On Wed, 29 Nov 2023, Christian Loehle wrote:
> Hi Mikulas,
> Agreed and thanks for fixing.
> Has this been selected for stable because of:
> 6fc45b6ed921 ("dm-delay: fix a race between delay_presuspend and delay_bio")
> If so, I would volunteer do the backports for that for you at least.
I wouldn't backport this patch - it is an enhancement, not a bugfix, so it
doesn't qualify for the stable kernel backports.
Mikulas
> BR,
> Christian
>
> On 29/11/2023 08:28, Mikulas Patocka wrote:
> > Hi
> >
> > This patch doesn't fix any bug (and introduces several serious bugs), so
> > it shouldn't be backported at all.
> >
> > Mikulas
> >
> >
> > On Tue, 28 Nov 2023, Sasha Levin wrote:
> >
> >> This is a note to let you know that I've just added the patch titled
> >>
> >> dm delay: for short delays, use kthread instead of timers and wq
> >>
> >> to the 6.6-stable tree which can be found at:
> >>
> >> http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
> >>
> >> The filename of the patch is:
> >> dm-delay-for-short-delays-use-kthread-instead-of-tim.patch
> >> and it can be found in the queue-6.6 subdirectory.
> >>
> >> If you, or anyone else, feels it should not be added to the stable tree,
> >> please let <[email protected]> know about it.
> >>
> >>
> >>
> >> commit 976fd593415e170a8ed5db68683b280d5876982d
> >> Author: Christian Loehle <[email protected]>
> >> Date: Fri Oct 20 12:46:05 2023 +0100
> >>
> >> dm delay: for short delays, use kthread instead of timers and wq
> >>
> >> [ Upstream commit 70bbeb29fab09d6ea6cfe64109db60a97d84d739 ]
> >>
> >> DM delay's current design of using timers and wq to realize the delays
> >> is insufficient for delays below ~50ms.
> >>
> >> This commit enhances the design to use a kthread to flush the expired
> >> delays, trading some CPU time (in some cases) for better delay
> >> accuracy and delays closer to what the user requested for smaller
> >> delays. The new design is chosen as long as all the delays are below
> >> 50ms.
> >>
> >> Since bios can't be completed in interrupt context using a kthread
> >> is probably the most reasonable way to approach this.
> >>
> >> Testing with
> >> echo "0 2097152 zero" | dmsetup create dm-zeros
> >> for i in $(seq 0 20);
> >> do
> >> echo "0 2097152 delay /dev/mapper/dm-zeros 0 $i" | dmsetup create
> >> dm-delay-${i}ms;
> >> done
> >>
> >> Some performance numbers for comparison, on beaglebone black (single
> >> core) CONFIG_HZ_1000=y:
> >>
> >> fio --name=1msread --rw=randread --bs=4k --runtime=60 --time_based \
> >> --filename=/dev/mapper/dm-delay-1ms
> >> Theoretical maximum: 1000 IOPS
> >> Previous: 250 IOPS
> >> Kthread: 500 IOPS
> >>
> >> fio --name=10msread --rw=randread --bs=4k --runtime=60 --time_based \
> >> --filename=/dev/mapper/dm-delay-10ms
> >> Theoretical maximum: 100 IOPS
> >> Previous: 45 IOPS
> >> Kthread: 50 IOPS
> >>
> >> fio --name=1mswrite --rw=randwrite --direct=1 --bs=4k --runtime=60 \
> >> --time_based --filename=/dev/mapper/dm-delay-1ms
> >> Theoretical maximum: 1000 IOPS
> >> Previous: 498 IOPS
> >> Kthread: 1000 IOPS
> >>
> >> fio --name=10mswrite --rw=randwrite --direct=1 --bs=4k --runtime=60 \
> >> --time_based --filename=/dev/mapper/dm-delay-10ms
> >> Theoretical maximum: 100 IOPS
> >> Previous: 90 IOPS
> >> Kthread: 100 IOPS
> >>
> >> (This one is just to prove the new design isn't impacting throughput,
> >> not really about delays):
> >> fio --name=10mswriteasync --rw=randwrite --direct=1 --bs=4k \
> >> --runtime=60 --time_based --filename=/dev/mapper/dm-delay-10ms \
> >> --numjobs=32 --iodepth=64 --ioengine=libaio --group_reporting
> >> Previous: 13.3k IOPS
