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SubjectRe: [PATCH] io stalls (was: -rc7 Re: Linux 2.4.21-rc6)
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Ok here's an updated patch, it changes the barriers around, updates
comments, and gets rid of the waited check in __get_request_wait. It is
still a combined patch with fix_pausing, queue_full and latency stats,
mostly because I want to make really sure any testers are using all
three.

So, if someone who saw io stalls in 2.4.21-rc could give this a try, I'd
be grateful. If you still see stalls with this applied, run elvtune
/dev/xxx and send along the resulting console output.

-chris

--- 1.9/drivers/block/blkpg.c Sat Mar 30 06:58:05 2002
+++ edited/drivers/block/blkpg.c Tue Jun 10 14:49:27 2003
@@ -261,6 +261,7 @@
return blkpg_ioctl(dev, (struct blkpg_ioctl_arg *) arg);

case BLKELVGET:
+ blk_print_stats(dev);
return blkelvget_ioctl(&blk_get_queue(dev)->elevator,
(blkelv_ioctl_arg_t *) arg);
case BLKELVSET:
--- 1.45/drivers/block/ll_rw_blk.c Wed May 28 03:50:02 2003
+++ edited/drivers/block/ll_rw_blk.c Wed Jun 11 13:36:10 2003
@@ -429,6 +429,8 @@
q->rq[READ].count = 0;
q->rq[WRITE].count = 0;
q->nr_requests = 0;
+ q->read_full = 0;
+ q->write_full = 0;

si_meminfo(&si);
megs = si.totalram >> (20 - PAGE_SHIFT);
@@ -442,6 +444,56 @@
spin_lock_init(&q->queue_lock);
}

+void blk_print_stats(kdev_t dev)
+{
+ request_queue_t *q;
+ unsigned long avg_wait;
+ unsigned long min_wait;
+ unsigned long high_wait;
+ unsigned long *d;
+
+ q = blk_get_queue(dev);
+ if (!q)
+ return;
+
+ min_wait = q->min_wait;
+ if (min_wait == ~0UL)
+ min_wait = 0;
+ if (q->num_wait)
+ avg_wait = q->total_wait / q->num_wait;
+ else
+ avg_wait = 0;
+ printk("device %s: num_req %lu, total jiffies waited %lu\n",
+ kdevname(dev), q->num_req, q->total_wait);
+ printk("\t%lu forced to wait\n", q->num_wait);
+ printk("\t%lu min wait, %lu max wait\n", min_wait, q->max_wait);
+ printk("\t%lu average wait\n", avg_wait);
+ d = q->deviation;
+ printk("\t%lu < 100, %lu < 200, %lu < 300, %lu < 400, %lu < 500\n",
+ d[0], d[1], d[2], d[3], d[4]);
+ high_wait = d[0] + d[1] + d[2] + d[3] + d[4];
+ high_wait = q->num_wait - high_wait;
+ printk("\t%lu waits longer than 500 jiffies\n", high_wait);
+}
+
+static void reset_stats(request_queue_t *q)
+{
+ q->max_wait = 0;
+ q->min_wait = ~0UL;
+ q->total_wait = 0;
+ q->num_req = 0;
+ q->num_wait = 0;
+ memset(q->deviation, 0, sizeof(q->deviation));
+}
+void blk_reset_stats(kdev_t dev)
+{
+ request_queue_t *q;
+ q = blk_get_queue(dev);
+ if (!q)
+ return;
+ printk("reset latency stats on device %s\n", kdevname(dev));
+ reset_stats(q);
+}
static int __make_request(request_queue_t * q, int rw, struct buffer_head * bh);

/**
@@ -491,6 +543,9 @@
q->plug_tq.routine = &generic_unplug_device;
q->plug_tq.data = q;
q->plugged = 0;
+
+ reset_stats(q);
+
/*
* These booleans describe the queue properties. We set the
* default (and most common) values here. Other drivers can
@@ -508,7 +563,7 @@
* Get a free request. io_request_lock must be held and interrupts
* disabled on the way in. Returns NULL if there are no free requests.
*/
-static struct request *get_request(request_queue_t *q, int rw)
+static struct request *__get_request(request_queue_t *q, int rw)
{
struct request *rq = NULL;
struct request_list *rl = q->rq + rw;
@@ -521,35 +576,48 @@
rq->cmd = rw;
rq->special = NULL;
rq->q = q;
- }
+ } else
+ set_queue_full(q, rw);

