On 4/27/22 16:41, Klaus Jensen wrote: > On Apr 27 05:08, Shinichiro Kawasaki wrote: >> On Apr 21, 2022 / 11:00, Luis Chamberlain wrote: >>> On Wed, Apr 20, 2022 at 05:54:29AM +0000, Shinichiro Kawasaki wrote: >>>> On Apr 14, 2022 / 15:02, Luis Chamberlain wrote: >>>>> Hey folks, >>>>> >>>>> While enhancing kdevops [0] to embrace automation of testing with >>>>> blktests for ZNS I ended up spotting a possible false positive RCU stall >>>>> when running zbd/006 after zbd/005. The curious thing though is that >>>>> this possible RCU stall is only possible when using the qemu >>>>> ZNS drive, not when using nbd. In so far as kdevops is concerned >>>>> it creates ZNS drives for you when you enable the config option >>>>> CONFIG_QEMU_ENABLE_NVME_ZNS=y. So picking any of the ZNS drives >>>>> suffices. When configuring blktests you can just enable the zbd >>>>> guest, so only a pair of guests are reated the zbd guest and the >>>>> respective development guest, zbd-dev guest. When using >>>>> CONFIG_KDEVOPS_HOSTS_PREFIX="linux517" this means you end up with >>>>> just two guests: >>>>> >>>>> * linux517-blktests-zbd >>>>> * linux517-blktests-zbd-dev >>>>> >>>>> The RCU stall can be triggered easily as follows: >>>>> >>>>> make menuconfig # make sure to enable CONFIG_QEMU_ENABLE_NVME_ZNS=y and blktests >>>>> make >>>>> make bringup # bring up guests >>>>> make linux # build and boot into v5.17-rc7 >>>>> make blktests # build and install blktests >>>>> >>>>> Now let's ssh to the guest while leaving a console attached >>>>> with `sudo virsh vagrant_linux517-blktests-zbd` in a window: >>>>> >>>>> ssh linux517-blktests-zbd >>>>> sudo su - >>>>> cd /usr/local/blktests >>>>> export TEST_DEVS=/dev/nvme9n1 >>>>> i=0; while true; do ./check zbd/005 zbd/006; if [[ $? -ne 0 ]]; then echo "BAD at $i"; break; else echo GOOOD $i ; fi; let i=$i+1; done; >>>>> >>>>> The above should never fail, but you should eventually see an RCU >>>>> stall candidate on the console. The full details can be observed on the >>>>> gist [1] but for completeness I list some of it below. It may be a false >>>>> positive at this point, not sure. >>>>> >>>>> [493272.711271] run blktests zbd/005 at 2022-04-14 20:03:22 >>>>> [493305.769531] run blktests zbd/006 at 2022-04-14 20:03:55 >>>>> [493336.979482] nvme nvme9: I/O 192 QID 5 timeout, aborting >>>>> [493336.981666] nvme nvme9: Abort status: 0x0 >>>>> [493367.699440] nvme nvme9: I/O 192 QID 5 timeout, reset controller >>>>> [493388.819341] rcu: INFO: rcu_preempt detected stalls on CPUs/tasks: >>>> >>>> Hello Luis, >>>> >>>> I run blktests zbd group on several QEMU ZNS emulation devices for every rcX >>>> kernel releases. But, I have not ever observed the symptom above. Now I'm >>>> repeating zbd/005 and zbd/006 using v5.18-rc3 and a QEMU ZNS device, and do >>>> not observe the symptom so far, after 400 times repeat. >>> >>> Did you try v5.17-rc7 ? >> >> I hadn't tried it. Then I tried v5.17-rc7 and observed the same symptom. >> >>> >>>> I would like to run the test using same ZNS set up as yours. Can you share how >>>> your ZNS device is set up? I would like to know device size and QEMU -device >>>> options, such as zoned.zone_size or zoned.max_active. >>> >>> It is as easy as the above make commands, and follow up login commands. >> >> I managed to run kdevops on my machine, and saw the I/O timeout and abort >> messages. Using similar QEMU ZNS set up as kdevops, the messages were recreated >> in my test environment also (the reset controller message and RCU relegated >> error were not observed). >> > > Can you extract the relevant part of the QEMU parameters? I tried to > reproduce this, but could not with a 10G, neither with discard=on or > off, qcow2 or raw. > >> [ 214.134083][ T1028] run blktests zbd/005 at 2022-04-22 21:29:54 >> [ 246.383978][ T1142] run blktests zbd/006 at 2022-04-22 21:30:26 >> [ 276.784284][ T386] nvme nvme6: I/O 494 QID 4 timeout, aborting >> [ 276.788391][ C0] nvme nvme6: Abort status: 0x0 >> >> The conditions to recreate the I/O timeout error are as follows: >> >> - Larger size of QEMU ZNS drive (10GB) >> - I use QEMU ZNS drives with 1GB size for my test runs. With this smaller >> size, the I/O timeout is not observed. >> >> - Issue zone reset command for all zones (with 'blkzone reset' command) just >> after zbd/005 completion to the drive. >> - The test case zbd/006 calls the zone reset command. It's enough to repeat >> zbd/005 and zone reset command to recreate the I/O timeout. >> - When 10 seconds sleep is added between zbd/005 run and zone reset command, >> the I/O timeout was not observed. >> - The data write pattern of zbd/005 looks important. Simple dd command to >> fill the device before 'blkzone reset' did not recreate the I/O timeout. >> >> I dug into QEMU code and found that it takes long time to complete zone reset >> command with all zones flag. It takes more than 30 seconds and looks triggering >> the I/O timeout in the block layer. The QEMU calls fallocate punch hole to the >> backend file for each zone, so that data of each zone is zero cleared. Each >> fallocate call is quick but between the calls, 0.7 second delay was observed >> often. I guess some fsync or fdatasync operation would be running and causing >> the delay. >> > > QEMU uses a write zeroes for zone reset. This is because of the > requirement that block in empty zones must be considered deallocated. > > When the drive is configured with `discard=on`, these write zeroes > *should* turn into discards. However, I also tested with discard=off and > I could not reproduce it. > > It might make sense to force QEMU to use a discard for zone reset in all > cases, and then change the reported DLFEAT appropriately, since we > cannot guarantee zeroes then. Why not punch a hole in the backing store file with fallocate() with mode set to FALLOC_FL_PUNCH_HOLE | FALLOC_FL_KEEP_SIZE ? That would be way faster than a write zeroes which potentially actually do the writes, leading to large command processing times. Reading in a hole in a file is guaranteed to return zeroes, at least on Linux. If the backingstore is a block device, then sure, write zeroes is the only solution. Discard should be used with caution since that is a hint only and some drives may actually do nothing. > >> In other words, QEMU ZNS zone reset for all zones is so slow depending on the >> ZNS drive's size and status. Performance improvement of zone reset is desired in >> QEMU. I will seek for the chance to work on it. >> > > Currently, each zone is a separate discard/write zero call. It would be > fair to special case all zones and do it in much larger chunks. Yep, for a backing file, a full file fallocate(FALLOC_FL_PUNCH_HOLE) would do nicely. Or truncate(0) + truncate(storage size) would do too. Since resets are always all zones or one zone, special optimized handling of the reset all case will definitely have huge benefits for that command. -- Damien Le Moal Western Digital Research
