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SubjectRe: [PATCH 18/45] writeback: introduce wait queue for balance_dirty_pages()
On Wed, 07 Oct 2009 15:38:36 +0800
Wu Fengguang <fengguang.wu@intel.com> wrote:

> As proposed by Chris, Dave and Jan, let balance_dirty_pages() wait for
> the per-bdi flusher to writeback enough pages for it, instead of
> starting foreground writeback by itself. By doing so we harvest two
> benefits:
> - avoid concurrent writeback of multiple inodes (Dave Chinner)
> If every thread doing writes and being throttled start foreground
> writeback, it leads to N IO submitters from at least N different
> inodes at the same time, end up with N different sets of IO being
> issued with potentially zero locality to each other, resulting in
> much lower elevator sort/merge efficiency and hence we seek the disk
> all over the place to service the different sets of IO.
> OTOH, if there is only one submission thread, it doesn't jump between
> inodes in the same way when congestion clears - it keeps writing to
> the same inode, resulting in large related chunks of sequential IOs
> being issued to the disk. This is more efficient than the above
> foreground writeback because the elevator works better and the disk
> seeks less.
> - avoid one constraint torwards huge per-file nr_to_write
> The write_chunk used by balance_dirty_pages() should be small enough to
> prevent user noticeable one-shot latency. Ie. each sleep/wait inside
> balance_dirty_pages() shall be small enough. When it starts its own
> writeback, it must specify a small nr_to_write. The throttle wait queue
> removes this dependancy by the way.
>

May I ask a question ? (maybe not directly related to this patch itself, sorry)

Recent works as "writeback: switch to per-bdi threads for flushing data"
removed congestion_wait() from balance_dirty_pages() and added
schedule_timeout_interruptible().

And this one replaces it with wake_up+wait_queue.

IIUC, "iowait" cpustat data was calculated by runqueue->nr_iowait as
== kernel/schec.c
void account_idle_time(cputime_t cputime)
{
struct cpu_usage_stat *cpustat = &kstat_this_cpu.cpustat;
cputime64_t cputime64 = cputime_to_cputime64(cputime);
struct rq *rq = this_rq();
if (atomic_read(&rq->nr_iowait) > 0)
cpustat->iowait = cputime64_add(cpustat->iowait, cputime64);
else
cpustat->idle = cputime64_add(cpustat->idle, cputime64);
}
==
Then, for showing "cpu is in iowait", runqueue->nr_iowait should be modified
at some places. In old kernel, congestion_wait() at el did that by calling
io_schedule_timeout().
How this runqueue->nr_iowait is handled now ?

