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Message-Id: <1370884611-3861-4-git-send-email-manfred@colorfullife.com>
Date: Mon, 10 Jun 2013 19:16:48 +0200
From: Manfred Spraul <manfred@...orfullife.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: LKML <linux-kernel@...r.kernel.org>,
Rik van Riel <riel@...hat.com>,
Davidlohr Bueso <davidlohr.bueso@...com>, hhuang@...hat.com,
Linus Torvalds <torvalds@...ux-foundation.org>,
Mike Galbraith <efault@....de>,
Manfred Spraul <manfred@...orfullife.com>
Subject: [PATCH 3/6] ipc/sem: seperate wait-for-zero and alter tasks into seperate queues
Introduce seperate queues for operations that do not modify the
semaphore values.
Advantages:
- Simpler logic in check_restart().
- Faster update_queue(): Right now, all wait-for-zero operations
are always tested, even if the semaphore value is not 0.
- wait-for-zero gets again priority, as in linux <=3.0.9
Signed-off-by: Manfred Spraul <manfred@...orfullife.com>
---
include/linux/sem.h | 5 +-
ipc/sem.c | 209 +++++++++++++++++++++++++++++++++++++---------------
2 files changed, 153 insertions(+), 61 deletions(-)
diff --git a/include/linux/sem.h b/include/linux/sem.h
index 53d4265..55e17f6 100644
--- a/include/linux/sem.h
+++ b/include/linux/sem.h
@@ -15,7 +15,10 @@ struct sem_array {
time_t sem_otime; /* last semop time */
time_t sem_ctime; /* last change time */
struct sem *sem_base; /* ptr to first semaphore in array */
- struct list_head sem_pending; /* pending operations to be processed */
+ struct list_head pending_alter; /* pending operations */
+ /* that alter the array */
+ struct list_head pending_const; /* pending complex operations */
+ /* that do not alter semvals */
struct list_head list_id; /* undo requests on this array */
int sem_nsems; /* no. of semaphores in array */
int complex_count; /* pending complex operations */
diff --git a/ipc/sem.c b/ipc/sem.c
index 1afbc57..e7f3d64 100644
--- a/ipc/sem.c
+++ b/ipc/sem.c
@@ -95,7 +95,10 @@ struct sem {
int semval; /* current value */
int sempid; /* pid of last operation */
spinlock_t lock; /* spinlock for fine-grained semtimedop */
- struct list_head sem_pending; /* pending single-sop operations */
+ struct list_head pending_alter; /* pending single-sop operations */
+ /* that alter the semaphore */
+ struct list_head pending_const; /* pending single-sop operations */
+ /* that do not alter the semaphore*/
} ____cacheline_aligned_in_smp;
/* One queue for each sleeping process in the system. */
@@ -152,7 +155,7 @@ static int sysvipc_sem_proc_show(struct seq_file *s, void *it);
/*
* linked list protection:
* sem_undo.id_next,
- * sem_array.sem_pending{,last},
+ * sem_array.pending{_alter,_cont},
* sem_array.sem_undo: sem_lock() for read/write
* sem_undo.proc_next: only "current" is allowed to read/write that field.
*
@@ -337,7 +340,7 @@ static inline void sem_rmid(struct ipc_namespace *ns, struct sem_array *s)
* Without the check/retry algorithm a lockless wakeup is possible:
* - queue.status is initialized to -EINTR before blocking.
* - wakeup is performed by
- * * unlinking the queue entry from sma->sem_pending
+ * * unlinking the queue entry from the pending list
* * setting queue.status to IN_WAKEUP
* This is the notification for the blocked thread that a
* result value is imminent.
