Re: 2.4.6 and ext3-2.4-0.9.1-246

Andrew Morton
2001-07-11T15:36:09Z

Mike Black wrote:
>
> My system is:
> Dual 1Ghz PIII
> 2G RAM
> 2x2G swapfiles
> And I ran tiobench as tiobench.pl --size 4000 (twice memory)
>
> Me thinkst SMP is probably the biggest difference in this list.

No, the problem is in RAID1. The buffer allocation in there is nowhere
near strong enough for these loads.

> I ran this on another "smaller" memory (still dual CPU though) machine and
> noticed this on top:
>
> 12983 root 15 0 548 544 448 D 73.6 0.2 0:11 tiotest
> 3 root 18 0 0 0 0 SW 72.6 0.0 0:52 kswapd
>
> kswapd is taking an awful lot of CPU time. Not sure why it should be
> hitting swap at all.

It's not trying to swap stuff out - it's trying to find pages
to recycle. kswapd often goes berzerk like this. I think it
was a design objective.

For me, RAID1 works OK with tiobench, but it is trivially deadlockable
with other workloads. The usual failure mode is for bdflush to be
stuck in raid1_alloc_r1bh() - can't allocate any more r1bh's, can't
move dirty buffers to disk. Dead.

The below patch increases the size of the reserved r1bh pool, scales it
by PAGE_CACHE_SIZE and introduces a reservation policy for PF_FLUSH
callers (ie: bdflush). That fixes the raid1_alloc_r1bh() deadlocks.

bdflush can also deadlock in raid1_alloc_bh(), trying to allocate
buffer_heads. So we do the same thing there.

Putting swap on RAID1 would definitely have exacerbated the problem.
The last thing we want to do when we're trying to push stuff out
of memory is to have to allocate more of it. So I allowed PF_MEMALLOC
tasks to bite into the reserves as well.

Please, if you have time, apply and retest.

--- linux-2.4.6/include/linux/sched.h Wed May 2 22:00:07 2001
+++ lk-ext3/include/linux/sched.h Thu Jul 12 01:03:20 2001
@@ -413,7 +418,7 @@ struct task_struct {
#define PF_SIGNALED 0x00000400 /* killed by a signal */
#define PF_MEMALLOC 0x00000800 /* Allocating memory */
#define PF_VFORK 0x00001000 /* Wake up parent in mm_release */
-
+#define PF_FLUSH 0x00002000 /* Flushes buffers to disk */
#define PF_USEDFPU 0x00100000 /* task used FPU this quantum (SMP) */

/*
--- linux-2.4.6/include/linux/raid/raid1.h Tue Dec 12 08:20:08 2000
+++ lk-ext3/include/linux/raid/raid1.h Thu Jul 12 01:15:39 2001
@@ -37,12 +37,12 @@ struct raid1_private_data {
/* buffer pool */
/* buffer_heads that we have pre-allocated have b_pprev -> &freebh
* and are linked into a stack using b_next
- * raid1_bh that are pre-allocated have R1BH_PreAlloc set.
* All these variable are protected by device_lock
*/
struct buffer_head *freebh;
int freebh_cnt; /* how many are on the list */
struct raid1_bh *freer1;
+ unsigned freer1_cnt;
struct raid1_bh *freebuf; /* each bh_req has a page allocated */
md_wait_queue_head_t wait_buffer;

@@ -87,5 +87,4 @@ struct raid1_bh {
/* bits for raid1_bh.state */
#define R1BH_Uptodate 1
#define R1BH_SyncPhase 2
-#define R1BH_PreAlloc 3 /* this was pre-allocated, add to free list */
#endif
--- linux-2.4.6/fs/buffer.c Wed Jul 4 18:21:31 2001
+++ lk-ext3/fs/buffer.c Thu Jul 12 01:03:57 2001
@@ -2685,6 +2748,7 @@ int bdflush(void *sem)
sigfillset(&tsk->blocked);
recalc_sigpending(tsk);
spin_unlock_irq(&tsk->sigmask_lock);
+ current->flags |= PF_FLUSH;

up((struct semaphore *)sem);

@@ -2726,6 +2790,7 @@ int kupdate(void *sem)
siginitsetinv(&current->blocked, sigmask(SIGCONT) | sigmask(SIGSTOP));
recalc_sigpending(tsk);
spin_unlock_irq(&tsk->sigmask_lock);
+ current->flags |= PF_FLUSH;

up((struct semaphore *)sem);

