Re: [PATCH v1] rcu: Fix and improve RCU read lock checks when !CONFIG_DEBUG_LOCK_ALLOC

[Paul E. McKenney]

On Thu, Jul 13, 2023 at 09:33:35AM -0700, Paul E. McKenney wrote:
> On Thu, Jul 13, 2023 at 11:33:24AM -0400, Joel Fernandes wrote:
> > On Thu, Jul 13, 2023 at 10:34 AM Gao Xiang <hsiangkao@xxxxxxxxxxxxxxxxx> wrote:
> > > On 2023/7/13 22:07, Joel Fernandes wrote:
> > > > On Thu, Jul 13, 2023 at 12:59 AM Gao Xiang <hsiangkao@xxxxxxxxxxxxxxxxx> wrote:
> > > >> On 2023/7/13 12:52, Paul E. McKenney wrote:
> > > >>> On Thu, Jul 13, 2023 at 12:41:09PM +0800, Gao Xiang wrote:
> > > >>
> > > >> ...
> > > >>
> > > >>>>
> > > >>>> There are lots of performance issues here and even a plumber
> > > >>>> topic last year to show that, see:
> > > >>>>
> > > >>>> [1] https://lore.kernel.org/r/20230519001709.2563-1-tj@xxxxxxxxxx
> > > >>>> [2] https://lore.kernel.org/r/CAHk-=wgE9kORADrDJ4nEsHHLirqPCZ1tGaEPAZejHdZ03qCOGg@xxxxxxxxxxxxxx
> > > >>>> [3] https://lore.kernel.org/r/CAB=BE-SBtO6vcoyLNA9F-9VaN5R0t3o_Zn+FW8GbO6wyUqFneQ@xxxxxxxxxxxxxx
> > > >>>> [4] https://lpc.events/event/16/contributions/1338/
> > > >>>> and more.
> > > >>>>
> > > >>>> I'm not sure if it's necessary to look info all of that,
> > > >>>> andSandeep knows more than I am (the scheduling issue
> > > >>>> becomes vital on some aarch64 platform.)
> > > >>>
> > > >>> Hmmm...  Please let me try again.
> > > >>>
> > > >>> Assuming that this approach turns out to make sense, the resulting
> > > >>> patch will need to clearly state the performance benefits directly in
> > > >>> the commit log.
> > > >>>
> > > >>> And of course, for the approach to make sense, it must avoid breaking
> > > >>> the existing lockdep-RCU debugging code.
> > > >>>
> > > >>> Is that more clear?
> > > >>
> > > >> Personally I'm not working on Android platform any more so I don't
> > > >> have a way to reproduce, hopefully Sandeep could give actually
> > > >> number _again_ if dm-verity is enabled and trigger another
> > > >> workqueue here and make a comparsion why the scheduling latency of
> > > >> the extra work becomes unacceptable.
> > > >>
> > > >
> > > > Question from my side, are we talking about only performance issues or
> > > > also a crash? It appears z_erofs_decompress_pcluster() takes
> > > > mutex_lock(&pcl->lock);
> > > >
> > > > So if it is either in an RCU read-side critical section or in an
> > > > atomic section, like the softirq path, then it may
> > > > schedule-while-atomic or trigger RCU warnings.
> > > >
> > > > z_erofs_decompressqueue_endio
> > > > -> z_erofs_decompress_kickoff
> > > >   ->z_erofs_decompressqueue_work
> > > >    ->z_erofs_decompress_queue
> > > >     -> z_erofs_decompress_pcluster
> > > >      -> mutex_lock
> > > >
> > >
> > > Why does the softirq path not trigger a workqueue instead?
> > 
> > I said "if it is". I was giving a scenario. mutex_lock() is not
> > allowed in softirq context or in an RCU-reader.
> > 
> > > > Per Sandeep in [1], this stack happens under RCU read-lock in:
> > > >
> > > > #define __blk_mq_run_dispatch_ops(q, check_sleep, dispatch_ops) \
> > > > [...]
> > > >                  rcu_read_lock();
> > > >                  (dispatch_ops);
> > > >                  rcu_read_unlock();
> > > > [...]
> > > >
> > > > Coming from:
> > > > blk_mq_flush_plug_list ->
> > > >                             blk_mq_run_dispatch_ops(q,
> > > >                                  __blk_mq_flush_plug_list(q, plug));
> > > >
> > > > and __blk_mq_flush_plug_list does this:
> > > >            q->mq_ops->queue_rqs(&plug->mq_list);
> > > >
> > > > This somehow ends up calling the bio_endio and the
> > > > z_erofs_decompressqueue_endio which grabs the mutex.
> > > >
> > > > So... I have a question, it looks like one of the paths in
> > > > __blk_mq_run_dispatch_ops() uses SRCU.  Where are as the alternate
> > > > path uses RCU. Why does this alternate want to block even if it is not
> > > > supposed to? Is the real issue here that the BLK_MQ_F_BLOCKING should
> > > > be set? It sounds like you want to block in the "else" path even
> > > > though BLK_MQ_F_BLOCKING is not set:
> > >
> > > BLK_MQ_F_BLOCKING is not a flag that a filesystem can do anything with.
> > > That is block layer and mq device driver stuffs. filesystems cannot set
> > > this value.
> > >
> > > As I said, as far as I understand, previously,
> > > .end_io() can only be called without RCU context, so it will be fine,
> > > but I don't know when .end_io() can be called under some RCU context
> > > now.
> > 
> > >From what Sandeep described, the code path is in an RCU reader. My
> > question is more, why doesn't it use SRCU instead since it clearly
> > does so if BLK_MQ_F_BLOCKING. What are the tradeoffs? IMHO, a deeper
> > dive needs to be made into that before concluding that the fix is to
> > use rcu_read_lock_any_held().
> 
> How can this be solved?
> 
> 1.	Always use a workqueue.  Simple, but is said to have performance
> 	issues.
> 
> 2.	Pass a flag in that indicates whether or not the caller is in an
> 	RCU read-side critical section.  Conceptually simple, but might
> 	or might not be reasonable to actually implement in the code as
> 	it exists now.	(You tell me!)
> 
> 3.	Create a function in z_erofs that gives you a decent
> 	approximation, maybe something like the following.
> 
> 4.	Other ideas here.

5.	#3 plus make the corresponding Kconfig option select
	PREEMPT_COUNT, assuming that any users needing compression in
	non-preemptible kernels are OK with PREEMPT_COUNT being set.
	(Some users of non-preemptible kernels object strenuously
	to the added overhead from CONFIG_PREEMPT_COUNT=y.)

							Thanx, Paul

> The following is untested, and is probably quite buggy, but it should
> provide you with a starting point.
> 
> static bool z_erofs_wq_needed(void)
> {
> 	if (IS_ENABLED(CONFIG_PREEMPTION) && rcu_preempt_depth())
> 		return true;  // RCU reader
> 	if (IS_ENABLED(CONFIG_PREEMPT_COUNT) && !preemptible())
> 		return true;  // non-preemptible
> 	if (!IS_ENABLED(CONFIG_PREEMPT_COUNT))
> 		return true;  // non-preeemptible kernel, so play it safe
> 	return false;
> }
> 
> You break it, you buy it!  ;-)
> 
> 							Thanx, Paul