Thanks,
-Kame




> CC: Chris Mason <chris.mason@oracle.com>
> CC: Dave Chinner <david@fromorbit.com>
> CC: Jan Kara <jack@suse.cz>
> CC: Peter Zijlstra <a.p.zijlstra@chello.nl>
> CC: Jens Axboe <jens.axboe@oracle.com>
> Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
> ---
> fs/fs-writeback.c | 71 ++++++++++++++++++++++++++++++++++
> include/linux/backing-dev.h | 15 +++++++
> mm/backing-dev.c | 4 +
> mm/page-writeback.c | 53 ++++++-------------------
> 4 files changed, 103 insertions(+), 40 deletions(-)
>
> --- linux.orig/mm/page-writeback.c 2009-10-06 23:38:30.000000000 +0800
> +++ linux/mm/page-writeback.c 2009-10-06 23:38:43.000000000 +0800
> @@ -218,6 +218,15 @@ static inline void __bdi_writeout_inc(st
> {
> __prop_inc_percpu_max(&vm_completions, &bdi->completions,
> bdi->max_prop_frac);
> +
> + /*
> + * The DIRTY_THROTTLE_PAGES_STOP test is an optional optimization, so
> + * it's OK to be racy. We set DIRTY_THROTTLE_PAGES_STOP*2 in other
> + * places to reduce the race possibility.
> + */
> + if (atomic_read(&bdi->throttle_pages) < DIRTY_THROTTLE_PAGES_STOP &&
> + atomic_dec_and_test(&bdi->throttle_pages))
> + bdi_writeback_wakeup(bdi);
> }
>
> void bdi_writeout_inc(struct backing_dev_info *bdi)
> @@ -458,20 +467,10 @@ static void balance_dirty_pages(struct a
> unsigned long background_thresh;
> unsigned long dirty_thresh;
> unsigned long bdi_thresh;
> - unsigned long pages_written = 0;
> - unsigned long pause = 1;
> int dirty_exceeded;
> struct backing_dev_info *bdi = mapping->backing_dev_info;
>
> for (;;) {
> - struct writeback_control wbc = {
> - .bdi = bdi,
> - .sync_mode = WB_SYNC_NONE,
> - .older_than_this = NULL,
> - .nr_to_write = write_chunk,
> - .range_cyclic = 1,
> - };
> -
> nr_reclaimable = global_page_state(NR_FILE_DIRTY) +
> global_page_state(NR_UNSTABLE_NFS);
> nr_writeback = global_page_state(NR_WRITEBACK) +
> @@ -518,39 +517,13 @@ static void balance_dirty_pages(struct a
> if (!bdi->dirty_exceeded)
> bdi->dirty_exceeded = 1;
>
> - /* Note: nr_reclaimable denotes nr_dirty + nr_unstable.
> - * Unstable writes are a feature of certain networked
> - * filesystems (i.e. NFS) in which data may have been
> - * written to the server's write cache, but has not yet
> - * been flushed to permanent storage.
> - * Only move pages to writeback if this bdi is over its
> - * threshold otherwise wait until the disk writes catch
> - * up.
> - */
> - if (bdi_nr_reclaimable > bdi_thresh) {
> - writeback_inodes_wbc(&wbc);
> - pages_written += write_chunk - wbc.nr_to_write;
> - /* don't wait if we've done enough */
> - if (pages_written >= write_chunk)
> - break;
> - }
> - schedule_timeout_interruptible(pause);
> -
> - /*
> - * Increase the delay for each loop, up to our previous
> - * default of taking a 100ms nap.
> - */
> - pause <<= 1;
> - if (pause > HZ / 10)
> - pause = HZ / 10;
> + bdi_writeback_wait(bdi, write_chunk);
> + break;
> }
>
> if (!dirty_exceeded && bdi->dirty_exceeded)
> bdi->dirty_exceeded = 0;
>
> - if (writeback_in_progress(bdi))
> - return;
> -
> /*
> * In laptop mode, we wait until hitting the higher threshold before
> * starting background writeout, and then write out all the way down
> @@ -559,8 +532,8 @@ static void balance_dirty_pages(struct a
> * In normal mode, we start background writeout at the lower
> * background_thresh, to keep the amount of dirty memory low.
> */
> - if ((laptop_mode && pages_written) ||
> - (!laptop_mode && (nr_reclaimable > background_thresh)))
> + if (!laptop_mode && (nr_reclaimable > background_thresh) &&
> + can_submit_background_writeback(bdi))
> bdi_start_writeback(bdi, NULL, 0);
> }
>
> --- linux.orig/include/linux/backing-dev.h 2009-10-06 23:38:43.000000000 +0800
> +++ linux/include/linux/backing-dev.h 2009-10-06 23:38:43.000000000 +0800
> @@ -86,6 +86,13 @@ struct backing_dev_info {
>
> struct list_head work_list;
>
> + /*
> + * dirtier process throttling
> + */
> + spinlock_t throttle_lock;
> + struct list_head throttle_list; /* nr to sync for each task */
