On 1/6/25 7:33 PM, Jens Axboe wrote: > On 1/6/25 4:53 PM, Jens Axboe wrote: >> On 1/6/25 1:03 PM, Haeuptle, Michael wrote: >>> Hello, >>> >>> I?m using the nvme multipath driver (NVMF/RDMA) and io-uring. When a >>> path goes away, I sometimes get a CQE.res = -EAGAIN in user space. >>> This is unexpected since the nvme multipath driver should handle this >>> transparently. It?s somewhat workload related but easy to reproduce >>> with fio. >>> >>> The multipath driver uses kblockd worker to re-queue the failed NVME >>> bios >>> (https://github.com/torvalds/linux/blob/13563da6ffcf49b8b45772e40b35f96926a7ee1e/drivers/nvme/host/multipath.c#L126). >>> The original request is ended. >>> >>> When the nvme_requeue_work callback is executed, the blk layer tries >>> to allocate a new request for the bios but that fails and the bio >>> status is set to BLK_STS_AGAIN >>> (https://elixir.bootlin.com/linux/v6.12.6/source/block/blk-mq.c#L2987). >>> The failure to allocate a new req seems to be due to all tags for the >>> queue being used up. >>> >>> Eventually, this makes it into io_uring?s io_rw_should_reissue and >>> hits same_thread_group(req->tctx->task, current) = false (in >>> https://github.com/torvalds/linux/blob/13563da6ffcf49b8b45772e40b35f96926a7ee1e/io_uring/rw.c#L437). >>> As a result, CQE.res = -EAGAIN and thrown back to the user space >>> program. >>> >>> Here?s a stack dump when we hit same_thread_group(req->tctx->task, >>> current) = false >>> >>> kernel: [237700.098733] dump_stack_lvl+0x44/0x5c >>> kernel: [237700.098737] io_rw_should_reissue.cold+0x5d/0x64 >>> kernel: [237700.098742] io_complete_rw+0x9a/0xc0 >>> kernel: [237700.098745] blkdev_bio_end_io_async+0x33/0x80 >>> kernel: [237700.098749] blk_mq_submit_bio+0x5b5/0x620 >>> kernel: [237700.098756] submit_bio_noacct_nocheck+0x163/0x370 >>> kernel: [237700.098760] ? submit_bio_noacct+0x79/0x4b0 >>> kernel: [237700.098764] nvme_requeue_work+0x4b/0x60 [nvme_core] >>> kernel: [237700.098776] process_one_work+0x1c7/0x380 >>> kernel: [237700.098782] worker_thread+0x4d/0x380 >>> kernel: [237700.098786] ? _raw_spin_lock_irqsave+0x23/0x50 >>> kernel: [237700.098791] ? rescuer_thread+0x3a0/0x3a0 >>> kernel: [237700.098794] kthread+0xe9/0x110 >>> kernel: [237700.098798] ? kthread_complete_and_exit+0x20/0x20 >>> kernel: [237700.098802] ret_from_fork+0x22/0x30 >>> kernel: [237700.098811] </TASK> >>> >>> Is the same_thread_group() check really needed in this case? The >>> thread groups are certainly different? Any side effects if this check >>> is being removed? >> >> It's their for safety reasons - across all request types, it's not >> always safe. For this case, absolutely the check does not need to be >> there. So probably best to ponder ways to bypass it selectively. >> >> Let me ponder a bit what the best approach would be here... > > Actually I think we can just remove it. The actual retry will happen out > of context anyway, and the comment about the import is no longer valid > as the import will have been done upfront since 6.10. > > Do you want to send a patch for that, or do you want me to send one out > referencing this report? Also see: commit 039a2e800bcd5beb89909d1a488abf3d647642cf Author: Jens Axboe <axboe@xxxxxxxxx> Date: Thu Apr 25 09:04:32 2024 -0600 io_uring/rw: reinstate thread check for retries let me take a closer look tomorrow... -- Jens Axboe