return rq;
}

/*
- * Here's the request allocation design:
+ * get a free request, honoring the queue_full condition
+ */
+static inline struct request *get_request(request_queue_t *q, int rw)
+{
+ if (queue_full(q, rw))
+ return NULL;
+ return __get_request(q, rw);
+}
+
+/*
+ * helper func to do memory barriers and wakeups when we finally decide
+ * to clear the queue full condition
+ */
+static inline void clear_full_and_wake(request_queue_t *q, int rw)
+{
+ clear_queue_full(q, rw);
+ mb();
+ if (unlikely(waitqueue_active(&q->wait_for_requests[rw])))
+ wake_up(&q->wait_for_requests[rw]);
+}
+
+/*
+ * Here's the request allocation design, low latency version:
*
* 1: Blocking on request exhaustion is a key part of I/O throttling.
*
* 2: We want to be `fair' to all requesters. We must avoid starvation, and
* attempt to ensure that all requesters sleep for a similar duration. Hence
* no stealing requests when there are other processes waiting.
- *
- * 3: We also wish to support `batching' of requests. So when a process is
- * woken, we want to allow it to allocate a decent number of requests
- * before it blocks again, so they can be nicely merged (this only really
- * matters if the process happens to be adding requests near the head of
- * the queue).
- *
- * 4: We want to avoid scheduling storms. This isn't really important, because
- * the system will be I/O bound anyway. But it's easy.
- *
- * There is tension between requirements 2 and 3. Once a task has woken,
- * we don't want to allow it to sleep as soon as it takes its second request.
- * But we don't want currently-running tasks to steal all the requests
- * from the sleepers. We handle this with wakeup hysteresis around
- * 0 .. batch_requests and with the assumption that request taking is much,
- * much faster than request freeing.
+ *
+ * There used to be more here, attempting to allow a process to send in a
+ * number of requests once it has woken up. But, there's no way to
+ * tell if a process has just been woken up, or if it is a new process
+ * coming in to steal requests from the waiters. So, we give up and force
+ * everyone to wait fairly.
*
* So here's what we do:
*
@@ -561,50 +629,78 @@
*
* When a process wants a new request:
*
- * b) If free_requests == 0, the requester sleeps in FIFO manner.
- *
- * b) If 0 < free_requests < batch_requests and there are waiters,
- * we still take a request non-blockingly. This provides batching.
- *
- * c) If free_requests >= batch_requests, the caller is immediately
- * granted a new request.
+ * b) If free_requests == 0, the requester sleeps in FIFO manner, and
+ * the queue full condition is set. The full condition is not
+ * cleared until there are no longer any waiters. Once the full
+ * condition is set, all new io must wait, hopefully for a very
+ * short period of time.
*
* When a request is released:
*
- * d) If free_requests < batch_requests, do nothing.
- *
- * f) If free_requests >= batch_requests, wake up a single waiter.
- *
- * The net effect is that when a process is woken at the batch_requests level,
- * it will be able to take approximately (batch_requests) requests before
- * blocking again (at the tail of the queue).
+ * c) If free_requests < batch_requests, do nothing.
*
- * This all assumes that the rate of taking requests is much, much higher
- * than the rate of releasing them. Which is very true.
+ * d) If free_requests >= batch_requests, wake up a single waiter.
*
- * -akpm, Feb 2002.
+ * As each waiter gets a request, he wakes another waiter. We do this
+ * to prevent a race where an unplug might get run before a request makes
+ * it's way onto the queue. The result is a cascade of wakeups, so delaying
+ * the initial wakeup until we've got batch_requests available helps avoid
+ * wakeups where there aren't any requests available yet.
*/

static struct request *__get_request_wait(request_queue_t *q, int rw)
{
register struct request *rq;
+ unsigned long wait_start = jiffies;
+ unsigned long time_waited;
DECLARE_WAITQUEUE(wait, current);

- add_wait_queue(&q->wait_for_requests[rw], &wait);
+ add_wait_queue_exclusive(&q->wait_for_requests[rw], &wait);
+
do {
set_current_state(TASK_UNINTERRUPTIBLE);
- generic_unplug_device(q);
- if (q->rq[rw].count == 0)
- schedule();
spin_lock_irq(&io_request_lock);
- rq = get_request(q, rw);
+ if (queue_full(q, rw) || q->rq[rw].count == 0) {
+ if (q->rq[rw].count == 0)
+ __generic_unplug_device(q);
+ spin_unlock_irq(&io_request_lock);
+ schedule();
+ spin_lock_irq(&io_request_lock);
+ }
+ rq = __get_request(q, rw);
spin_unlock_irq(&io_request_lock);
} while (rq == NULL);
remove_wait_queue(&q->wait_for_requests[rw], &wait);
current->state = TASK_RUNNING;
+
+ if (!waitqueue_active(&q->wait_for_requests[rw]))
+ clear_full_and_wake(q, rw);
+
+ time_waited = jiffies - wait_start;
+ if (time_waited > q->max_wait)
+ q->max_wait = time_waited;
+ if (time_waited && time_waited < q->min_wait)
+ q->min_wait = time_waited;
+ q->total_wait += time_waited;
+ q->num_wait++;
+ if (time_waited < 500) {
+ q->deviation[time_waited/100]++;
+ }
+
return rq;
}

