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

[Alan Huang <mmpgouride@gmail.com>]

> 2023年7月15日 01:02，Paul E. McKenney <paulmck@kernel.org> 写道：
> 
> On Fri, Jul 14, 2023 at 11:54:47PM +0800, Alan Huang wrote:
>> 
>>> 2023年7月14日 23:35，Alan Huang <mmpgouride@gmail.com> 写道：
>>> 
>>>> 
>>>> 2023年7月14日 10:16，Paul E. McKenney <paulmck@kernel.org> 写道：
>>>> 
>>>> 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@linux.alibaba.com> wrote:
>>>>>>> On 2023/7/13 22:07, Joel Fernandes wrote:
>>>>>>>> On Thu, Jul 13, 2023 at 12:59 AM Gao Xiang <hsiangkao@linux.alibaba.com> 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@kernel.org
>>>>>>>>>>> [2] https://lore.kernel.org/r/CAHk-=wgE9kORADrDJ4nEsHHLirqPCZ1tGaEPAZejHdZ03qCOGg@mail.gmail.com
>>>>>>>>>>> [3] https://lore.kernel.org/r/CAB=BE-SBtO6vcoyLNA9F-9VaN5R0t3o_Zn+FW8GbO6wyUqFneQ@mail.gmail.com
>>>>>>>>>>> [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.)
>>> 
>>> 6. Set one bit in bio->bi_private, check the bit and flip it in rcu_read_lock() path,
>>> then in z_erofs_decompressqueue_endio, check if the bit has changed.
>> 
>> Seems bad, read and modify bi_private is a bad idea.
> 
> Is there some other field that would work?

Maybe bio->bi_opf, btrfs uses some bits of it.

> 
> Thanx, Paul
> 
>>> Not sure if this is feasible or acceptable. :)
>>> 
>>>> 
>>>> 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