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SubjectRe: [PATCH] writeback: Per-block device bdi->dirty_writeback_interval and bdi->dirty_expire_interval.
Hi Kautuk,

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On Thu, Aug 11, 2011 at 05:50:56PM +0530, Kautuk Consul wrote:
> Hi,
>
> Currently the /proc/sys/vm/dirty_writeback_centisecs and
> /proc/sys/vm/dirty_expire_centisecs values are
> global to the system.
> All the BDI flush-* threads are controlled by these central values.

Yes.

> However, the user/admin might want to set different writeback speeds
> for different block devices based on
> their page write-back performance.

How can the above two sysctl values impact "writeback speeds"?
In particular, what's the "speed" you mean?

> For example, the user might want to write-back pages in smaller
> intervals to a block device which has a
> faster known writeback speed.

That's not a complete rational. What does the user ultimately want by
setting a smaller interval? What would be the problems to the other
slow devices if the user does so by simply setting a small value
_globally_?

We need strong use cases for doing such user interface changes.
Would you detail the problem and the pains that can only (or best)
be addressed by this patch?

Thanks,
Fengguang

> This patch creates 3 new counters (in centisecs) for all the BDI
> threads that were controlled centrally by these
> 2 counters:
> i) /sys/block/<block_dev>/bdi/dirty_writeback_interval,
> ii) /sys/block/<block_dev>/bdi/dirty_expire_interval,
> iii) /proc/sys/vm/sync_supers_centisecs.
>
> Although these new counters can be tuned individually, I have taken
> care that they be centrally reset by changes
> to the /proc/sys/vm/dirty_expire_centisecs and
> /proc/sys/vm/dirty_writeback_centisecs so that the earlier
> functionality is not broken by distributions using these central values.
> After resetting all values centrally, these values can be tuned
> individually without altering the central values.
>
> Signed-off-by: Kautuk Consul <consul.kautuk@gmail.com>
> ---
>
> diff -uprN a/fs/fs-writeback.c b/fs/fs-writeback.c
> --- a/fs/fs-writeback.c 2011-08-05 10:29:21.000000000 +0530
> +++ b/fs/fs-writeback.c 2011-08-09 09:15:37.093041675 +0530
> @@ -638,8 +638,8 @@ static inline bool over_bground_thresh(v
> * just walks the superblock inode list, writing back any inodes which are
> * older than a specific point in time.
> *
> - * Try to run once per dirty_writeback_interval. But if a writeback event
> - * takes longer than a dirty_writeback_interval interval, then leave a
> + * Try to run once per bdi->dirty_writeback_interval. But if a writeback event
> + * takes longer than a bdi->dirty_writeback_interval interval, then leave a
> * one-second gap.
> *
> * older_than_this takes precedence over nr_to_write. So we'll only write back
> @@ -663,7 +663,7 @@ static long wb_writeback(struct bdi_writ
> if (wbc.for_kupdate) {
> wbc.older_than_this = &oldest_jif;
> oldest_jif = jiffies -
> - msecs_to_jiffies(dirty_expire_interval * 10);
> + msecs_to_jiffies(wb->bdi->dirty_expire_interval * 10);
> }
> if (!wbc.range_cyclic) {
> wbc.range_start = 0;
> @@ -811,15 +811,16 @@ static long wb_check_old_data_flush(stru
> {
> unsigned long expired;
> long nr_pages;
> + struct backing_dev_info *bdi = wb->bdi;
>
> /*
> * When set to zero, disable periodic writeback
> */
> - if (!dirty_writeback_interval)
