Re: [PATCHv2 11/11] writeback: prevent unnecessary bdi threads wakeups

[Nick Piggin <npiggin@suse.de>]

On Wed, Jul 21, 2010 at 12:31:46PM +0300, Artem Bityutskiy wrote:
> @@ -973,22 +981,37 @@ void __mark_inode_dirty(struct inode *inode, int flags)
>  		 * reposition it (that would break b_dirty time-ordering).
>  		 */
>  		if (!was_dirty) {
> -			struct bdi_writeback *wb = &inode_to_bdi(inode)->wb;
> -			struct backing_dev_info *bdi = wb->bdi;
> -
> -			if (bdi_cap_writeback_dirty(bdi) &&
> -			    !test_bit(BDI_registered, &bdi->state)) {
> -				WARN_ON(1);
> -				printk(KERN_ERR "bdi-%s not registered\n",
> -								bdi->name);
> +			bdi = inode_to_bdi(inode);
> +
> +			if (bdi_cap_writeback_dirty(bdi)) {
> +				WARN(!test_bit(BDI_registered, &bdi->state),
> +				     "bdi-%s not registered\n", bdi->name);
> +
> +				/*
> +				 * If this is the first dirty inode for this
> +				 * bdi, we have to wake-up the corresponding
> +				 * bdi thread to make sure background
> +				 * write-back happens later.
> +				 */
> +				if (!wb_has_dirty_io(&bdi->wb))
> +					wakeup_bdi = true;
>  			}
>  
>  			inode->dirtied_when = jiffies;
> -			list_move(&inode->i_list, &wb->b_dirty);
> +			list_move(&inode->i_list, &bdi->wb.b_dirty);
>  		}
>  	}
>  out:
>  	spin_unlock(&inode_lock);
> +
> +	if (wakeup_bdi) {
> +		spin_lock(&bdi->wb_lock);
> +		if (!bdi->wb.task)
> +			wake_up_process(default_backing_dev_info.wb.task);
> +		else
> +			wake_up_process(bdi->wb.task);
> +		spin_unlock(&bdi->wb_lock);
> +	}
>  }

We really want to wake up the bdi right away when first dirtying
the inode? I haven't looked at where the state of the bdi code is
now, but isn't it better to have a a delay there?

And rather than spreading details of how bdi tasks are managed
would you consider putting this into its own function?

Other than that, I like your patches. Out of interest, is 5 seconds
very detremental to power usage? What is a reasonable goal for
wakeups? (eg. 95%+ of possible efficiency)