Re: [PATCH 2/2] md/raid5: exclusive wait_for_stripe

From: NeilBrown
Date: Sun Apr 26 2015 - 20:24:31 EST

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On Fri, 24 Apr 2015 21:39:04 +0800 Yuanhan Liu <yuanhan.liu@xxxxxxxxxxxxxxx>
wrote:

> I noticed heavy spin lock contention at get_active_stripe() with fsmark
> multiple thread write workloads.
>
> Here is how this hot contention comes from. We have limited stripes, and
> it's a multiple thread write workload. Hence, those stripes will be taken
> soon, which puts later processes to sleep for waiting free stripes. When
> enough stripes(> 1/4 total stripes) are released, all process are woken,
> trying to get the lock. But there is one only being able to get this lock
> for each hash lock, making other processes spinning out there for acquiring
> the lock.
>
> Thus, it's effectiveless to wakeup all processes and let them battle for
> a lock that permits one to access only each time. Instead, we could make
> it be a exclusive wake up: wake up one process only. That avoids the heavy
> spin lock contention naturally.
>
> Here are some test results I have got with this patch applied(all test run
> 3 times):
>
> `fsmark.files_per_sec'
> =====================
>
> next-20150317 this patch
> ------------------------- -------------------------
> metric_value Âstddev metric_value Âstddev change testbox/benchmark/testcase-params
> ------------------------- ------------------------- -------- ------------------------------
> 25.600 Â0.0 92.700 Â2.5 262.1% ivb44/fsmark/1x-64t-4BRD_12G-RAID5-btrfs-4M-30G-fsyncBeforeClose
> 25.600 Â0.0 77.800 Â0.6 203.9% ivb44/fsmark/1x-64t-9BRD_6G-RAID5-btrfs-4M-30G-fsyncBeforeClose
> 32.000 Â0.0 93.800 Â1.7 193.1% ivb44/fsmark/1x-64t-4BRD_12G-RAID5-ext4-4M-30G-fsyncBeforeClose
> 32.000 Â0.0 81.233 Â1.7 153.9% ivb44/fsmark/1x-64t-9BRD_6G-RAID5-ext4-4M-30G-fsyncBeforeClose
> 48.800 Â14.5 99.667 Â2.0 104.2% ivb44/fsmark/1x-64t-4BRD_12G-RAID5-xfs-4M-30G-fsyncBeforeClose
> 6.400 Â0.0 12.800 Â0.0 100.0% ivb44/fsmark/1x-64t-3HDD-RAID5-btrfs-4M-40G-fsyncBeforeClose
> 63.133 Â8.2 82.800 Â0.7 31.2% ivb44/fsmark/1x-64t-9BRD_6G-RAID5-xfs-4M-30G-fsyncBeforeClose
> 245.067 Â0.7 306.567 Â7.9 25.1% ivb44/fsmark/1x-64t-4BRD_12G-RAID5-f2fs-4M-30G-fsyncBeforeClose
> 17.533 Â0.3 21.000 Â0.8 19.8% ivb44/fsmark/1x-1t-3HDD-RAID5-xfs-4M-40G-fsyncBeforeClose
> 188.167 Â1.9 215.033 Â3.1 14.3% ivb44/fsmark/1x-1t-4BRD_12G-RAID5-btrfs-4M-30G-NoSync
> 254.500 Â1.8 290.733 Â2.4 14.2% ivb44/fsmark/1x-1t-9BRD_6G-RAID5-btrfs-4M-30G-NoSync
>
> `time.system_time'
> =====================
>
> next-20150317 this patch
> ------------------------- -------------------------
> metric_value Âstddev metric_value Âstddev change testbox/benchmark/testcase-params
> ------------------------- ------------------------- -------- ------------------------------
> 7235.603 Â1.2 185.163 Â1.9 -97.4% ivb44/fsmark/1x-64t-4BRD_12G-RAID5-btrfs-4M-30G-fsyncBeforeClose
> 7666.883 Â2.9 202.750 Â1.0 -97.4% ivb44/fsmark/1x-64t-9BRD_6G-RAID5-btrfs-4M-30G-fsyncBeforeClose
> 14567.893 Â0.7 421.230 Â0.4 -97.1% ivb44/fsmark/1x-64t-3HDD-RAID5-btrfs-4M-40G-fsyncBeforeClose
> 3697.667 Â14.0 148.190 Â1.7 -96.0% ivb44/fsmark/1x-64t-4BRD_12G-RAID5-xfs-4M-30G-fsyncBeforeClose
> 5572.867 Â3.8 310.717 Â1.4 -94.4% ivb44/fsmark/1x-64t-9BRD_6G-RAID5-ext4-4M-30G-fsyncBeforeClose
> 5565.050 Â0.5 313.277 Â1.5 -94.4% ivb44/fsmark/1x-64t-4BRD_12G-RAID5-ext4-4M-30G-fsyncBeforeClose
> 2420.707 Â17.1 171.043 Â2.7 -92.9% ivb44/fsmark/1x-64t-9BRD_6G-RAID5-xfs-4M-30G-fsyncBeforeClose
> 3743.300 Â4.6 379.827 Â3.5 -89.9% ivb44/fsmark/1x-64t-3HDD-RAID5-ext4-4M-40G-fsyncBeforeClose
> 3308.687 Â6.3 363.050 Â2.0 -89.0% ivb44/fsmark/1x-64t-3HDD-RAID5-xfs-4M-40G-fsyncBeforeClose
>
> Where,
>
> 1x: where 'x' means iterations or loop, corresponding to the 'L' option of fsmark
>
> 1t, 64t: where 't' means thread
>
> 4M: means the single file size, corresponding to the '-s' option of fsmark
> 40G, 30G, 120G: means the total test size
>
> 4BRD_12G: BRD is the ramdisk, where '4' means 4 ramdisk, and where '12G' means
> the size of one ramdisk. So, it would be 48G in total. And we made a
> raid on those ramdisk
>
> As you can see, though there are no much performance gain for hard disk
> workload, the system time is dropped heavily, up to 97%. And as expected,
> the performance increased a lot, up to 260%, for fast device(ram disk).
>
> Signed-off-by: Yuanhan Liu <yuanhan.liu@xxxxxxxxxxxxxxx>
> ---
> drivers/md/raid5.c | 46 +++++++++++++++++++++++++++++++++++-----------
> drivers/md/raid5.h | 2 +-
> 2 files changed, 36 insertions(+), 12 deletions(-)
>
> diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
> index b7e385f..2d8fcc1 100644
> --- a/drivers/md/raid5.c
> +++ b/drivers/md/raid5.c
> @@ -344,7 +344,8 @@ static void release_inactive_stripe_list(struct r5conf *conf,
> int hash)
> {
> int size;
> - bool do_wakeup = false;
> + bool do_wakeup[NR_STRIPE_HASH_LOCKS] = { false, };

