lkml.org 
[lkml]   [2023]   [Jul]   [14]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
SubjectRe: [PATCH v1] rcu: Fix and improve RCU read lock checks when !CONFIG_DEBUG_LOCK_ALLOC
From


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

\
 
 \ /
  Last update: 2023-07-16 20:50    [W:0.098 / U:0.348 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site