@@ -418,12 +421,14 @@ static int newary(struct ipc_namespace *ns, struct ipc_params *params)
sma->sem_base = (struct sem *) &sma[1];
for (i = 0; i < nsems; i++) {
- INIT_LIST_HEAD(&sma->sem_base[i].sem_pending);
+ INIT_LIST_HEAD(&sma->sem_base[i].pending_alter);
+ INIT_LIST_HEAD(&sma->sem_base[i].pending_const);
spin_lock_init(&sma->sem_base[i].lock);
}
sma->complex_count = 0;
- INIT_LIST_HEAD(&sma->sem_pending);
+ INIT_LIST_HEAD(&sma->pending_alter);
+ INIT_LIST_HEAD(&sma->pending_const);
INIT_LIST_HEAD(&sma->list_id);
sma->sem_nsems = nsems;
sma->sem_ctime = get_seconds();
@@ -609,60 +614,130 @@ static void unlink_queue(struct sem_array *sma, struct sem_queue *q)
* update_queue is O(N^2) when it restarts scanning the whole queue of
* waiting operations. Therefore this function checks if the restart is
* really necessary. It is called after a previously waiting operation
- * was completed.
+ * modified the array.
+ * Note that wait-for-zero operations are handled without restart.
*/
static int check_restart(struct sem_array *sma, struct sem_queue *q)
{
- struct sem *curr;
- struct sem_queue *h;
-
- /* if the operation didn't modify the array, then no restart */
- if (q->alter == 0)
- return 0;
-
- /* pending complex operations are too difficult to analyse */
- if (sma->complex_count)
+ /* pending complex alter operations are too difficult to analyse */
+ if (!list_empty(&sma->pending_alter))
return 1;
/* we were a sleeping complex operation. Too difficult */
if (q->nsops > 1)
return 1;
- curr = sma->sem_base + q->sops[0].sem_num;
+ /* It is impossible that someone waits for the new value:
+ * - complex operations always restart.
+ * - wait-for-zero are handled seperately.
+ * - q is a previously sleeping simple operation that
+ * altered the array. It must be a decrement, because
+ * simple increments never sleep.
+ * - If there are older (higher priority) decrements
+ * in the queue, then they have observed the original
+ * semval value and couldn't proceed. The operation
+ * decremented to value - thus they won't proceed either.
+ */
+ return 0;
+}
- /* No-one waits on this queue */
- if (list_empty(&curr->sem_pending))
- return 0;
+/**
+ * wake_const_ops(sma, semnum, pt) - Wake up non-alter tasks
+ * @sma: semaphore array.
+ * @semnum: semaphore that was modified.
+ * @pt: list head for the tasks that must be woken up.
+ *
+ * wake_const_ops must be called after a semaphore in a semaphore array
+ * was set to 0. If complex const operations are pending, wake_const_ops must
+ * be called with semnum = -1, as well as with the number of each modified
+ * semaphore.
+ * The tasks that must be woken up are added to @pt. The return code
+ * is stored in q->pid.
+ * The function returns 1 if at least one operation was completed successfully.
+ */
+static int wake_const_ops(struct sem_array *sma, int semnum,
+ struct list_head *pt)
+{
+ struct sem_queue *q;
+ struct list_head *walk;
+ struct list_head *pending_list;
+ int semop_completed = 0;
+
+ if (semnum == -1)
+ pending_list = &sma->pending_const;
+ else
+ pending_list = &sma->sem_base[semnum].pending_const;
+
+ walk = pending_list->next;
+ while (walk != pending_list) {
+ int error;
+
+ q = container_of(walk, struct sem_queue, list);
+ walk = walk->next;
+
+ error = try_atomic_semop(sma, q->sops, q->nsops,
+ q->undo, q->pid);
+
+ if (error <= 0) {
+ /* operation completed, remove from queue & wakeup */
+
+ unlink_queue(sma, q);
+
+ wake_up_sem_queue_prepare(pt, q, error);
+ if (error == 0)
+ semop_completed = 1;
+ }
+ }
+ return semop_completed;
+}
- /* the new semaphore value */
- if (curr->semval) {
- /* It is impossible that someone waits for the new value:
- * - q is a previously sleeping simple operation that
- * altered the array. It must be a decrement, because
- * simple increments never sleep.
- * - The value is not 0, thus wait-for-zero won't proceed.
- * - If there are older (higher priority) decrements
- * in the queue, then they have observed the original
- * semval value and couldn't proceed. The operation
- * decremented to value - thus they won't proceed either.
