From mboxrd@z Thu Jan  1 00:00:00 1970
From: Christoph Hellwig <hch@infradead.org>
Subject: Re: [PATCH 6/6] vfs: Avoid unnecessary WB_SYNC_NONE writeback during
 sys_sync and reorder sync passes
Date: Thu, 20 Oct 2011 05:57:25 -0400
Message-ID: <20111020095725.GF11291@infradead.org>
References: <1318020055-4450-1-git-send-email-jack@suse.cz>
 <1318020055-4450-7-git-send-email-jack@suse.cz>
Mime-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Cc: Christoph Hellwig <hch@infradead.org>,
	Curt Wohlgemuth <curtw@google.com>,
	linux-fsdevel@vger.kernel.org, Al Viro <viro@ZenIV.linux.org.uk>
To: Jan Kara <jack@suse.cz>
Return-path: <linux-fsdevel-owner@vger.kernel.org>
Received: from 173-166-109-252-newengland.hfc.comcastbusiness.net ([173.166.109.252]:43520
	"EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK)
	by vger.kernel.org with ESMTP id S1754147Ab1JTJ51 (ORCPT
	<rfc822;linux-fsdevel@vger.kernel.org>);
	Thu, 20 Oct 2011 05:57:27 -0400
Content-Disposition: inline
In-Reply-To: <1318020055-4450-7-git-send-email-jack@suse.cz>
Sender: linux-fsdevel-owner@vger.kernel.org
List-ID: <linux-fsdevel.vger.kernel.org>

On Fri, Oct 07, 2011 at 10:40:55PM +0200, Jan Kara wrote:
> wakeup_flusher_threads(0) will queue work doing complete writeback for each
> flusher thread. Thus there is not much point in submitting another work doing
> full inode WB_SYNC_NONE writeback by writeback_inodes_sb().
> 
> After this change it does not make sense to call nonblocking ->sync_fs and
> block device flush before calling sync_inodes_sb() because
> wakeup_flusher_threads() is completely asynchronous and thus these functions
> would be called in parallel with inode writeback running which will effectively
> void any work they do. So we move sync_inodes_sb() call before these two
> functions.

>  	wakeup_flusher_threads(0);
> -	iterate_supers(sync_inodes_one_sb, &nowait);
> +	iterate_supers(sync_inodes_one_sb, &wait);
>  	iterate_supers(sync_fs_one_sb, &nowait);
>  	sync_all_bdevs(nowait);
> -	iterate_supers(sync_inodes_one_sb, &wait);
>  	iterate_supers(sync_fs_one_sb, &wait);
>  	sync_all_bdevs(wait);
>  	if (unlikely(laptop_mode))

This looks a bit half-assed to me.  Why do we still do the non-blocking
sync_all_bdevs call?  This really only starts async writeback on the
block device inodes, which wakeup_flusher_threads already did.
Similarly I don't think the non-blocking ->sync_fs call really make
much sense anymore here.

Also we really need some good comment on why the order is
like the one chose here in this function.