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13日 23:33，Joel Fernandes <joel@joelfernandes.org> 写道：
> 
> 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().

Copied from [1]:

"Background: Historically erofs would always schedule a kworker for
 decompression which would incur the scheduling cost regardless of
 the context. But z_erofs_decompressqueue_endio() may not always
 be in atomic context and we could actually benefit from doing the
 decompression in z_erofs_decompressqueue_endio() if we are in
 thread context, for example when running with dm-verity.
 This optimization was later added in patch [2] which has shown
 improvement in performance benchmarks.”

I’m not sure if it is a design issue.


[1] https://lore.kernel.org/all/20230621220848.3379029-1-dhavale@google.com/

> 
> - Joel



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