> + if (!bdi->dirty_writeback_interval)
> return 0;
>
> expired = wb->last_old_flush +
> - msecs_to_jiffies(dirty_writeback_interval * 10);
> + msecs_to_jiffies(bdi->dirty_writeback_interval * 10);
> if (time_before(jiffies, expired))
> return 0;
>
> @@ -923,8 +924,8 @@ int bdi_writeback_thread(void *data)
> continue;
> }
>
> - if (wb_has_dirty_io(wb) && dirty_writeback_interval)
> - schedule_timeout(msecs_to_jiffies(dirty_writeback_interval * 10));
> + if (wb_has_dirty_io(wb) && bdi->dirty_writeback_interval)
> + schedule_timeout(msecs_to_jiffies(bdi->dirty_writeback_interval * 10));
> else {
> /*
> * We have nothing to do, so can go sleep without any
> diff -uprN a/include/linux/backing-dev.h b/include/linux/backing-dev.h
> --- a/include/linux/backing-dev.h 2011-08-05 10:29:21.000000000 +0530
> +++ b/include/linux/backing-dev.h 2011-08-09 09:15:37.094041619 +0530
> @@ -76,6 +76,8 @@ struct backing_dev_info {
>
> unsigned int min_ratio;
> unsigned int max_ratio, max_prop_frac;
> + unsigned int dirty_writeback_interval;
> + unsigned int dirty_expire_interval;
>
> struct bdi_writeback wb; /* default writeback info for this bdi */
> spinlock_t wb_lock; /* protects work_list */
> @@ -333,4 +335,5 @@ static inline int bdi_sched_wait(void *w
> return 0;
> }
>
> +extern unsigned int shortest_dirty_writeback_interval;
> #endif /* _LINUX_BACKING_DEV_H */
> diff -uprN a/include/linux/writeback.h b/include/linux/writeback.h
> --- a/include/linux/writeback.h 2011-08-05 10:29:21.000000000 +0530
> +++ b/include/linux/writeback.h 2011-08-09 10:09:23.581268260 +0530
> @@ -100,6 +100,7 @@ extern unsigned long dirty_background_by
> extern int vm_dirty_ratio;
> extern unsigned long vm_dirty_bytes;
> extern unsigned int dirty_writeback_interval;
> +extern unsigned int sync_supers_interval;
> extern unsigned int dirty_expire_interval;
> extern int vm_highmem_is_dirtyable;
> extern int block_dump;
> @@ -123,6 +124,10 @@ extern int dirty_bytes_handler(struct ct
> struct ctl_table;
> int dirty_writeback_centisecs_handler(struct ctl_table *, int,
> void __user *, size_t *, loff_t *);
> +int sync_supers_centisecs_handler(struct ctl_table *, int,
> + void __user *, size_t *, loff_t *);
> +int dirty_expire_centisecs_handler(struct ctl_table *, int,
> + void __user *, size_t *, loff_t *);
>
> void global_dirty_limits(unsigned long *pbackground, unsigned long *pdirty);
> unsigned long bdi_dirty_limit(struct backing_dev_info *bdi,
> diff -uprN a/kernel/sysctl.c b/kernel/sysctl.c
> --- a/kernel/sysctl.c 2011-08-05 10:29:21.000000000 +0530
> +++ b/kernel/sysctl.c 2011-08-09 12:39:43.453087554 +0530
> @@ -1076,12 +1076,19 @@ static struct ctl_table vm_table[] = {
> .mode = 0644,
> .proc_handler = dirty_writeback_centisecs_handler,
> },
> + {
> + .procname = "sync_supers_centisecs",
> + .data = &sync_supers_interval,
> + .maxlen = sizeof(sync_supers_interval),
> + .mode = 0644,
> + .proc_handler = sync_supers_centisecs_handler,
> + },
> {
> .procname = "dirty_expire_centisecs",
> .data = &dirty_expire_interval,
> .maxlen = sizeof(dirty_expire_interval),
> .mode = 0644,
> - .proc_handler = proc_dointvec_minmax,
> + .proc_handler = dirty_expire_centisecs_handler,
> .extra1 = &zero,
> },
> {
> diff -uprN a/mm/backing-dev.c b/mm/backing-dev.c
> --- a/mm/backing-dev.c 2011-08-05 10:29:21.000000000 +0530
> +++ b/mm/backing-dev.c 2011-08-09 12:08:06.287079027 +0530
> @@ -39,6 +39,10 @@ DEFINE_SPINLOCK(bdi_lock);
