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

[Luis Chamberlain]

On Fri, Jan 26, 2024 at 08:32:28PM +0000, Matthew Wilcox wrote:
> 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?

The RCU warning was due to my getting an try call missing an RCU lock,
I fixed that. The pending issue was a soft lockup that I get on low end systems
testing test_xarray with higher order but after testing on a 2 vcpus
with only 2 GiB of RAM I cannot reproduce so we can address this later.
I forget the exact type of low end system I tested this on... but anyway
I can't reproduce now. I suspect it may have been similar to the issue
0-day had found long ago and you noted an overloaded system [0]

[0] https://lore.kernel.org/all/20190306120843.GI13380@xxxxxxxxxxxxxxxxxxxxxx/

  Luis