Re: [PATCH 7/7] fs-writeback: only allow one inflight and pending full flush

[Jan Kara]

On Thu 28-09-17 20:09:50, Jens Axboe wrote:
> 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.

Thanks for looking into this! I like the change.

								Honza

> 
> 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
> 
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR