On Fri 04-11-22 14:25:32, Khazhy Kumykov wrote: > On Thu, Nov 3, 2022 at 1:47 AM Jan Kara <jack@xxxxxxx> wrote: > > > > On Thu 03-11-22 11:51:15, Yu Kuai wrote: > > > Hi, > > > > > > 在 2022/11/03 11:05, Khazhy Kumykov 写道: > > > > On Wed, Nov 2, 2022 at 7:56 PM Yu Kuai <yukuai1@xxxxxxxxxxxxxxx> wrote: > > > > > > > > > > Hi, > > > > > > > > > > 在 2022/11/03 9:39, Khazhismel Kumykov 写道: > > > > > > This fixes crashes in bfq_add_bfqq_busy due to waker_bfqq being NULL, > > > > > > but woken_list_node still being hashed. This would happen when > > > > > > bfq_init_rq() expects a brand new allocated queue to be returned from > > > > > > > > > > From what I see, bfqq->waker_bfqq is updated in bfq_init_rq() only if > > > > > 'new_queue' is false, but if 'new_queue' is false, the returned 'bfqq' > > > > > from bfq_get_bfqq_handle_split() will never be oom_bfqq, so I'm confused > > > > > here... > > > > There's two calls for bfq_get_bfqq_handle_split in this function - the > > > > second one is after the check you mentioned, and is the problematic > > > > one. > > > Yes, thanks for the explanation. Now I understand how the problem > > > triggers. > > > > > > > > > > > > > > bfq_get_bfqq_handle_split() and unconditionally updates waker_bfqq > > > > > > without resetting woken_list_node. Since we can always return oom_bfqq > > > > > > when attempting to allocate, we cannot assume waker_bfqq starts as NULL. > > > > > > We must either reset woken_list_node, or avoid setting woken_list at all > > > > > > for oom_bfqq - opt to do the former. > > > > > > > > > > Once oom_bfqq is used, I think the io is treated as issued from root > > > > > group. Hence I don't think it's necessary to set woken_list or > > > > > waker_bfqq for oom_bfqq. > > > > Ack, I was wondering what's right here since, evidently, *someone* had > > > > already set oom_bfqq->waker_bfqq to *something* (although... maybe it > > > > was an earlier init_rq). But maybe it's better to do nothing if we > > > > *know* it's oom_bfqq. > > > > > > I need to have a check how oom_bfqq get involved with waker_bfqq, and > > > then see if it's reasonable. > > > > > > Probably Jan and Paolo will have better view on this. > > > > Thanks for the CC Kuai and thanks to Khazy for spotting the bug. The > > oom_bfqq is just a fallback bfqq and as such it should be extempted from > > all special handling like waker detection etc. All this stuff is just for > > optimizing performance and when we are OOM, we have far larger troubles > > than to optimize performance. > > > > So how I think we should really fix this is that we extempt oom_bfqq from > > waker detection in bfq_check_waker() by adding: > > > > bfqq == bfqd->oom_bfqq || > > bfqd->last_completed_rq_bfq == bfqd->oom_bfqq) > > > > to the initial check and then also if bfq_get_bfqq_handle_split() returns > > oom_bfqq we should just skip carrying over the waker information. > Thanks for the tip! I'll send a followup, including your suggestions. > > I do have some other questions in this area, if you could help me > understand. Looking again at bfq_init_rq, inside of the !new_queue > section - we call bfq_split_bfqq() to "split" our bfqq, then in the > next line bfq_get_bfqq_handle_split inspects bic_to_bfqq(bic, > is_sync), and if it's NULL, allocates a new queue. However, if we have > an async rq, this call will return the pre-existing async bfqq, as the > call to bfq_split_bfqq() only clears the sync bfqq, ever. The best > understanding I have now is: "bic->bfqq[aync] is never NULL (and we > don't merge async queues) so we'll never reach this !new_queue section > anyways if it's async". Is that accurate? So you are right that async queues are never merged or split. In fact, if you have a look at bfq_get_queue(), you'll notice that async queue is common for all processes with the same ioprio & blkcg. So all these games with splitting, merging, waker detection etc. impact only sync queues. Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
