| Message ID | 20200420162454.48679-1-dianders@chromium.org (mailing list archive) |
|---|---|
| Headers | show |
| Series | blk-mq: Fix two causes of IO stalls found in reboot testing | expand |
On 4/20/20 10:24 AM, Douglas Anderson wrote: > While doing reboot testing, I found that occasionally my device would > trigger the hung task detector. Many tasks were stuck waiting for the > a blkdev mutex, but at least one task in the system was always sitting > waiting for IO to complete (and holding the blkdev mutex). One > example of a task that was just waiting for its IO to complete on one > reboot: > > udevd D 0 2177 306 0x00400209 > Call trace: > __switch_to+0x15c/0x17c > __schedule+0x6e0/0x928 > schedule+0x8c/0xbc > schedule_timeout+0x9c/0xfc > io_schedule_timeout+0x24/0x48 > do_wait_for_common+0xd0/0x160 > wait_for_completion_io_timeout+0x54/0x74 > blk_execute_rq+0x9c/0xd8 > __scsi_execute+0x104/0x198 > scsi_test_unit_ready+0xa0/0x154 > sd_check_events+0xb4/0x164 > disk_check_events+0x58/0x154 > disk_clear_events+0x74/0x110 > check_disk_change+0x28/0x6c > sd_open+0x5c/0x130 > __blkdev_get+0x20c/0x3d4 > blkdev_get+0x74/0x170 > blkdev_open+0x94/0xa8 > do_dentry_open+0x268/0x3a0 > vfs_open+0x34/0x40 > path_openat+0x39c/0xdf4 > do_filp_open+0x90/0x10c > do_sys_open+0x150/0x3c8 > ... > > I've reproduced this on two systems: one boots from an internal UFS > disk and one from eMMC. Each has a card reader attached via USB with > an SD card plugged in. On the USB-attached SD card is a disk with 12 > partitions (a Chrome OS test image), if it matters. The system > doesn't do much with the USB disk other than probe it (it's plugged in > my system to help me recover). > > From digging, I believe that there are two separate but related > issues. Both issues relate to the SCSI code saying that there is no > budget. > > I have done testing with only one or the other of the two patches in > this series and found that I could still encounter hung tasks if only > one of the two patches was applied. This deserves a bit of > explanation. To me, it's fairly obvious that the first fix wouldn't > fix the problems talked about in the second patch. However, it's less > obvious why the second patch doesn't fix the problems in > blk_mq_dispatch_rq_list(). It turns out that it _almost_ does > (problems become much more rare), but I did manage to get a single > trace where the "kick" scheduled by the second patch happened really > quickly. The scheduled kick then ran and found nothing to do. This > happened in parallel to a task running in blk_mq_dispatch_rq_list() > which hadn't gotten around to splicing the list back into > hctx->dispatch. This is why we need both fixes. > > Most of my testing has been atop Chrome OS 5.4's kernel tree which > currently has v5.4.30 merged in. The Chrome OS 5.4 tree also has a > patch by Salman Qazi, namely ("block: Limit number of items taken from > the I/O scheduler in one go"). Reverting that patch didn't make the > hung tasks go away, so I kept it in for most of my testing. > > I have also done some testing on mainline Linux (most on what git > describe calls v5.6-rc7-227-gf3e69428b5e2) even without Salman's > patch. I found that I could reproduce the problems there and that > traces looked about the same as I saw on the downstream branch. These > patches were also confirmed to fix the problems on mainline. > > Chrome OS is currently setup to use the BFQ scheduler and I found that > I couldn't reproduce the problems without BFQ. As discussed in the > second patch this is believed to be because BFQ sometimes returns > "true" from has_work() but then NULL from dispatch_request(). > > I'll insert my usual caveat that I'm sending patches to code that I > know very little about. If I'm making a total bozo patch here, please > help me figure out how I should fix the problems I found in a better > way. > > If you want to see a total ridiculous amount of chatter where I > stumbled around a whole bunch trying to figure out what was wrong and > how to fix it, feel free to read <https://crbug.com/1061950>. I > promise it will make your eyes glaze over right away if this cover > letter didn't already do that. Specifically comment 79 in that bug > includes a link to my ugly prototype of making BFQ's has_work() more > exact (I only managed it by actually defining _both_ an exact and > inexact function to avoid circular locking problems when it was called > directly from blk_mq_hctx_has_pending()). Comment 79 also has more > thoughts about alternatives considered. > > I don't know if these fixes represent a regression of some sort or are > new. As per above I could only reproduce with BFQ enabled which makes > it nearly impossible to go too far back with this. I haven't listed > any "Fixes" tags here, but if someone felt it was appropriate to > backport this to some stable trees that seems like it'd be nice. > Presumably at least 5.4 stable would make sense. > > Thanks to Salman Qazi, Paolo Valente, and Guenter Roeck who spent a > bunch of time helping me trawl through some of this code and reviewing > early versions of this patch. Applied, thanks.
