Re: [RFC] Bypass filesystems for reading cached pages

[Andreas Gruenbacher <agruenba@redhat.com>]

On Tue, Jun 23, 2020 at 2:52 AM Dave Chinner <david@fromorbit.com> wrote:
> On Mon, Jun 22, 2020 at 04:35:05PM +0200, Andreas Gruenbacher wrote:
> > On Mon, Jun 22, 2020 at 2:32 AM Dave Chinner <david@fromorbit.com> wrote:
> > > On Fri, Jun 19, 2020 at 08:50:36AM -0700, Matthew Wilcox wrote:
> > > >
> > > > This patch lifts the IOCB_CACHED idea expressed by Andreas to the VFS.
> > > > The advantage of this patch is that we can avoid taking any filesystem
> > > > lock, as long as the pages being accessed are in the cache (and we don't
> > > > need to readahead any pages into the cache).  We also avoid an indirect
> > > > function call in these cases.
> > >
> > > What does this micro-optimisation actually gain us except for more
> > > complexity in the IO path?
> > >
> > > i.e. if a filesystem lock has such massive overhead that it slows
> > > down the cached readahead path in production workloads, then that's
> > > something the filesystem needs to address, not unconditionally
> > > bypass the filesystem before the IO gets anywhere near it.
> >
> > I'm fine with not moving that functionality into the VFS. The problem
> > I have in gfs2 is that taking glocks is really expensive. Part of that
> > overhead is accidental, but we definitely won't be able to fix it in
> > the short term. So something like the IOCB_CACHED flag that prevents
> > generic_file_read_iter from issuing readahead I/O would save the day
> > for us. Does that idea stand a chance?
>
> I have no problem with a "NOREADAHEAD" flag being passed to
> generic_file_read_iter(). It's not a "already cached" flag though,
> it's a "don't start any IO" directive, just like the NOWAIT flag is
> a "don't block on locks or IO in progress" directive and not an
> "already cached" flag. Readahead is something we should be doing,
> unless a filesystem has a very good reason not to, such as the gfs2
> locking case here...

The requests coming in can have the IOCB_NOWAIT flag set or cleared.
The idea was to have an additional flag that implies IOCB_NOWAIT so
that you can do:

    iocb->ki_flags |= IOCB_NOIO;
    generic_file_read_iter()
    if ("failed because of IOCB_NOIO") {
        if ("failed because of IOCB_NOWAIT")
            return -EAGAIN;
        iocb->ki_flags &= ~IOCB_NOIO;
        "locking"
         generic_file_read_iter()
        "unlocking"
    }

without having to save iocb->ki_flags. The alternative would be:

    int flags = iocb->ki_flags;
    iocb->ki_flags |= IOCB_NOIO | IOCB_NOWAIT;
    ret = generic_file_read_iter()
    if ("failed because of IOCB_NOIO or IOCB_NOWAIT") {
        if ("failed because of IOCB_NOWAIT" && (flags & IOCB_NOWAIT))
            return -EAGAIN;
        iocb->ki_flags &= ~IOCB_NOIO;
        "locking"
         generic_file_read_iter()
        "unlocking"
    }

Andreas