30514bd2dd
Commit4acb83417c("sbitmap: fix batched wait_cnt accounting") is a big improvement: without it, I had to revert to before commit040b83fcec("sbitmap: fix possible io hung due to lost wakeup") to avoid the high system time and freezes which that had introduced. Now okay on the NVME laptop, but4acb83417cis a disaster for heavy swapping (kernel builds in low memory) on another: soon locking up in sbitmap_queue_wake_up() (into which __sbq_wake_up() is inlined), cycling around with waitqueue_active() but wait_cnt 0 . Here is a backtrace, showing the common pattern of outer sbitmap_queue_wake_up() interrupted before setting wait_cnt 0 back to wake_batch (in some cases other CPUs are idle, in other cases they're spinning for a lock in dd_bio_merge()): sbitmap_queue_wake_up < sbitmap_queue_clear < blk_mq_put_tag < __blk_mq_free_request < blk_mq_free_request < __blk_mq_end_request < scsi_end_request < scsi_io_completion < scsi_finish_command < scsi_complete < blk_complete_reqs < blk_done_softirq < __do_softirq < __irq_exit_rcu < irq_exit_rcu < common_interrupt < asm_common_interrupt < _raw_spin_unlock_irqrestore < __wake_up_common_lock < __wake_up < sbitmap_queue_wake_up < sbitmap_queue_clear < blk_mq_put_tag < __blk_mq_free_request < blk_mq_free_request < dd_bio_merge < blk_mq_sched_bio_merge < blk_mq_attempt_bio_merge < blk_mq_submit_bio < __submit_bio < submit_bio_noacct_nocheck < submit_bio_noacct < submit_bio < __swap_writepage < swap_writepage < pageout < shrink_folio_list < evict_folios < lru_gen_shrink_lruvec < shrink_lruvec < shrink_node < do_try_to_free_pages < try_to_free_pages < __alloc_pages_slowpath < __alloc_pages < folio_alloc < vma_alloc_folio < do_anonymous_page < __handle_mm_fault < handle_mm_fault < do_user_addr_fault < exc_page_fault < asm_exc_page_fault See how the process-context sbitmap_queue_wake_up() has been interrupted, after bringing wait_cnt down to 0 (and in this example, after doing its wakeups), before advancing wake_index and refilling wake_cnt: an interrupt-context sbitmap_queue_wake_up() of the same sbq gets stuck. I have almost no grasp of all the possible sbitmap races, and their consequences: but __sbq_wake_up() can do nothing useful while wait_cnt 0, so it is better if sbq_wake_ptr() skips on to the next ws in that case: which fixes the lockup and shows no adverse consequence for me. The check for wait_cnt being 0 is obviously racy, and ultimately can lead to lost wakeups: for example, when there is only a single waitqueue with waiters. However, lost wakeups are unlikely to matter in these cases, and a proper fix requires redesign (and benchmarking) of the batched wakeup code: so let's plug the hole with this bandaid for now. Signed-off-by: Hugh Dickins <hughd@google.com> Reviewed-by: Jan Kara <jack@suse.cz> Reviewed-by: Keith Busch <kbusch@kernel.org> Link: https://lore.kernel.org/r/9c2038a7-cdc5-5ee-854c-fbc6168bf16@google.com Signed-off-by: Jens Axboe <axboe@kernel.dk>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
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Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
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https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.