> >> Kthread: 13.3k IOPS
> >>
> >> Signed-off-by: Christian Loehle <[email protected]>
> >> [Harshit: kthread_create error handling fix in delay_ctr]
> >> Signed-off-by: Harshit Mogalapalli <[email protected]>
> >> Signed-off-by: Mike Snitzer <[email protected]>
> >> Stable-dep-of: 6fc45b6ed921 ("dm-delay: fix a race between
> >> delay_presuspend and delay_bio")
> >> Signed-off-by: Sasha Levin <[email protected]>
> >>
> >> diff --git a/drivers/md/dm-delay.c b/drivers/md/dm-delay.c
> >> index 7433525e59856..efd510984e259 100644
> >> --- a/drivers/md/dm-delay.c
> >> +++ b/drivers/md/dm-delay.c
> >> @@ -13,6 +13,7 @@
> >> #include <linux/blkdev.h>
> >> #include <linux/bio.h>
> >> #include <linux/slab.h>
> >> +#include <linux/kthread.h>
> >>
> >> #include <linux/device-mapper.h>
> >>
> >> @@ -31,6 +32,7 @@ struct delay_c {
> >> struct workqueue_struct *kdelayd_wq;
> >> struct work_struct flush_expired_bios;
> >> struct list_head delayed_bios;
> >> + struct task_struct *worker;
> >> atomic_t may_delay;
> >>
> >> struct delay_class read;
> >> @@ -66,6 +68,44 @@ static void queue_timeout(struct delay_c *dc, unsigned
> >> long expires)
> >> mutex_unlock(&dc->timer_lock);
> >> }
> >>
> >> +static inline bool delay_is_fast(struct delay_c *dc)
> >> +{
> >> + return !!dc->worker;
> >> +}
> >> +
> >> +static void flush_delayed_bios_fast(struct delay_c *dc, bool flush_all)
> >> +{
> >> + struct dm_delay_info *delayed, *next;
> >> +
> >> + mutex_lock(&delayed_bios_lock);
> >> + list_for_each_entry_safe(delayed, next, &dc->delayed_bios, list) {
> >> + if (flush_all || time_after_eq(jiffies, delayed->expires)) {
> >> + struct bio *bio = dm_bio_from_per_bio_data(delayed,
> >> + sizeof(struct dm_delay_info));
> >> + list_del(&delayed->list);
> >> + dm_submit_bio_remap(bio, NULL);
> >> + delayed->class->ops--;
> >> + }
> >> + }
> >> + mutex_unlock(&delayed_bios_lock);
> >> +}
> >> +
> >> +static int flush_worker_fn(void *data)
> >> +{
> >> + struct delay_c *dc = data;
> >> +
> >> + while (1) {
> >> + flush_delayed_bios_fast(dc, false);
> >> + if (unlikely(list_empty(&dc->delayed_bios))) {
> >> + set_current_state(TASK_INTERRUPTIBLE);
> >> + schedule();
> >> + } else
> >> + cond_resched();
> >> + }
> >> +
> >> + return 0;
> >> +}
> >> +
> >> static void flush_bios(struct bio *bio)
> >> {
> >> struct bio *n;
> >> @@ -78,7 +118,7 @@ static void flush_bios(struct bio *bio)
> >> }
> >> }
> >>
> >> -static struct bio *flush_delayed_bios(struct delay_c *dc, int flush_all)
> >> +static struct bio *flush_delayed_bios(struct delay_c *dc, bool flush_all)
> >> {
> >> struct dm_delay_info *delayed, *next;
> >> unsigned long next_expires = 0;
> >> @@ -115,7 +155,10 @@ static void flush_expired_bios(struct work_struct
> >> *work)
> >> struct delay_c *dc;
> >>
> >> dc = container_of(work, struct delay_c, flush_expired_bios);
> >> - flush_bios(flush_delayed_bios(dc, 0));
> >> + if (delay_is_fast(dc))
> >> + flush_delayed_bios_fast(dc, false);
> >> + else
> >> + flush_bios(flush_delayed_bios(dc, false));
> >> }
> >>
> >> static void delay_dtr(struct dm_target *ti)
> >> @@ -131,8 +174,11 @@ static void delay_dtr(struct dm_target *ti)
> >> dm_put_device(ti, dc->write.dev);
> >> if (dc->flush.dev)
> >> dm_put_device(ti, dc->flush.dev);
> >> + if (dc->worker)
> >> + kthread_stop(dc->worker);
> >>
> >> - mutex_destroy(&dc->timer_lock);
> >> + if (!delay_is_fast(dc))
> >> + mutex_destroy(&dc->timer_lock);
> >>
> >> kfree(dc);
> >> }
> >> @@ -175,6 +221,7 @@ static int delay_ctr(struct dm_target *ti, unsigned
> >> int argc, char **argv)
> >> {
> >> struct delay_c *dc;
> >> int ret;
> >> + unsigned int max_delay;
