 		}
 
@@ -460,7 +467,7 @@ static void slow_work_new_thread_execute(struct slow_work *work)
 		if (atomic_dec_and_test(&slow_work_thread_count))
 			BUG(); /* we're running on a slow work thread... */
 		mod_timer(&slow_work_oom_timer,
-			  jiffies + SLOW_WORK_OOM_TIMEOUT);
+			  round_jiffies(jiffies + SLOW_WORK_OOM_TIMEOUT));
 	} else {
 		/* ratelimit the starting of new threads */
 		mod_timer(&slow_work_oom_timer, jiffies + 1);
@@ -504,8 +511,7 @@ static int slow_work_min_threads_sysctl(struct ctl_table *table, int write,
 			if (n < 0 && !slow_work_may_not_start_new_thread)
 				slow_work_enqueue(&slow_work_new_thread);
 			else if (n > 0)
-				mod_timer(&slow_work_cull_timer,
-					  jiffies + SLOW_WORK_CULL_TIMEOUT);
+				slow_work_schedule_cull();
 		}
 		mutex_unlock(&slow_work_user_lock);
 	}
@@ -531,8 +537,7 @@ static int slow_work_max_threads_sysctl(struct ctl_table *table, int write,
 				atomic_read(&slow_work_thread_count);
 
 			if (n < 0)
-				mod_timer(&slow_work_cull_timer,
-					  jiffies + SLOW_WORK_CULL_TIMEOUT);
+				slow_work_schedule_cull();
 		}
 		mutex_unlock(&slow_work_user_lock);
 	}