While doing reboot testing, I found that occasionally my device would trigger the hung task detector. Many tasks were stuck waiting for the a blkdev mutex, but at least one task in the system was always sitting waiting for IO to complete (and holding the blkdev mutex). One example of a task that was just waiting for its IO to complete on one reboot: udevd D 0 2177 306 0x00400209 Call trace: __switch_to+0x15c/0x17c __schedule+0x6e0/0x928 schedule+0x8c/0xbc schedule_timeout+0x9c/0xfc io_schedule_timeout+0x24/0x48 do_wait_for_common+0xd0/0x160 wait_for_completion_io_timeout+0x54/0x74 blk_execute_rq+0x9c/0xd8 __scsi_execute+0x104/0x198 scsi_test_unit_ready+0xa0/0x154 sd_check_events+0xb4/0x164 disk_check_events+0x58/0x154 disk_clear_events+0x74/0x110 check_disk_change+0x28/0x6c sd_open+0x5c/0x130 __blkdev_get+0x20c/0x3d4 blkdev_get+0x74/0x170 blkdev_open+0x94/0xa8 do_dentry_open+0x268/0x3a0 vfs_open+0x34/0x40 path_openat+0x39c/0xdf4 do_filp_open+0x90/0x10c do_sys_open+0x150/0x3c8 ... I've reproduced this on two systems: one boots from an internal UFS disk and one from eMMC. Each has a card reader attached via USB with an SD card plugged in. On the USB-attached SD card is a disk with 12 partitions (a Chrome OS test image), if it matters. The system doesn't do much with the USB disk other than probe it (it's plugged in my system to help me recover). From digging, I believe that there are two separate but related issues. Both issues relate to the SCSI code saying that there is no budget. I have done testing with only one or the other of the two patches in this series and found that I could still encounter hung tasks if only one of the two patches was applied. This deserves a bit of explanation. To me, it's fairly obvious that the first fix wouldn't fix the problems talked about in the second patch. However, it's less obvious why the second patch doesn't fix the problems in blk_mq_dispatch_rq_list(). It turns out that it _almost_ does (problems become much more rare), but I did manage to get a single trace where the "kick" scheduled by the second patch happened really quickly. The scheduled kick then ran and found nothing to do. This happened in parallel to a task running in blk_mq_dispatch_rq_list() which hadn't gotten around to splicing the list back into hctx->dispatch. This is why we need both fixes. Most of my testing has been atop Chrome OS 5.4's kernel tree which currently has v5.4.30 merged in. The Chrome OS 5.4 tree also has a patch by Salman Qazi, namely ("block: Limit number of items taken from the I/O scheduler in one go"). Reverting that patch didn't make the hung tasks go away, so I kept it in for most of my testing. I have also done some testing on mainline Linux (most on what git describe calls v5.6-rc7-227-gf3e69428b5e2) even without Salman's patch. I found that I could reproduce the problems there and that traces looked about the same as I saw on the downstream branch. These patches were also confirmed to fix the problems on mainline. Chrome OS is currently setup to use the BFQ scheduler and I found that I couldn't reproduce the problems without BFQ. As discussed in the second patch this is believed to be because BFQ sometimes returns "true" from has_work() but then NULL from dispatch_request(). I'll insert my usual caveat that I'm sending patches to code that I know very little about. If I'm making a total bozo patch here, please help me figure out how I should fix the problems I found in a better way. If you want to see a total ridiculous amount of chatter where I stumbled around a whole bunch trying to figure out what was wrong and how to fix it, feel free to read <https://crbug.com/1061950>. I promise it will make your eyes glaze over right away if this cover letter didn't already do that. Specifically comment 79 in that bug includes a link to my ugly prototype of making BFQ's has_work() more exact (I only managed it by actually defining _both_ an exact and inexact function to avoid circular locking problems when it was called directly from blk_mq_hctx_has_pending()). Comment 79 also has more thoughts about alternatives considered. I don't know if these fixes represent a regression of some sort or are new. As per above I could only reproduce with BFQ enabled which makes it nearly impossible to go too far back with this. I haven't listed any "Fixes" tags here, but if someone felt it was appropriate to backport this to some stable trees that seems like it'd be nice. Presumably at least 5.4 stable would make sense. Thanks to Salman Qazi, Paolo Valente, and Guenter Roeck who spent a bunch of time helping me trawl through some of this code and reviewing early versions of this patch. Changes in v5: - Rebase atop commit 5fe56de799ad ("...Put driver tag...when no budget") Changes in v4: - Only kick in blk_mq_do_dispatch_ctx() / blk_mq_do_dispatch_sched(). Changes in v3: - Note why blk_mq_dispatch_rq_list() change is needed. - ("blk-mq: Add blk_mq_delay_run_hw_queues() API call") new for v3 - Always kick when putting the budget. - Delay blk_mq_do_dispatch_sched() kick by 3 ms for inexact has_work(). - Totally rewrote commit message. - ("Revert "scsi: core: run queue...") new for v3. Changes in v2: - Replace ("scsi: core: Fix stall...") w/ ("blk-mq: Rerun dispatch...") Douglas Anderson (4): blk-mq: In blk_mq_dispatch_rq_list() "no budget" is a reason to kick blk-mq: Add blk_mq_delay_run_hw_queues() API call blk-mq: Rerun dispatching in the case of budget contention Revert "scsi: core: run queue if SCSI device queue isn't ready and queue is idle" block/blk-mq-sched.c | 18 ++++++++++++++++++ block/blk-mq.c | 27 +++++++++++++++++++++++++-- drivers/scsi/scsi_lib.c | 7 +------ include/linux/blk-mq.h | 1 + 4 files changed, 45 insertions(+), 8 deletions(-)