I think I'd rather use an 'unsigned long' and set bits.

> + int i = 0;
> unsigned long flags;
>
> if (hash == NR_STRIPE_HASH_LOCKS) {
> @@ -365,17 +366,22 @@ static void release_inactive_stripe_list(struct r5conf *conf,
> !list_empty(list))
> atomic_dec(&conf->empty_inactive_list_nr);
> list_splice_tail_init(list, conf->inactive_list + hash);
> - do_wakeup = true;
> + do_wakeup[size - 1] = true;

... so this becomes
do_wakeup |= 1 << (size - 1);

> spin_unlock_irqrestore(conf->hash_locks + hash, flags);
> }
> size--;
> hash--;
> }
>
> - if (do_wakeup) {
> - wake_up(&conf->wait_for_stripe);
> - if (conf->retry_read_aligned)
> - md_wakeup_thread(conf->mddev->thread);
> + for (i = 0; i < NR_STRIPE_HASH_LOCKS; i++) {
> + bool waked_thread = false;
> + if (do_wakeup[i]) {
> + wake_up(&conf->wait_for_stripe[i]);
> + if (!waked_thread) {
> + waked_thread = true;
> + md_wakeup_thread(conf->mddev->thread);
> + }
> + }

I don't think you want waked_thread to be local to this loop.
As it is, the "if (!waked_thread)" test *always* succeeds.

You can discard it if do_wakeup becomes and unsigned long, and just do

if (do_wakeup && conf->retry_read_aligned)
md_wakeup_thread(conf->mddev->thread);

And why have you removed the test on conf->retry_read_aligned??