+static void get_request_wait_wakeup(request_queue_t *q, int rw)
+{
+ /*
+ * avoid losing an unplug if a second __get_request_wait did the
+ * generic_unplug_device while our __get_request_wait was running
+ * w/o the queue_lock held and w/ our request out of the queue.
+ */
+ if (waitqueue_active(&q->wait_for_requests[rw]))
+ wake_up(&q->wait_for_requests[rw]);
+}
+
/* RO fail safe mechanism */

static long ro_bits[MAX_BLKDEV][8];
@@ -829,8 +925,14 @@
*/
if (q) {
list_add(&req->queue, &q->rq[rw].free);
- if (++q->rq[rw].count >= q->batch_requests)
- wake_up(&q->wait_for_requests[rw]);
+ q->rq[rw].count++;
+ if (q->rq[rw].count >= q->batch_requests) {
+ smp_mb();
+ if (waitqueue_active(&q->wait_for_requests[rw]))
+ wake_up(&q->wait_for_requests[rw]);
+ else
+ clear_full_and_wake(q, rw);
+ }
}
}

@@ -948,7 +1050,6 @@
*/
max_sectors = get_max_sectors(bh->b_rdev);

-again:
req = NULL;
head = &q->queue_head;
/*
@@ -957,6 +1058,7 @@
*/
spin_lock_irq(&io_request_lock);

+again:
insert_here = head->prev;
if (list_empty(head)) {
q->plug_device_fn(q, bh->b_rdev); /* is atomic */
@@ -1042,6 +1144,9 @@
if (req == NULL) {
spin_unlock_irq(&io_request_lock);
freereq = __get_request_wait(q, rw);
+ head = &q->queue_head;
+ spin_lock_irq(&io_request_lock);
+ get_request_wait_wakeup(q, rw);
goto again;
}
}
@@ -1063,6 +1168,7 @@
req->rq_dev = bh->b_rdev;
req->start_time = jiffies;
req_new_io(req, 0, count);
+ q->num_req++;
blk_started_io(count);
add_request(q, req, insert_here);
out:
@@ -1196,8 +1302,15 @@
bh->b_rdev = bh->b_dev;
bh->b_rsector = bh->b_blocknr * count;

+ get_bh(bh);
generic_make_request(rw, bh);

+ /* fix race condition with wait_on_buffer() */
+ smp_mb(); /* spin_unlock may have inclusive semantics */
+ if (waitqueue_active(&bh->b_wait))
+ wake_up(&bh->b_wait);
+
+ put_bh(bh);
switch (rw) {
case WRITE:
kstat.pgpgout += count;
--- 1.83/fs/buffer.c Wed May 14 12:51:00 2003
+++ edited/fs/buffer.c Wed Jun 11 09:56:27 2003
@@ -153,10 +153,23 @@
get_bh(bh);
add_wait_queue(&bh->b_wait, &wait);
do {
- run_task_queue(&tq_disk);
set_task_state(tsk, TASK_UNINTERRUPTIBLE);
if (!buffer_locked(bh))
break;
+ /*
+ * We must read tq_disk in TQ_ACTIVE after the
+ * add_wait_queue effect is visible to other cpus.
+ * We could unplug some line above it wouldn't matter
+ * but we can't do that right after add_wait_queue
+ * without an smp_mb() in between because spin_unlock
+ * has inclusive semantics.
+ * Doing it here is the most efficient place so we
+ * don't do a suprious unplug if we get a racy
+ * wakeup that make buffer_locked to return 0, and
+ * doing it here avoids an explicit smp_mb() we
+ * rely on the implicit one in set_task_state.
+ */
+ run_task_queue(&tq_disk);
schedule();
} while (buffer_locked(bh));
tsk->state = TASK_RUNNING;
@@ -1507,6 +1520,9 @@

/* Done - end_buffer_io_async will unlock */
SetPageUptodate(page);
+
+ wakeup_page_waiters(page);
+
return 0;

out:
@@ -1538,6 +1554,7 @@
} while (bh != head);
if (need_unlock)
UnlockPage(page);
+ wakeup_page_waiters(page);
return err;
}

@@ -1765,6 +1782,8 @@
else
submit_bh(READ, bh);
}
+
+ wakeup_page_waiters(page);

return 0;
}
@@ -2378,6 +2397,7 @@
submit_bh(rw, bh);
bh = next;
} while (bh != head);
+ wakeup_page_waiters(page);
return 0;
}

