Re: [PATCH 1/3] fuse: remove reliance on bdi congestion

[Matthew Wilcox <willy@infradead.org>]

On Mon, Jan 31, 2022 at 03:47:41PM +1100, NeilBrown wrote:
> On Mon, 31 Jan 2022, Matthew Wilcox wrote:
> > > +++ b/fs/fuse/file.c
> > > @@ -958,6 +958,8 @@ static void fuse_readahead(struct readahead_control *rac)
> > >  
> > >  	if (fuse_is_bad(inode))
> > >  		return;
> > > +	if (fc->num_background >= fc->congestion_threshold)
> > > +		return;
> > 
> > This seems like a bad idea to me.  If we don't even start reads on
> > readahead pages, they'll get ->readpage called on them one at a time
> > and the reading thread will block.  It's going to lead to some nasty
> > performance problems, exactly when you don't want them.  Better to
> > queue the reads internally and wait for congestion to ease before
> > submitting the read.
> > 
> 
> Isn't that exactly what happens now? page_cache_async_ra() sees that
> inode_read_congested() returns true, so it doesn't start readahead.
> ???

It's rather different.  Imagine the readahead window has expanded to
256kB (64 pages).  Today, we see congestion and don't do anything.
That means we miss the async readahed opportunity, find a missing
page and end up calling into page_cache_sync_ra(), by which time
we may or may not be congested.

If the inode_read_congested() in page_cache_async_ra() is removed and
the patch above is added to replace it, we'll allocate those 64 pages and
add them to the page cache.  But then we'll return without starting IO.
When we hit one of those !uptodate pages, we'll call ->readpage on it,
but we won't do anything to the other 63 pages.  So we'll go through a
protracted slow period of sending 64 reads, one at a time, whether or
not congestion has eased.  Then we'll hit a missing page and proceed
to the sync ra case as above.

(I'm assuming this is a workload which does a linear scan and so
readahead is actually effective)