> }
> }
>
> @@ -655,6 +661,18 @@ static int has_failed(struct r5conf *conf)
> return 0;
> }
>
> +/* XXX: might put it to linux/wait.h to be a public API? */

Yes, definitely put it in linux/wait.h

Thanks,
NeilBrown

> +#define raid_wait_event_exclusive_cmd(wq, condition, cmd1, cmd2) \
> +do { \
> + if (condition) \
> + break; \
> + (void)___wait_event(wq, condition, TASK_UNINTERRUPTIBLE, 1, 0, \
> + cmd1; \
> + schedule(); \
> + cmd2); \
> +} while (0)
> +
> +
> static struct stripe_head *
> get_active_stripe(struct r5conf *conf, sector_t sector,
> int previous, int noblock, int noquiesce)
> @@ -684,14 +702,15 @@ get_active_stripe(struct r5conf *conf, sector_t sector,
> if (!sh) {
> set_bit(R5_INACTIVE_BLOCKED,
> &conf->cache_state);
> - wait_event_lock_irq(
> - conf->wait_for_stripe,
> + raid_wait_event_exclusive_cmd(
> + conf->wait_for_stripe[hash],
> !list_empty(conf->inactive_list + hash) &&
> (atomic_read(&conf->active_stripes)
> < (conf->max_nr_stripes * 3 / 4)
> || !test_bit(R5_INACTIVE_BLOCKED,
> &conf->cache_state)),
> - *(conf->hash_locks + hash));
> + spin_unlock_irq(conf->hash_locks + hash),
> + spin_lock_irq(conf->hash_locks + hash));
> clear_bit(R5_INACTIVE_BLOCKED,
> &conf->cache_state);
> } else {
> @@ -716,6 +735,9 @@ get_active_stripe(struct r5conf *conf, sector_t sector,
> }
> } while (sh == NULL);
>
> + if (!list_empty(conf->inactive_list + hash))
> + wake_up(&conf->wait_for_stripe[hash]);
> +
> spin_unlock_irq(conf->hash_locks + hash);
> return sh;
> }
> @@ -2136,7 +2158,7 @@ static int resize_stripes(struct r5conf *conf, int newsize)
> cnt = 0;
> list_for_each_entry(nsh, &newstripes, lru) {
> lock_device_hash_lock(conf, hash);
> - wait_event_cmd(conf->wait_for_stripe,
> + raid_wait_event_exclusive_cmd(conf->wait_for_stripe[hash],
> !list_empty(conf->inactive_list + hash),
> unlock_device_hash_lock(conf, hash),
> lock_device_hash_lock(conf, hash));
> @@ -6391,7 +6413,9 @@ static struct r5conf *setup_conf(struct mddev *mddev)
> spin_lock_init(&conf->device_lock);
> seqcount_init(&conf->gen_lock);
> init_waitqueue_head(&conf->wait_for_quiesce);
> - init_waitqueue_head(&conf->wait_for_stripe);
> + for (i = 0; i < NR_STRIPE_HASH_LOCKS; i++) {
> + init_waitqueue_head(&conf->wait_for_stripe[i]);
> + }
> init_waitqueue_head(&conf->wait_for_overlap);
> INIT_LIST_HEAD(&conf->handle_list);
> INIT_LIST_HEAD(&conf->hold_list);
> diff --git a/drivers/md/raid5.h b/drivers/md/raid5.h
> index fab53a3..cdad2d2 100644
> --- a/drivers/md/raid5.h
> +++ b/drivers/md/raid5.h
> @@ -509,7 +509,7 @@ struct r5conf {
> atomic_t empty_inactive_list_nr;
> struct llist_head released_stripes;
> wait_queue_head_t wait_for_quiesce;
> - wait_queue_head_t wait_for_stripe;
> + wait_queue_head_t wait_for_stripe[NR_STRIPE_HASH_LOCKS];
> wait_queue_head_t wait_for_overlap;
> unsigned long cache_state;
> #define R5_INACTIVE_BLOCKED 1 /* release of inactive stripes blocked,

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Next message: Dave Airlie: "[git pull] drm intel fixes"
Previous message: NeilBrown: "Re: [PATCH 1/2] md/raid5: split wait_for_stripe and introduce wait_for_quiesce"
In reply to: Yuanhan Liu: "[PATCH 2/2] md/raid5: exclusive wait_for_stripe"
Next in thread: Yuanhan Liu: "Re: [PATCH 2/2] md/raid5: exclusive wait_for_stripe"
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