> >>
> >> if (argc != 3 && argc != 6 && argc != 9) {
> >> ti->error = "Requires exactly 3, 6 or 9 arguments";
> >> @@ -188,16 +235,14 @@ static int delay_ctr(struct dm_target *ti, unsigned
> >> int argc, char **argv)
> >> }
> >>
> >> ti->private = dc;
> >> - timer_setup(&dc->delay_timer, handle_delayed_timer, 0);
> >> - INIT_WORK(&dc->flush_expired_bios, flush_expired_bios);
> >> INIT_LIST_HEAD(&dc->delayed_bios);
> >> - mutex_init(&dc->timer_lock);
> >> atomic_set(&dc->may_delay, 1);
> >> dc->argc = argc;
> >>
> >> ret = delay_class_ctr(ti, &dc->read, argv);
> >> if (ret)
> >> goto bad;
> >> + max_delay = dc->read.delay;
> >>
> >> if (argc == 3) {
> >> ret = delay_class_ctr(ti, &dc->write, argv);
> >> @@ -206,6 +251,8 @@ static int delay_ctr(struct dm_target *ti, unsigned
> >> int argc, char **argv)
> >> ret = delay_class_ctr(ti, &dc->flush, argv);
> >> if (ret)
> >> goto bad;
> >> + max_delay = max(max_delay, dc->write.delay);
> >> + max_delay = max(max_delay, dc->flush.delay);
> >> goto out;
> >> }
> >>
> >> @@ -216,19 +263,37 @@ static int delay_ctr(struct dm_target *ti, unsigned
> >> int argc, char **argv)
> >> ret = delay_class_ctr(ti, &dc->flush, argv + 3);
> >> if (ret)
> >> goto bad;
> >> + max_delay = max(max_delay, dc->flush.delay);
> >> goto out;
> >> }
> >>
> >> ret = delay_class_ctr(ti, &dc->flush, argv + 6);
> >> if (ret)
> >> goto bad;
> >> + max_delay = max(max_delay, dc->flush.delay);
> >>
> >> out:
> >> - dc->kdelayd_wq = alloc_workqueue("kdelayd", WQ_MEM_RECLAIM, 0);
> >> - if (!dc->kdelayd_wq) {
> >> - ret = -EINVAL;
> >> - DMERR("Couldn't start kdelayd");
> >> - goto bad;
> >> + if (max_delay < 50) {
> >> + /*
> >> + * In case of small requested delays, use kthread instead of
> >> + * timers and workqueue to achieve better latency.
> >> + */
> >> + dc->worker = kthread_create(&flush_worker_fn, dc,
> >> + "dm-delay-flush-worker");
> >> + if (IS_ERR(dc->worker)) {
> >> + ret = PTR_ERR(dc->worker);
> >> + goto bad;
> >> + }
> >> + } else {
> >> + timer_setup(&dc->delay_timer, handle_delayed_timer, 0);
> >> + INIT_WORK(&dc->flush_expired_bios, flush_expired_bios);
> >> + mutex_init(&dc->timer_lock);
> >> + dc->kdelayd_wq = alloc_workqueue("kdelayd", WQ_MEM_RECLAIM, 0);
> >> + if (!dc->kdelayd_wq) {
> >> + ret = -EINVAL;
> >> + DMERR("Couldn't start kdelayd");
> >> + goto bad;
> >> + }
> >> }
> >>
> >> ti->num_flush_bios = 1;
> >> @@ -260,7 +325,10 @@ static int delay_bio(struct delay_c *dc, struct
> >> delay_class *c, struct bio *bio)
> >> list_add_tail(&delayed->list, &dc->delayed_bios);
> >> mutex_unlock(&delayed_bios_lock);
> >>
> >> - queue_timeout(dc, expires);
> >> + if (delay_is_fast(dc))
> >> + wake_up_process(dc->worker);
> >> + else
> >> + queue_timeout(dc, expires);
> >>
> >> return DM_MAPIO_SUBMITTED;
> >> }
> >> @@ -270,8 +338,13 @@ static void delay_presuspend(struct dm_target *ti)
> >> struct delay_c *dc = ti->private;
> >>
> >> atomic_set(&dc->may_delay, 0);
> >> - del_timer_sync(&dc->delay_timer);
> >> - flush_bios(flush_delayed_bios(dc, 1));
> >> +
> >> + if (delay_is_fast(dc))
> >> + flush_delayed_bios_fast(dc, true);
> >> + else {
> >> + del_timer_sync(&dc->delay_timer);
> >> + flush_bios(flush_delayed_bios(dc, true));
> >> + }
> >> }
> >>
> >> static void delay_resume(struct dm_target *ti)
> >> @@ -356,7 +429,7 @@ static int delay_iterate_devices(struct dm_target *ti,
> >>
> >> static struct target_type delay_target = {
> >> .name = "delay",
> >> - .version = {1, 3, 0},
> >> + .version = {1, 4, 0},
> >> .features = DM_TARGET_PASSES_INTEGRITY,
> >> .module = THIS_MODULE,
> >> .ctr = delay_ctr,
> >>
> >
>
