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SubjectRe: [PATCH 7/7] fs-writeback: only allow one inflight and pending full flush
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On 09/25/2017 11:35 AM, Jan Kara wrote:
> On Thu 21-09-17 10:00:25, Jens Axboe wrote:
>> On 09/21/2017 09:36 AM, Jens Axboe wrote:
>>>> But more importantly once we are not guaranteed that we only have
>>>> a single global wb_writeback_work per bdi_writeback we should just
>>>> embedd that into struct bdi_writeback instead of dynamically
>>>> allocating it.
>>>
>>> We could do this as a followup. But right now the logic is that we
>>> can have on started (inflight), and still have one new queued.
>>
>> Something like the below would fit on top to do that. Gets rid of the
>> allocation and embeds the work item for global start-all in the
>> bdi_writeback structure.
>
> Hum, so when we consider stuff like embedded work item, I would somewhat
> prefer to handle this like we do for for_background and for_kupdate style
> writeback so that we don't have another special case. For these don't queue
> any item, we just queue writeback work into the workqueue (via
> wb_wakeup()). When flusher work gets processed wb_do_writeback() checks
> (after processing all normal writeback requests) whether conditions for
> these special writeback styles are met and if yes, it creates on-stack work
> item and processes it (see wb_check_old_data_flush() and
> wb_check_background_flush()).
>
> So in this case we would just set some flag in bdi_writeback when memory
> reclaim needs help and wb_do_writeback() would check for this flag and
> create and process writeback-all style writeback work. Granted this does
> not preserve ordering of requests (basically any specific request gets
> priority over writeback-whole-world request) but memory gets cleaned in
> either case so flusher should be doing what is needed.

How about something like the below? It's on top of the latest series,
which is in my wb-start-all branch. It handles start_all like the
background/kupdate style writeback, reusing the WB_start_all bit for
that.

On a plane, so untested, but it seems pretty straight forward. It
changes the logic a little bit, as the WB_start_all bit isn't cleared
until after we're done with a flush-all request. At this point it's
truly on inflight at any point in time, not one inflight and one
potentially queued.

diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index 399619c97567..9e24d604c59c 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -53,7 +53,6 @@ struct wb_writeback_work {
unsigned int for_background:1;
unsigned int for_sync:1; /* sync(2) WB_SYNC_ALL writeback */
unsigned int auto_free:1; /* free on completion */
- unsigned int start_all:1; /* nr_pages == 0 (all) writeback */
enum wb_reason reason; /* why was writeback initiated? */

struct list_head list; /* pending work list */
@@ -947,8 +946,6 @@ static unsigned long get_nr_dirty_pages(void)

static void wb_start_writeback(struct bdi_writeback *wb, enum wb_reason reason)
{
- struct wb_writeback_work *work;
-
if (!wb_has_dirty_io(wb))
return;

@@ -958,35 +955,14 @@ static void wb_start_writeback(struct bdi_writeback *wb, enum wb_reason reason)
* high frequency, causing pointless allocations of tons of
* work items and keeping the flusher threads busy retrieving
* that work. Ensure that we only allow one of them pending and
- * inflight at the time. It doesn't matter if we race a little
- * bit on this, so use the faster separate test/set bit variants.
+ * inflight at the time.
*/
- if (test_bit(WB_start_all, &wb->state))
+ if (test_bit(WB_start_all, &wb->state) ||
+ test_and_set_bit(WB_start_all, &wb->state))
return;

- set_bit(WB_start_all, &wb->state);
-
- /*
- * This is WB_SYNC_NONE writeback, so if allocation fails just
- * wakeup the thread for old dirty data writeback
- */
- work = kzalloc(sizeof(*work),
- GFP_NOWAIT | __GFP_NOMEMALLOC | __GFP_NOWARN);
- if (!work) {
- clear_bit(WB_start_all, &wb->state);
- trace_writeback_nowork(wb);
- wb_wakeup(wb);
- return;
- }
-
- work->sync_mode = WB_SYNC_NONE;
- work->nr_pages = wb_split_bdi_pages(wb, get_nr_dirty_pages());
- work->range_cyclic = 1;
- work->reason = reason;
- work->auto_free = 1;
- work->start_all = 1;
-
- wb_queue_work(wb, work);
+ wb->start_all_reason = reason;
+ wb_wakeup(wb);
}

/**
@@ -1838,14 +1814,6 @@ static struct wb_writeback_work *get_next_work_item(struct bdi_writeback *wb)
list_del_init(&work->list);
}
spin_unlock_bh(&wb->work_lock);
-
- /*
- * Once we start processing a work item that had !nr_pages,
- * clear the wb state bit for that so we can allow more.
- */
- if (work && work->start_all)
- clear_bit(WB_start_all, &wb->state);
-
return work;
}

@@ -1901,6 +1869,30 @@ static long wb_check_old_data_flush(struct bdi_writeback *wb)
return 0;
}

+static long wb_check_start_all(struct bdi_writeback *wb)
+{
+ long nr_pages;
+
+ if (!test_bit(WB_start_all, &wb->state))
+ return 0;
+
+ nr_pages = get_nr_dirty_pages();
+ if (nr_pages) {
+ struct wb_writeback_work work = {
+ .nr_pages = wb_split_bdi_pages(wb, nr_pages),
+ .sync_mode = WB_SYNC_NONE,
+ .range_cyclic = 1,
+ .reason = wb->start_all_reason,
+ };
+
+ nr_pages = wb_writeback(wb, &work);
+ }
+
+ clear_bit(WB_start_all, &wb->state);
+ return nr_pages;
+}
+
+
/*
* Retrieve work items and do the writeback they describe
*/
@@ -1917,6 +1909,11 @@ static long wb_do_writeback(struct bdi_writeback *wb)
}

/*
+ * Check for a flush-everything request
+ */
+ wrote += wb_check_start_all(wb);
+
+ /*
* Check for periodic writeback, kupdated() style
*/
wrote += wb_check_old_data_flush(wb);
diff --git a/include/linux/backing-dev-defs.h b/include/linux/backing-dev-defs.h
index 420de5c7c7f9..f0f1df29d6b8 100644
--- a/include/linux/backing-dev-defs.h
+++ b/include/linux/backing-dev-defs.h
@@ -116,6 +116,7 @@ struct bdi_writeback {

struct fprop_local_percpu completions;
int dirty_exceeded;
+ int start_all_reason;

spinlock_t work_lock; /* protects work_list & dwork scheduling */
struct list_head work_list;
diff --git a/include/trace/events/writeback.h b/include/trace/events/writeback.h
index 9b57f014d79d..19a0ea08e098 100644
--- a/include/trace/events/writeback.h
+++ b/include/trace/events/writeback.h
@@ -286,7 +286,6 @@ DEFINE_EVENT(writeback_class, name, \
TP_PROTO(struct bdi_writeback *wb), \
TP_ARGS(wb))

-DEFINE_WRITEBACK_EVENT(writeback_nowork);
DEFINE_WRITEBACK_EVENT(writeback_wake_background);

TRACE_EVENT(writeback_bdi_register,
--
Jens Axboe

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