> 2023年7月14日 23:35，Alan Huang <mmpgouride@xxxxxxxxx> 写道： > >> >> 2023年7月14日 10:16，Paul E. McKenney <paulmck@xxxxxxxxxx> 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@xxxxxxxxxxxxxxxxx> wrote: >>>>> On 2023/7/13 22:07, Joel Fernandes wrote: >>>>>> On Thu, Jul 13, 2023 at 12:59 AM Gao Xiang <hsiangkao@xxxxxxxxxxxxxxxxx> 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@xxxxxxxxxx >>>>>>>>> [2] https://lore.kernel.org/r/CAHk-=wgE9kORADrDJ4nEsHHLirqPCZ1tGaEPAZejHdZ03qCOGg@xxxxxxxxxxxxxx >>>>>>>>> [3] https://lore.kernel.org/r/CAB=BE-SBtO6vcoyLNA9F-9VaN5R0t3o_Zn+FW8GbO6wyUqFneQ@xxxxxxxxxxxxxx >>>>>>>>> [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. > > 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
