Re: [PATCH 1/2] test_xarray: add tests for advanced multi-index use

[Matthew Wilcox <willy@infradead.org>]

On Fri, Jan 26, 2024 at 12:04:44PM -0800, Luis Chamberlain wrote:
> > We have a perfectly good system for "relaxing":
> > 
> >         xas_for_each_marked(&xas, page, end, PAGECACHE_TAG_DIRTY) {
> >                 xas_set_mark(&xas, PAGECACHE_TAG_TOWRITE);
> >                 if (++tagged % XA_CHECK_SCHED)
> >                         continue;
> > 
> >                 xas_pause(&xas);
> >                 xas_unlock_irq(&xas);
> >                 cond_resched();
> >                 xas_lock_irq(&xas);
> >         }
> 
> And yet we can get a soft lockup with order 20 (1,048,576 entries),
> granted busy looping over 1 million entries is insane, but it seems it
> the existing code may not be enough to avoid the soft lockup. Also
> cond_resched() may be eventually removed [0].

what?  you're in charge of when you sleep.  you can do this:

unsigned i = 0;
rcu_read_lock();
xas_for_each(...) {
	...
	if (iter++ % XA_CHECK_SCHED)
		continue;
	xas_pause();
	rcu_read_unlock();
	rcu_read_lock();
}
rcu_read_unlock();

and that will get rid of the rcu warnings.  right?