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authorTim Chen <tim.c.chen@linux.intel.com>2017-08-25 09:13:54 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2017-09-14 09:56:17 -0700
commit2554db916586b228ce93e6f74a12fd7fe430a004 (patch)
treecebbd0d46f172da6a690ecc573069d3f21fdac29
parent46c1e79fee417f151547aa46fae04ab06cb666f4 (diff)
sched/wait: Break up long wake list walk
We encountered workloads that have very long wake up list on large systems. A waker takes a long time to traverse the entire wake list and execute all the wake functions. We saw page wait list that are up to 3700+ entries long in tests of large 4 and 8 socket systems. It took 0.8 sec to traverse such list during wake up. Any other CPU that contends for the list spin lock will spin for a long time. It is a result of the numa balancing migration of hot pages that are shared by many threads. Multiple CPUs waking are queued up behind the lock, and the last one queued has to wait until all CPUs did all the wakeups. The page wait list is traversed with interrupt disabled, which caused various problems. This was the original cause that triggered the NMI watch dog timer in: https://patchwork.kernel.org/patch/9800303/ . Only extending the NMI watch dog timer there helped. This patch bookmarks the waker's scan position in wake list and break the wake up walk, to allow access to the list before the waker resume its walk down the rest of the wait list. It lowers the interrupt and rescheduling latency. This patch also provides a performance boost when combined with the next patch to break up page wakeup list walk. We saw 22% improvement in the will-it-scale file pread2 test on a Xeon Phi system running 256 threads. [ v2: Merged in Linus' changes to remove the bookmark_wake_function, and simply access to flags. ] Reported-by: Kan Liang <kan.liang@intel.com> Tested-by: Kan Liang <kan.liang@intel.com> Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
-rw-r--r--include/linux/wait.h1
-rw-r--r--kernel/sched/wait.c78
2 files changed, 64 insertions, 15 deletions
diff --git a/include/linux/wait.h b/include/linux/wait.h
index dc19880c02f5..78401ef02d29 100644
--- a/include/linux/wait.h
+++ b/include/linux/wait.h
@@ -18,6 +18,7 @@ int default_wake_function(struct wait_queue_entry *wq_entry, unsigned mode, int
/* wait_queue_entry::flags */
#define WQ_FLAG_EXCLUSIVE 0x01
#define WQ_FLAG_WOKEN 0x02
+#define WQ_FLAG_BOOKMARK 0x04
/*
* A single wait-queue entry structure:
diff --git a/kernel/sched/wait.c b/kernel/sched/wait.c
index d6afed6d0752..70701ef50465 100644
--- a/kernel/sched/wait.c
+++ b/kernel/sched/wait.c
@@ -53,6 +53,12 @@ void remove_wait_queue(struct wait_queue_head *wq_head, struct wait_queue_entry
}
EXPORT_SYMBOL(remove_wait_queue);
+/*
+ * Scan threshold to break wait queue walk.
+ * This allows a waker to take a break from holding the
+ * wait queue lock during the wait queue walk.
+ */
+#define WAITQUEUE_WALK_BREAK_CNT 64
/*
* The core wakeup function. Non-exclusive wakeups (nr_exclusive == 0) just
@@ -63,18 +69,67 @@ EXPORT_SYMBOL(remove_wait_queue);
* started to run but is not in state TASK_RUNNING. try_to_wake_up() returns
* zero in this (rare) case, and we handle it by continuing to scan the queue.