> LIST_HEAD(bdi_list);
> LIST_HEAD(bdi_pending_list);
>
> +/* Same value as the dirty_writeback_interval as this is what our
> + * initial shortest_dirty_writeback_interval. */
> +unsigned int shortest_dirty_writeback_interval = 5 * 100;
> +
> static struct task_struct *sync_supers_tsk;
> static struct timer_list sync_supers_timer;
>
> @@ -204,12 +208,50 @@ static ssize_t max_ratio_store(struct de
> }
> BDI_SHOW(max_ratio, bdi->max_ratio)
>
> +static ssize_t dirty_writeback_interval_store(struct device *dev,
> + struct device_attribute *attr, const char *buf, size_t count)
> +{
> + struct backing_dev_info *bdi = dev_get_drvdata(dev);
> + char *end;
> + unsigned int interval;
> + ssize_t ret = -EINVAL;
> +
> + interval = simple_strtoul(buf, &end, 10);
> + if (*buf && (end[0] == '\0' || (end[0] == '\n' && end[1] == '\0'))) {
> + bdi->dirty_writeback_interval = interval;
> + shortest_dirty_writeback_interval =
> + min(shortest_dirty_writeback_interval,interval);
> + ret = count;
> + }
> + return ret;
> +}
> +BDI_SHOW(dirty_writeback_interval, bdi->dirty_writeback_interval)
> +
> +static ssize_t dirty_expire_interval_store (struct device *dev,
> + struct device_attribute *attr, const char *buf, size_t count)
> +{
> + struct backing_dev_info *bdi = dev_get_drvdata(dev);
> + char *end;
> + unsigned int interval;
> + ssize_t ret = -EINVAL;
> +
> + interval = simple_strtoul(buf, &end, 10);
> + if (*buf && (end[0] == '\0' || (end[0] == '\n' && end[1] == '\0'))) {
> + bdi->dirty_expire_interval = interval;
> + ret = count;
> + }
> + return ret;
> +}
> +BDI_SHOW(dirty_expire_interval, bdi->dirty_expire_interval)
> +
> #define __ATTR_RW(attr) __ATTR(attr, 0644, attr##_show, attr##_store)
>
> static struct device_attribute bdi_dev_attrs[] = {
> __ATTR_RW(read_ahead_kb),
> __ATTR_RW(min_ratio),
> __ATTR_RW(max_ratio),
> + __ATTR_RW(dirty_writeback_interval),
> + __ATTR_RW(dirty_expire_interval),
> __ATTR_NULL,
> };
>
> @@ -291,7 +333,7 @@ void bdi_arm_supers_timer(void)
> if (!dirty_writeback_interval)
> return;
>
> - next = msecs_to_jiffies(dirty_writeback_interval * 10) + jiffies;
> + next = msecs_to_jiffies(sync_supers_interval* 10) + jiffies;
> mod_timer(&sync_supers_timer, round_jiffies_up(next));
> }
>
> @@ -336,7 +378,7 @@ void bdi_wakeup_thread_delayed(struct ba
> {
> unsigned long timeout;
>
> - timeout = msecs_to_jiffies(dirty_writeback_interval * 10);
> + timeout = msecs_to_jiffies(bdi->dirty_writeback_interval * 10);
> mod_timer(&bdi->wb.wakeup_timer, jiffies + timeout);
> }
>
> @@ -348,7 +390,19 @@ static unsigned long bdi_longest_inactiv
> {
> unsigned long interval;
>
> - interval = msecs_to_jiffies(dirty_writeback_interval * 10);
> + interval = msecs_to_jiffies(shortest_dirty_writeback_interval * 10);
> + return max(5UL * 60 * HZ, interval);
> +}
> +
> +/*
> + * Calculate the longest interval (jiffies) this bdi thread is allowed to be
> + * inactive.
> + */
> +static unsigned long bdi_longest_inactive_this(struct backing_dev_info *bdi)
> +{
> + unsigned long interval;
> +
> + interval = msecs_to_jiffies(bdi->dirty_writeback_interval * 10);
> return max(5UL * 60 * HZ, interval);
> }
>
> @@ -422,7 +476,7 @@ static int bdi_forker_thread(void *ptr)
> */
> if (bdi->wb.task && !have_dirty_io &&
> time_after(jiffies, bdi->wb.last_active +
> - bdi_longest_inactive())) {
> + bdi_longest_inactive_this(bdi))) {
> task = bdi->wb.task;