+/**
+ * do_smart_wakeup_zero(sma, sops, nsops, pt) - wakeup all wait for zero tasks
+ * @sma: semaphore array
+ * @sops: operations that were performed
+ * @nsops: number of operations
+ * @pt: list head of the tasks that must be woken up.
+ *
+ * do_smart_wakeup_zero() checks all required queue for wait-for-zero
+ * operations, based on the actual changes that were performed on the
+ * semaphore array.
+ * The function returns 1 if at least one operation was completed successfully.
+ */
+static int do_smart_wakeup_zero(struct sem_array *sma, struct sembuf *sops,
+ int nsops, struct list_head *pt)
+{
+ int i;
+ int semop_completed = 0;
+ int got_zero = 0;
+
+ /* first: the per-semaphore queues, if known */
+ if (sops) {
+ for (i = 0; i < nsops; i++) {
+ int num = sops[i].sem_num;
+
+ if (sma->sem_base[num].semval == 0) {
+ got_zero = 1;
+ semop_completed |= wake_const_ops(sma, num, pt);
+ }
+ }
+ } else {
+ /*
+ * No sops means modified semaphores not known.
+ * Assume all were changed.
*/
- BUG_ON(q->sops[0].sem_op >= 0);
- return 0;
+ for (i = 0; i < sma->sem_nsems; i++) {
+ if (sma->sem_base[i].semval == 0)
+ semop_completed |= wake_const_ops(sma, i, pt);
+ }
}
/*
- * semval is 0. Check if there are wait-for-zero semops.
- * They must be the first entries in the per-semaphore queue
+ * If one of the modified semaphores got 0,
+ * then check the global queue, too.
*/
- h = list_first_entry(&curr->sem_pending, struct sem_queue, list);
- BUG_ON(h->nsops != 1);
- BUG_ON(h->sops[0].sem_num != q->sops[0].sem_num);
+ if (got_zero)
+ semop_completed |= wake_const_ops(sma, -1, pt);
- /* Yes, there is a wait-for-zero semop. Restart */
- if (h->sops[0].sem_op == 0)
- return 1;
-
- /* Again - no-one is waiting for the new value. */
- return 0;
+ return semop_completed;
}
@@ -678,6 +753,8 @@ static int check_restart(struct sem_array *sma, struct sem_queue *q)
* semaphore.
* The tasks that must be woken up are added to @pt. The return code
* is stored in q->pid.
+ * The function internally checks if const operations can now succeed.
+ *
* The function return 1 if at least one semop was completed successfully.
*/
static int update_queue(struct sem_array *sma, int semnum, struct list_head *pt)
@@ -688,9 +765,9 @@ static int update_queue(struct sem_array *sma, int semnum, struct list_head *pt)
int semop_completed = 0;
if (semnum == -1)
- pending_list = &sma->sem_pending;
+ pending_list = &sma->pending_alter;
else
- pending_list = &sma->sem_base[semnum].sem_pending;
+ pending_list = &sma->sem_base[semnum].pending_alter;
again:
walk = pending_list->next;
@@ -702,13 +779,12 @@ again:
/* If we are scanning the single sop, per-semaphore list of
* one semaphore and that semaphore is 0, then it is not
- * necessary to scan the "alter" entries: simple increments
+ * necessary to scan further: simple increments
* that affect only one entry succeed immediately and cannot
* be in the per semaphore pending queue, and decrements
* cannot be successful if the value is already 0.
*/
- if (semnum != -1 && sma->sem_base[semnum].semval == 0 &&
- q->alter)
+ if (semnum != -1 && sma->sem_base[semnum].semval == 0)
break;
error = try_atomic_semop(sma, q->sops, q->nsops,
@@ -724,6 +800,7 @@ again:
restart = 0;
} else {
semop_completed = 1;
+ do_smart_wakeup_zero(sma, q->sops, q->nsops, pt);
restart = check_restart(sma, q);
}
@@ -742,8 +819,8 @@ again:
* @otime: force setting otime
* @pt: list head of the tasks that must be woken up.
*
- * do_smart_update() does the required called to update_queue, based on the
- * actual changes that were performed on the semaphore array.
+ * do_smart_update() does the required calls to update_queue and wakeup_zero,
+ * based on the actual changes that were performed on the semaphore array.
