Date:   Mon, 13 Dec 2021 16:07:36 +1100
From:   Dave Chinner <david@fromorbit.com>
To:     NeilBrown <neilb@suse.de>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Mel Gorman <mgorman@suse.de>,
        Philipp Reisner <philipp.reisner@linbit.com>,
        Lars Ellenberg <lars.ellenberg@linbit.com>,
        Jan Kara <jack@suse.com>,
        Ryusuke Konishi <konishi.ryusuke@gmail.com>,
        "Darrick J. Wong" <djwong@kernel.org>, linux-xfs@vger.kernel.org,
        linux-ext4@vger.kernel.org, linux-nilfs@vger.kernel.org,
        linux-mm@kvack.org, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2/2] Remove bdi_congested() and wb_congested() and
 related functions
Message-ID: <20211213050736.GS449541@dread.disaster.area>
References: <163936868317.23860.5037433897004720387.stgit@noble.brown>
 <163936886727.23860.5245364396572576756.stgit@noble.brown>
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On Mon, Dec 13, 2021 at 03:14:27PM +1100, NeilBrown wrote:
> These functions are no longer useful as the only bdis that report
> congestion are in ceph, fuse, and nfs.  None of those bdis can be the
> target of the calls in drbd, ext2, nilfs2, or xfs.
> 
> Removing the test on bdi_write_contested() in current_may_throttle()
> could cause a small change in behaviour, but only when PF_LOCAL_THROTTLE
> is set.
> 
> So replace the calls by 'false' and simplify the code - and remove the
> functions.
> 
> Signed-off-by: NeilBrown <neilb@suse.de>
....
> diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c
> index 631c5a61d89b..22f73b3e888e 100644
> --- a/fs/xfs/xfs_buf.c
> +++ b/fs/xfs/xfs_buf.c
> @@ -843,9 +843,6 @@ xfs_buf_readahead_map(
>  {
>  	struct xfs_buf		*bp;
>  
> -	if (bdi_read_congested(target->bt_bdev->bd_disk->bdi))
> -		return;

Ok, but this isn't a "throttle writeback" test here - it's trying to
avoid having speculative readahead blocking on a full request queue
instead of just skipping the readahead IO. i.e. prevent readahead
thrashing and/or adding unnecessary read load when we already have a
full read queue...

So what is the replacement for that? We want to skip the entire
buffer lookup/setup/read overhead if we're likely to block on IO
submission - is there anything we can use to do this these days?

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com