> + atomic_t throttle_pages; /* nr to sync for head task */
> +
> struct device *dev;
>
> #ifdef CONFIG_DEBUG_FS
> @@ -99,6 +106,12 @@ struct backing_dev_info {
> */
> #define WB_FLAG_BACKGROUND_WORK 30
>
> +/*
> + * when no task is throttled, set throttle_pages to larger than this,
> + * to avoid unnecessary atomic decreases.
> + */
> +#define DIRTY_THROTTLE_PAGES_STOP (1 << 22)
> +
> int bdi_init(struct backing_dev_info *bdi);
> void bdi_destroy(struct backing_dev_info *bdi);
>
> @@ -110,6 +123,8 @@ void bdi_start_writeback(struct backing_
> long nr_pages);
> int bdi_writeback_task(struct bdi_writeback *wb);
> int bdi_has_dirty_io(struct backing_dev_info *bdi);
> +int bdi_writeback_wakeup(struct backing_dev_info *bdi);
> +void bdi_writeback_wait(struct backing_dev_info *bdi, long nr_pages);
>
> extern spinlock_t bdi_lock;
> extern struct list_head bdi_list;
> --- linux.orig/fs/fs-writeback.c 2009-10-06 23:38:43.000000000 +0800
> +++ linux/fs/fs-writeback.c 2009-10-06 23:38:43.000000000 +0800
> @@ -25,6 +25,7 @@
> #include <linux/blkdev.h>
> #include <linux/backing-dev.h>
> #include <linux/buffer_head.h>
> +#include <linux/completion.h>
> #include "internal.h"
>
> #define inode_to_bdi(inode) ((inode)->i_mapping->backing_dev_info)
> @@ -265,6 +266,72 @@ void bdi_start_writeback(struct backing_
> bdi_alloc_queue_work(bdi, &args);
> }
>
> +struct dirty_throttle_task {
> + long nr_pages;
> + struct list_head list;
> + struct completion complete;
> +};
> +
> +void bdi_writeback_wait(struct backing_dev_info *bdi, long nr_pages)
> +{
> + struct dirty_throttle_task tt = {
> + .nr_pages = nr_pages,
> + .complete = COMPLETION_INITIALIZER_ONSTACK(tt.complete),
> + };
> + unsigned long flags;
> +
> + /*
> + * register throttle pages
> + */
> + spin_lock_irqsave(&bdi->throttle_lock, flags);
> + if (list_empty(&bdi->throttle_list))
> + atomic_set(&bdi->throttle_pages, nr_pages);
> + list_add(&tt.list, &bdi->throttle_list);
> + spin_unlock_irqrestore(&bdi->throttle_lock, flags);
> +
> + /*
> + * make sure we will be woke up by someone
> + */
> + if (can_submit_background_writeback(bdi))
> + bdi_start_writeback(bdi, NULL, 0);
> +
> + wait_for_completion(&tt.complete);
> +}
> +
> +/*
> + * return 1 if there are more waiting tasks.
> + */
> +int bdi_writeback_wakeup(struct backing_dev_info *bdi)
> +{
> + struct dirty_throttle_task *tt;
> + unsigned long flags;
> +
> + spin_lock_irqsave(&bdi->throttle_lock, flags);
> + /*
> + * remove and wakeup head task
> + */
> + if (!list_empty(&bdi->throttle_list)) {
> + tt = list_entry(bdi->throttle_list.prev,
> + struct dirty_throttle_task, list);
> + list_del(&tt->list);
> + complete(&tt->complete);
> + }
> + /*
> + * update throttle pages
> + */
> + if (!list_empty(&bdi->throttle_list)) {
> + tt = list_entry(bdi->throttle_list.prev,
> + struct dirty_throttle_task, list);
> + atomic_set(&bdi->throttle_pages, tt->nr_pages);
> + } else {
> + tt = NULL;
> + atomic_set(&bdi->throttle_pages, DIRTY_THROTTLE_PAGES_STOP * 2);
> + }
> + spin_unlock_irqrestore(&bdi->throttle_lock, flags);
> +
> + return tt != NULL;
> +}
> +
> /*
> * Redirty an inode: set its when-it-was dirtied timestamp and move it to the
> * furthest end of its superblock's dirty-inode list.
> @@ -760,6 +827,10 @@ static long wb_writeback(struct bdi_writ
> spin_unlock(&inode_lock);
> }
>
> + if (args->for_background)
> + while (bdi_writeback_wakeup(wb->bdi))
> + ; /* unthrottle all tasks */
> +
> return wrote;
> }
>
> --- linux.orig/mm/backing-dev.c 2009-10-06 23:37:47.000000000 +0800
> +++ linux/mm/backing-dev.c 2009-10-06 23:38:43.000000000 +0800
> @@ -646,6 +646,10 @@ int bdi_init(struct backing_dev_info *bd
> bdi->wb_mask = 1;
> bdi->wb_cnt = 1;
>
> + spin_lock_init(&bdi->throttle_lock);
> + INIT_LIST_HEAD(&bdi->throttle_list);
> + atomic_set(&bdi->throttle_pages, DIRTY_THROTTLE_PAGES_STOP * 2);
> +
> for (i = 0; i < NR_BDI_STAT_ITEMS; i++) {
> err = percpu_counter_init(&bdi->bdi_stat[i], 0);
> if (err)
>
>
> --
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