> 2023年7月15日 01:02，Paul E. McKenney <paulmck@xxxxxxxxxx> 写道： > > On Fri, Jul 14, 2023 at 11:54:47PM +0800, Alan Huang wrote: >> >>> 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. > > Is there some other field that would work? Maybe bio->bi_opf, btrfs uses some bits of it. > > Thanx, Paul > >>> 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
