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

[Gao Xiang <hsiangkao@linux.alibaba.com>]

On 2023/7/14 21:42, Joel Fernandes wrote:
> On Thu, Jul 13, 2023 at 11:17 PM Gao Xiang <hsiangkao@linux.alibaba.com> wrote:
>>
>>
>>
>> On 2023/7/14 10:16, Paul E. McKenney wrote:
>>> 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:
>>
>> ...
>>
>>>>>
>>>>> >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.)
>>
>> I'm not sure if it's a good idea
> 
> I think it is a fine idea.
> 
>> we need to work on
>> CONFIG_PREEMPT_COUNT=n (why not?), we could just always trigger a
>> workqueue for this.
>>
> 
> So CONFIG_PREEMPT_COUNT=n users don't deserve good performance? ;-)

I'm not sure if non-preemptible kernel users really care about
such sensitive latencies, I don't know, my 2 cents.

Thanks,
Gao Xiang

> 
> thanks,
> 
>   - Joel