--- linux-2.4.6/drivers/md/raid1.c Wed Jul 4 18:21:26 2001
+++ lk-ext3/drivers/md/raid1.c Thu Jul 12 01:28:58 2001
@@ -51,6 +51,28 @@ static mdk_personality_t raid1_personali
static md_spinlock_t retry_list_lock = MD_SPIN_LOCK_UNLOCKED;
struct raid1_bh *raid1_retry_list = NULL, **raid1_retry_tail;

+/*
+ * We need to scale the number of reserved buffers by the page size
+ * to make writepage()s sucessful. --akpm
+ */
+#define R1_BLOCKS_PP (PAGE_CACHE_SIZE / 1024)
+#define FREER1_MEMALLOC_RESERVED (16 * R1_BLOCKS_PP)
+
+/*
+ * Return true if the caller make take a bh from the list.
+ * PF_FLUSH and PF_MEMALLOC tasks are allowed to use the reserves, because
+ * they're trying to *free* some memory.
+ *
+ * Requires that conf->device_lock be held.
+ */
+static int may_take_bh(raid1_conf_t *conf, int cnt)
+{
+ int min_free = (current->flags & (PF_FLUSH|PF_MEMALLOC)) ?
+ cnt :
+ (cnt + FREER1_MEMALLOC_RESERVED * conf->raid_disks);
+ return conf->freebh_cnt >= min_free;
+}
+
static struct buffer_head *raid1_alloc_bh(raid1_conf_t *conf, int cnt)
{
/* return a linked list of "cnt" struct buffer_heads.
@@ -62,7 +84,7 @@ static struct buffer_head *raid1_alloc_b
while(cnt) {
struct buffer_head *t;
md_spin_lock_irq(&conf->device_lock);
- if (conf->freebh_cnt >= cnt)
+ if (may_take_bh(conf, cnt))
while (cnt) {
t = conf->freebh;
conf->freebh = t->b_next;
@@ -83,7 +105,7 @@ static struct buffer_head *raid1_alloc_b
cnt--;
} else {
PRINTK("raid1: waiting for %d bh\n", cnt);
- wait_event(conf->wait_buffer, conf->freebh_cnt >= cnt);
+ wait_event(conf->wait_buffer, may_take_bh(conf, cnt));
}
}
return bh;
@@ -96,9 +118,9 @@ static inline void raid1_free_bh(raid1_c
while (bh) {
struct buffer_head *t = bh;
bh=bh->b_next;
- if (t->b_pprev == NULL)
+ if (conf->freebh_cnt >= FREER1_MEMALLOC_RESERVED) {
kfree(t);
- else {
+ } else {
t->b_next= conf->freebh;
conf->freebh = t;
conf->freebh_cnt++;
@@ -108,29 +130,6 @@ static inline void raid1_free_bh(raid1_c
wake_up(&conf->wait_buffer);
}

-static int raid1_grow_bh(raid1_conf_t *conf, int cnt)
-{
- /* allocate cnt buffer_heads, possibly less if kalloc fails */
- int i = 0;
-
- while (i < cnt) {
- struct buffer_head *bh;
- bh = kmalloc(sizeof(*bh), GFP_KERNEL);
- if (!bh) break;
- memset(bh, 0, sizeof(*bh));
-
- md_spin_lock_irq(&conf->device_lock);
- bh->b_pprev = &conf->freebh;
- bh->b_next = conf->freebh;
- conf->freebh = bh;
- conf->freebh_cnt++;
- md_spin_unlock_irq(&conf->device_lock);
-
- i++;
- }
- return i;
-}
-
static int raid1_shrink_bh(raid1_conf_t *conf, int cnt)
{
/* discard cnt buffer_heads, if we can find them */
@@ -147,7 +146,16 @@ static int raid1_shrink_bh(raid1_conf_t
md_spin_unlock_irq(&conf->device_lock);
return i;
}
-
+
+/*
+ * Return true if the caller make take a raid1_bh from the list.
+ * Requires that conf->device_lock be held.
+ */
+static int may_take_r1bh(raid1_conf_t *conf)
+{
+ return ((conf->freer1_cnt > FREER1_MEMALLOC_RESERVED) ||
+ (current->flags & (PF_FLUSH|PF_MEMALLOC))) && conf->freer1;
+}