*/
-static void __wake_up_common(struct wait_queue_head *wq_head, unsigned int mode,
- int nr_exclusive, int wake_flags, void *key)
+static int __wake_up_common(struct wait_queue_head *wq_head, unsigned int mode,
+ int nr_exclusive, int wake_flags, void *key,
+ wait_queue_entry_t *bookmark)
{
wait_queue_entry_t *curr, *next;
+ int cnt = 0;
+
+ if (bookmark && (bookmark->flags & WQ_FLAG_BOOKMARK)) {
+ curr = list_next_entry(bookmark, entry);
- list_for_each_entry_safe(curr, next, &wq_head->head, entry) {
+ list_del(&bookmark->entry);
+ bookmark->flags = 0;
+ } else
+ curr = list_first_entry(&wq_head->head, wait_queue_entry_t, entry);
+
+ if (&curr->entry == &wq_head->head)
+ return nr_exclusive;
+
+ list_for_each_entry_safe_from(curr, next, &wq_head->head, entry) {
unsigned flags = curr->flags;
- int ret = curr->func(curr, mode, wake_flags, key);
+ int ret;
+
+ if (flags & WQ_FLAG_BOOKMARK)
+ continue;
+
+ ret = curr->func(curr, mode, wake_flags, key);
if (ret < 0)
break;
if (ret && (flags & WQ_FLAG_EXCLUSIVE) && !--nr_exclusive)
break;
+
+ if (bookmark && (++cnt > WAITQUEUE_WALK_BREAK_CNT) &&
+ (&next->entry != &wq_head->head)) {
+ bookmark->flags = WQ_FLAG_BOOKMARK;
+ list_add_tail(&bookmark->entry, &next->entry);
+ break;
+ }
+ }
+ return nr_exclusive;
+}
+
+static void __wake_up_common_lock(struct wait_queue_head *wq_head, unsigned int mode,
+ int nr_exclusive, int wake_flags, void *key)
+{
+ unsigned long flags;
+ wait_queue_entry_t bookmark;
+
+ bookmark.flags = 0;
+ bookmark.private = NULL;
+ bookmark.func = NULL;
+ INIT_LIST_HEAD(&bookmark.entry);
+
+ spin_lock_irqsave(&wq_head->lock, flags);
+ nr_exclusive = __wake_up_common(wq_head, mode, nr_exclusive, wake_flags, key, &bookmark);
+ spin_unlock_irqrestore(&wq_head->lock, flags);
+
+ while (bookmark.flags & WQ_FLAG_BOOKMARK) {
+ spin_lock_irqsave(&wq_head->lock, flags);
+ nr_exclusive = __wake_up_common(wq_head, mode, nr_exclusive,
+ wake_flags, key, &bookmark);
+ spin_unlock_irqrestore(&wq_head->lock, flags);
}
}
@@ -91,11 +146,7 @@ static void __wake_up_common(struct wait_queue_head *wq_head, unsigned int mode,
void __wake_up(struct wait_queue_head *wq_head, unsigned int mode,
int nr_exclusive, void *key)
{
- unsigned long flags;
-
- spin_lock_irqsave(&wq_head->lock, flags);
- __wake_up_common(wq_head, mode, nr_exclusive, 0, key);
- spin_unlock_irqrestore(&wq_head->lock, flags);
+ __wake_up_common_lock(wq_head, mode, nr_exclusive, 0, key);
}
EXPORT_SYMBOL(__wake_up);
@@ -104,13 +155,13 @@ EXPORT_SYMBOL(__wake_up);
*/
void __wake_up_locked(struct wait_queue_head *wq_head, unsigned int mode, int nr)
{
- __wake_up_common(wq_head, mode, nr, 0, NULL);
+ __wake_up_common(wq_head, mode, nr, 0, NULL, NULL);
}
EXPORT_SYMBOL_GPL(__wake_up_locked);
void __wake_up_locked_key(struct wait_queue_head *wq_head, unsigned int mode, void *key)
{
- __wake_up_common(wq_head, mode, 1, 0, key);
+ __wake_up_common(wq_head, mode, 1, 0, key, NULL);
}
EXPORT_SYMBOL_GPL(__wake_up_locked_key);
@@ -134,7 +185,6 @@ EXPORT_SYMBOL_GPL(__wake_up_locked_key);
void __wake_up_sync_key(struct wait_queue_head *wq_head, unsigned int mode,
int nr_exclusive, void *key)
{
- unsigned long flags;
int wake_flags = 1; /* XXX WF_SYNC */
if (unlikely(!wq_head))
@@ -143,9 +193,7 @@ void __wake_up_sync_key(struct wait_queue_head *wq_head, unsigned int mode,
if (unlikely(nr_exclusive != 1))
wake_flags = 0;
- spin_lock_irqsave(&wq_head->lock, flags);
- __wake_up_common(wq_head, mode, nr_exclusive, wake_flags, key);
- spin_unlock_irqrestore(&wq_head->lock, flags);
+ __wake_up_common_lock(wq_head, mode, nr_exclusive, wake_flags, key);
}
EXPORT_SYMBOL_GPL(__wake_up_sync_key);