--- 1.49/fs/super.c Wed Dec 18 21:34:24 2002
+++ edited/fs/super.c Tue Jun 10 14:49:27 2003
@@ -726,6 +726,7 @@
if (!fs_type->read_super(s, data, flags & MS_VERBOSE ? 1 : 0))
goto Einval;
s->s_flags |= MS_ACTIVE;
+ blk_reset_stats(dev);
path_release(&nd);
return s;

--- 1.45/fs/reiserfs/inode.c Thu May 22 16:35:02 2003
+++ edited/fs/reiserfs/inode.c Tue Jun 10 14:49:27 2003
@@ -2048,6 +2048,7 @@
*/
if (nr) {
submit_bh_for_writepage(arr, nr) ;
+ wakeup_page_waiters(page);
} else {
UnlockPage(page) ;
}
--- 1.23/include/linux/blkdev.h Fri Nov 29 17:03:01 2002
+++ edited/include/linux/blkdev.h Wed Jun 11 09:56:55 2003
@@ -126,6 +126,14 @@
*/
char head_active;

+ /*
+ * Booleans that indicate whether the queue's free requests have
+ * been exhausted and is waiting to drop below the batch_requests
+ * threshold
+ */
+ char read_full;
+ char write_full;
+
unsigned long bounce_pfn;

/*
@@ -138,8 +146,17 @@
* Tasks wait here for free read and write requests
*/
wait_queue_head_t wait_for_requests[2];
+ unsigned long max_wait;
+ unsigned long min_wait;
+ unsigned long total_wait;
+ unsigned long num_req;
+ unsigned long num_wait;
+ unsigned long deviation[5];
};

+void blk_reset_stats(kdev_t dev);
+void blk_print_stats(kdev_t dev);
+
#define blk_queue_plugged(q) (q)->plugged
#define blk_fs_request(rq) ((rq)->cmd == READ || (rq)->cmd == WRITE)
#define blk_queue_empty(q) list_empty(&(q)->queue_head)
@@ -156,6 +173,30 @@
}
}

+static inline void set_queue_full(request_queue_t *q, int rw)
+{
+ if (rw == READ)
+ q->read_full = 1;
+ else
+ q->write_full = 1;
+}
+
+static inline void clear_queue_full(request_queue_t *q, int rw)
+{
+ if (rw == READ)
+ q->read_full = 0;
+ else
+ q->write_full = 0;
+}
+
+static inline int queue_full(request_queue_t *q, int rw)
+{
+ if (rw == READ)
+ return q->read_full;
+ else
+ return q->write_full;
+}
+
extern unsigned long blk_max_low_pfn, blk_max_pfn;

#define BLK_BOUNCE_HIGH (blk_max_low_pfn << PAGE_SHIFT)
@@ -217,6 +258,7 @@
extern void generic_make_request(int rw, struct buffer_head * bh);
extern inline request_queue_t *blk_get_queue(kdev_t dev);
extern void blkdev_release_request(struct request *);
+extern void blk_print_stats(kdev_t dev);

/*
* Access functions for manipulating queue properties
--- 1.19/include/linux/pagemap.h Sun Aug 25 15:32:11 2002
+++ edited/include/linux/pagemap.h Wed Jun 11 08:57:12 2003
@@ -97,6 +97,8 @@
___wait_on_page(page);
}

+extern void FASTCALL(wakeup_page_waiters(struct page * page));
+
/*
* Returns locked page at given index in given cache, creating it if needed.
*/
--- 1.68/kernel/ksyms.c Fri May 23 17:40:47 2003
+++ edited/kernel/ksyms.c Tue Jun 10 14:49:27 2003
@@ -295,6 +295,7 @@
EXPORT_SYMBOL(filemap_fdatawait);
EXPORT_SYMBOL(lock_page);
EXPORT_SYMBOL(unlock_page);
+EXPORT_SYMBOL(wakeup_page_waiters);

/* device registration */
EXPORT_SYMBOL(register_chrdev);
--- 1.77/mm/filemap.c Thu Apr 24 11:05:10 2003
+++ edited/mm/filemap.c Tue Jun 10 14:49:28 2003
@@ -812,6 +812,20 @@
return &wait[hash];
}

+/*
+ * This must be called after every submit_bh with end_io
+ * callbacks that would result into the blkdev layer waking
+ * up the page after a queue unplug.
+ */
+void wakeup_page_waiters(struct page * page)
+{
+ wait_queue_head_t * head;
+
+ head = page_waitqueue(page);
+ if (waitqueue_active(head))
+ wake_up(head);
+}
+
/*
* Wait for a page to get unlocked.
*
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