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dm-delay: change locking to avoid contention

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The delayed_bios list is protected by one mutex shared by all dm-delay
devices. This mutex must be held whenever a bio is added or expired bios
are removed from the list.  Since a large number of expired bios could
be on the list, flush_delayed_bios() can schedule while holding the
mutex. This means a flush_delayed_bios() call on any dm-delay device can
slow down delay_map() calls on any other dm-delay device.

To keep dm-delay devices from slowing each other down and keep
processing delay bios from slowing adding delayed bios, the global mutex
has been removed, and each dm-delay device now has two locks.
delayed_bios_lock is a spinlock that must be held whenever the
delayed_bios list is accessed. process_bios_lock is a mutex that must be
held whenever a process has temporarily pulled bios off the delayed_bios
list to check which ones should be processed. It must be held until all
the bios that won't be processed are returned to the list. This is what
flush_delayed_bios() now does. The mutex is necessary to guarantee that
delay_presuspend() sees the entire list of delayed bios when it calls
flush_delayed_bios().

Signed-off-by: Benjamin Marzinski <bmarzins@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@kernel.org>
This commit is contained in:
Benjamin Marzinski 2024-05-07 17:16:24 -04:00 committed by Mike Snitzer
parent 64eb88d6ca
commit c542ee1492
1 changed files with 23 additions and 13 deletions
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36
drivers/md/dm-delay.c
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@ -29,6 +29,8 @@ struct delay_class {
struct delay_c {
struct timer_list delay_timer;
struct mutex timer_lock;
struct mutex process_bios_lock; /* hold while removing bios to be processed from list */
spinlock_t delayed_bios_lock; /* hold on all accesses to delayed_bios list */
struct workqueue_struct *kdelayd_wq;
struct work_struct flush_expired_bios;
struct list_head delayed_bios;
@ -49,8 +51,6 @@ struct dm_delay_info {
unsigned long expires;
};
static DEFINE_MUTEX(delayed_bios_lock);
static void handle_delayed_timer(struct timer_list *t)
{
struct delay_c *dc = from_timer(dc, t, delay_timer);
@ -89,12 +89,16 @@ static void flush_delayed_bios(struct delay_c *dc, bool flush_all)
{
struct dm_delay_info *delayed, *next;
struct bio_list flush_bio_list;
LIST_HEAD(local_list);
unsigned long next_expires = 0;
bool start_timer = false;
bio_list_init(&flush_bio_list);
mutex_lock(&delayed_bios_lock);
list_for_each_entry_safe(delayed, next, &dc->delayed_bios, list) {
mutex_lock(&dc->process_bios_lock);
spin_lock(&dc->delayed_bios_lock);
list_replace_init(&dc->delayed_bios, &local_list);
spin_unlock(&dc->delayed_bios_lock);
list_for_each_entry_safe(delayed, next, &local_list, list) {
cond_resched();
if (flush_all || time_after_eq(jiffies, delayed->expires)) {
struct bio *bio = dm_bio_from_per_bio_data(delayed,
@ -114,7 +118,10 @@ static void flush_delayed_bios(struct delay_c *dc, bool flush_all)
}
}
}
mutex_unlock(&delayed_bios_lock);
spin_lock(&dc->delayed_bios_lock);
list_splice(&local_list, &dc->delayed_bios);
spin_unlock(&dc->delayed_bios_lock);
mutex_unlock(&dc->process_bios_lock);
if (start_timer)
queue_timeout(dc, next_expires);
@ -128,13 +135,13 @@ static int flush_worker_fn(void *data)
while (!kthread_should_stop()) {
flush_delayed_bios(dc, false);
mutex_lock(&delayed_bios_lock);
spin_lock(&dc->delayed_bios_lock);
if (unlikely(list_empty(&dc->delayed_bios))) {
set_current_state(TASK_INTERRUPTIBLE);
mutex_unlock(&delayed_bios_lock);
spin_unlock(&dc->delayed_bios_lock);
schedule();
} else {
mutex_unlock(&delayed_bios_lock);
spin_unlock(&dc->delayed_bios_lock);
cond_resched();
}
}
@ -168,6 +175,7 @@ static void delay_dtr(struct dm_target *ti)
if (dc->worker)
kthread_stop(dc->worker);
mutex_destroy(&dc->process_bios_lock);
mutex_destroy(&dc->timer_lock);
kfree(dc);
@ -227,6 +235,8 @@ static int delay_ctr(struct dm_target *ti, unsigned int argc, char **argv)
ti->private = dc;
INIT_LIST_HEAD(&dc->delayed_bios);
mutex_init(&dc->timer_lock);
mutex_init(&dc->process_bios_lock);
spin_lock_init(&dc->delayed_bios_lock);
dc->may_delay = true;
dc->argc = argc;
@ -309,14 +319,14 @@ static int delay_bio(struct delay_c *dc, struct delay_class *c, struct bio *bio)
delayed->context = dc;
delayed->expires = expires = jiffies + msecs_to_jiffies(c->delay);
mutex_lock(&delayed_bios_lock);
spin_lock(&dc->delayed_bios_lock);
if (unlikely(!dc->may_delay)) {
mutex_unlock(&delayed_bios_lock);
spin_unlock(&dc->delayed_bios_lock);
return DM_MAPIO_REMAPPED;
}
c->ops++;
list_add_tail(&delayed->list, &dc->delayed_bios);
mutex_unlock(&delayed_bios_lock);
spin_unlock(&dc->delayed_bios_lock);
if (delay_is_fast(dc))
wake_up_process(dc->worker);
@ -330,9 +340,9 @@ static void delay_presuspend(struct dm_target *ti)
{
struct delay_c *dc = ti->private;
mutex_lock(&delayed_bios_lock);
spin_lock(&dc->delayed_bios_lock);
dc->may_delay = false;
mutex_unlock(&delayed_bios_lock);
spin_unlock(&dc->delayed_bios_lock);
if (!delay_is_fast(dc))
timer_delete(&dc->delay_timer);