> bdi->wb.task = NULL;
> spin_unlock(&bdi->wb_lock);
> @@ -469,7 +523,7 @@ static int bdi_forker_thread(void *ptr)
> break;
>
> case NO_ACTION:
> - if (!wb_has_dirty_io(me) || !dirty_writeback_interval)
> + if (!wb_has_dirty_io(me) || !me->bdi->dirty_writeback_interval)
> /*
> * There are no dirty data. The only thing we
> * should now care about is checking for
> @@ -479,7 +533,7 @@ static int bdi_forker_thread(void *ptr)
> */
> schedule_timeout(bdi_longest_inactive());
> else
> - schedule_timeout(msecs_to_jiffies(dirty_writeback_interval * 10));
> + schedule_timeout(msecs_to_jiffies(me->bdi->dirty_writeback_interval * 10));
> try_to_freeze();
> /* Back to the main loop */
> continue;
> @@ -641,6 +695,8 @@ int bdi_init(struct backing_dev_info *bd
> bdi->min_ratio = 0;
> bdi->max_ratio = 100;
> bdi->max_prop_frac = PROP_FRAC_BASE;
> + bdi->dirty_writeback_interval = dirty_writeback_interval;
> + bdi->dirty_expire_interval = dirty_expire_interval;
> spin_lock_init(&bdi->wb_lock);
> INIT_LIST_HEAD(&bdi->bdi_list);
> INIT_LIST_HEAD(&bdi->work_list);
> diff -uprN a/mm/page-writeback.c b/mm/page-writeback.c
> --- a/mm/page-writeback.c 2011-08-05 10:29:21.000000000 +0530
> +++ b/mm/page-writeback.c 2011-08-09 13:09:37.985919961 +0530
> @@ -92,6 +92,11 @@ unsigned long vm_dirty_bytes;
> unsigned int dirty_writeback_interval = 5 * 100; /* centiseconds */
>
> /*
> + * The interval between sync_supers thread writebacks
> + */
> +unsigned int sync_supers_interval = 5 * 100; /* centiseconds */
> +
> +/*
> * The longest time for which data is allowed to remain dirty
> */
> unsigned int dirty_expire_interval = 30 * 100; /* centiseconds */
> @@ -686,8 +691,60 @@ void throttle_vm_writeout(gfp_t gfp_mask
> int dirty_writeback_centisecs_handler(ctl_table *table, int write,
> void __user *buffer, size_t *length, loff_t *ppos)
> {
> + struct backing_dev_info *bdi;
> +
> + proc_dointvec(table, write, buffer, length, ppos);
> +
> + if (write) {
> + /* Traverse all the BDIs registered to the BDI list and reset their
> + * bdi->dirty_writeback_interval to this value. */
> + spin_lock_bh(&bdi_lock);
> + list_for_each_entry(bdi, &bdi_list, bdi_list)
> + bdi->dirty_writeback_interval = dirty_writeback_interval;
> + spin_unlock_bh(&bdi_lock);
> +
> + sync_supers_interval =
> + shortest_dirty_writeback_interval = dirty_writeback_interval;
> +
> + }
> +
> + bdi_arm_supers_timer();
> +
> + return 0;
> +}
> +
> +/*
> + * sysctl handler for /proc/sys/vm/sync_supers_centisecs
> + */
> +int sync_supers_centisecs_handler(ctl_table *table, int write,
> + void __user *buffer, size_t *length, loff_t *ppos)
> +{
> proc_dointvec(table, write, buffer, length, ppos);
> +
> bdi_arm_supers_timer();
> +
> + return 0;
> +}
> +
> +/*
> + * sysctl handler for /proc/sys/vm/dirty_expire_centisecs
> + */
> +int dirty_expire_centisecs_handler(ctl_table *table, int write,
> + void __user *buffer, size_t *length, loff_t *ppos)
> +{
> + struct backing_dev_info *bdi;
> +
> + proc_dointvec_minmax(table, write, buffer, length, ppos);
> +
> + if (write) {
> + /* Traverse all the BDIs registered to the BDI list and reset their
> + * bdi->dirty_expire_interval to this value. */
> + spin_lock_bh(&bdi_lock);
> + list_for_each_entry(bdi, &bdi_list, bdi_list)
> + bdi->dirty_expire_interval = dirty_expire_interval;
> + spin_unlock_bh(&bdi_lock);
> + }
> +
> return 0;
> }
> --
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