Messages in this thread | | | Date | Fri, 14 Jul 2023 21:51:16 +0800 | Subject | Re: [PATCH v1] rcu: Fix and improve RCU read lock checks when !CONFIG_DEBUG_LOCK_ALLOC | From | Gao Xiang <> |
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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
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