* Note that the function does not do the actual wake-up: the caller is
* responsible for calling wake_up_sem_queue_do(@pt).
* It is safe to perform this call after dropping all locks.
@@ -754,6 +831,8 @@ static void do_smart_update(struct sem_array *sma, struct sembuf *sops, int nsop
int i;
int progress;
+ otime |= do_smart_wakeup_zero(sma, sops, nsops, pt);
+
progress = 1;
retry_global:
if (sma->complex_count) {
@@ -813,14 +892,14 @@ static int count_semncnt (struct sem_array * sma, ushort semnum)
struct sem_queue * q;
semncnt = 0;
- list_for_each_entry(q, &sma->sem_base[semnum].sem_pending, list) {
+ list_for_each_entry(q, &sma->sem_base[semnum].pending_alter, list) {
struct sembuf * sops = q->sops;
BUG_ON(sops->sem_num != semnum);
if ((sops->sem_op < 0) && !(sops->sem_flg & IPC_NOWAIT))
semncnt++;
}
- list_for_each_entry(q, &sma->sem_pending, list) {
+ list_for_each_entry(q, &sma->pending_alter, list) {
struct sembuf * sops = q->sops;
int nsops = q->nsops;
int i;
@@ -839,14 +918,14 @@ static int count_semzcnt (struct sem_array * sma, ushort semnum)
struct sem_queue * q;
semzcnt = 0;
- list_for_each_entry(q, &sma->sem_base[semnum].sem_pending, list) {
+ list_for_each_entry(q, &sma->sem_base[semnum].pending_const, list) {
struct sembuf * sops = q->sops;
BUG_ON(sops->sem_num != semnum);
if ((sops->sem_op == 0) && !(sops->sem_flg & IPC_NOWAIT))
semzcnt++;
}
- list_for_each_entry(q, &sma->sem_pending, list) {
+ list_for_each_entry(q, &sma->pending_const, list) {
struct sembuf * sops = q->sops;
int nsops = q->nsops;
int i;
@@ -884,13 +963,22 @@ static void freeary(struct ipc_namespace *ns, struct kern_ipc_perm *ipcp)
/* Wake up all pending processes and let them fail with EIDRM. */
INIT_LIST_HEAD(&tasks);
- list_for_each_entry_safe(q, tq, &sma->sem_pending, list) {
+ list_for_each_entry_safe(q, tq, &sma->pending_const, list) {
+ unlink_queue(sma, q);
+ wake_up_sem_queue_prepare(&tasks, q, -EIDRM);
+ }
+
+ list_for_each_entry_safe(q, tq, &sma->pending_alter, list) {
unlink_queue(sma, q);
wake_up_sem_queue_prepare(&tasks, q, -EIDRM);
}
for (i = 0; i < sma->sem_nsems; i++) {
struct sem *sem = sma->sem_base + i;
- list_for_each_entry_safe(q, tq, &sem->sem_pending, list) {
+ list_for_each_entry_safe(q, tq, &sem->pending_const, list) {
+ unlink_queue(sma, q);
+ wake_up_sem_queue_prepare(&tasks, q, -EIDRM);
+ }
+ list_for_each_entry_safe(q, tq, &sem->pending_alter, list) {
unlink_queue(sma, q);
wake_up_sem_queue_prepare(&tasks, q, -EIDRM);
}
@@ -1654,14 +1742,15 @@ SYSCALL_DEFINE4(semtimedop, int, semid, struct sembuf __user *, tsops,
curr = &sma->sem_base[sops->sem_num];
if (alter)
- list_add_tail(&queue.list, &curr->sem_pending);
+ list_add_tail(&queue.list, &curr->pending_alter);
else
- list_add(&queue.list, &curr->sem_pending);
+ list_add_tail(&queue.list, &curr->pending_const);
} else {
if (alter)
- list_add_tail(&queue.list, &sma->sem_pending);
+ list_add_tail(&queue.list, &sma->pending_alter);
else
- list_add(&queue.list, &sma->sem_pending);
+ list_add_tail(&queue.list, &sma->pending_const);
+
sma->complex_count++;
}
--
1.8.1.4
--
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