static struct raid1_bh *raid1_alloc_r1bh(raid1_conf_t *conf)
{
@@ -155,8 +163,9 @@ static struct raid1_bh *raid1_alloc_r1bh

do {
md_spin_lock_irq(&conf->device_lock);
- if (conf->freer1) {
+ if (may_take_r1bh(conf)) {
r1_bh = conf->freer1;
+ conf->freer1_cnt--;
conf->freer1 = r1_bh->next_r1;
r1_bh->next_r1 = NULL;
r1_bh->state = 0;
@@ -170,7 +179,7 @@ static struct raid1_bh *raid1_alloc_r1bh
memset(r1_bh, 0, sizeof(*r1_bh));
return r1_bh;
}
- wait_event(conf->wait_buffer, conf->freer1);
+ wait_event(conf->wait_buffer, may_take_r1bh(conf));
} while (1);
}

@@ -178,49 +187,30 @@ static inline void raid1_free_r1bh(struc
{
struct buffer_head *bh = r1_bh->mirror_bh_list;
raid1_conf_t *conf = mddev_to_conf(r1_bh->mddev);
+ unsigned long flags;

r1_bh->mirror_bh_list = NULL;

- if (test_bit(R1BH_PreAlloc, &r1_bh->state)) {
- unsigned long flags;
- spin_lock_irqsave(&conf->device_lock, flags);
+ spin_lock_irqsave(&conf->device_lock, flags);
+ if (conf->freer1_cnt < FREER1_MEMALLOC_RESERVED) {
r1_bh->next_r1 = conf->freer1;
conf->freer1 = r1_bh;
+ conf->freer1_cnt++;
spin_unlock_irqrestore(&conf->device_lock, flags);
} else {
+ spin_unlock_irqrestore(&conf->device_lock, flags);
kfree(r1_bh);
}
raid1_free_bh(conf, bh);
}

-static int raid1_grow_r1bh (raid1_conf_t *conf, int cnt)
-{
- int i = 0;
-
- while (i < cnt) {
- struct raid1_bh *r1_bh;
- r1_bh = (struct raid1_bh*)kmalloc(sizeof(*r1_bh), GFP_KERNEL);
- if (!r1_bh)
- break;
- memset(r1_bh, 0, sizeof(*r1_bh));
-
- md_spin_lock_irq(&conf->device_lock);
- set_bit(R1BH_PreAlloc, &r1_bh->state);
- r1_bh->next_r1 = conf->freer1;
- conf->freer1 = r1_bh;
- md_spin_unlock_irq(&conf->device_lock);
-
- i++;
- }
- return i;
-}
-
static void raid1_shrink_r1bh(raid1_conf_t *conf)
{
md_spin_lock_irq(&conf->device_lock);
while (conf->freer1) {
struct raid1_bh *r1_bh = conf->freer1;
conf->freer1 = r1_bh->next_r1;
+ conf->freer1_cnt--; /* pedantry */
kfree(r1_bh);
}
md_spin_unlock_irq(&conf->device_lock);
@@ -1610,21 +1600,6 @@ static int raid1_run (mddev_t *mddev)
goto out_free_conf;
}

-
- /* pre-allocate some buffer_head structures.
- * As a minimum, 1 r1bh and raid_disks buffer_heads
- * would probably get us by in tight memory situations,
- * but a few more is probably a good idea.
- * For now, try 16 r1bh and 16*raid_disks bufferheads
- * This will allow at least 16 concurrent reads or writes
- * even if kmalloc starts failing
- */
- if (raid1_grow_r1bh(conf, 16) < 16 ||
- raid1_grow_bh(conf, 16*conf->raid_disks)< 16*conf->raid_disks) {
- printk(MEM_ERROR, mdidx(mddev));
- goto out_free_conf;
- }
-
for (i = 0; i < MD_SB_DISKS; i++) {

descriptor = sb->disks+i;
@@ -1713,6 +1688,8 @@ out_free_conf:
raid1_shrink_r1bh(conf);
raid1_shrink_bh(conf, conf->freebh_cnt);
raid1_shrink_buffers(conf);
+ if (conf->freer1_cnt != 0)
+ BUG();
kfree(conf);
mddev->private = NULL;