Re: large pause when opening file descriptor which is power of 2

[Al Viro <viro@zeniv.linux.org.uk>]

On Wed, Apr 26, 2023 at 08:13:37PM +0100, Matthew Wilcox wrote:
> On Wed, Apr 26, 2023 at 05:58:06PM +0000, Kernel.org Bugbot wrote:
> > When running a threaded program, and opening a file descriptor that
> > is a power of 2 (starting at 64), the call takes a very long time to
> > complete. Normally such a call takes less than 2us. However with this
> > issue, I've seen the call take up to around 50ms. Additionally this only
> > happens the first time, and not subsequent times that file descriptor is
> > used. I'm guessing there might be some expansion of some internal data
> > structures going on. But I cannot see why this process would take so long.
> 
> Because we allocate a new block of memory and then memcpy() the old
> block of memory into it.  This isn't surprising behaviour to me.
> I don't think there's much we can do to change it (Allocating a
> segmented array of file descriptors has previously been vetoed by
> people who have programs with a million file descriptors).  Is it
> causing you problems?

FWIW, I suspect that this is not so much allocation + memcpy.
        /* make sure all fd_install() have seen resize_in_progress
	 * or have finished their rcu_read_lock_sched() section.
	 */
	if (atomic_read(&files->count) > 1)
		synchronize_rcu();

in expand_fdtable() is a likelier source of delays.