FYI for aarch64 main [14] running a mid March version: I ended up with [usb{usbus2}] stuck at (near) 100% cpu

Note: The context was using a non-debug main build
      from mid-2021-Mar. (More details identified
      later.)

The issue happend while attempting a:

# zfs send -R zpold@for-copy | zfs recv -Fdv zpnew

where the drives involved in the command were:

zpold: a USB3 SSD, using /dev/da0p3
zpnew: an 480 GiByte Optane in the PCIe slot, using /dev/nda0p3

with:

# gpart show -pl
=>       40  468862048    da0  GPT  (224G)
         40     532480  da0p1  4C8GCA72EFI  (260M)
     532520       2008         - free -  (1.0M)
     534528   29360128  da0p2  4C8GCA72swp14  (14G)
   29894656    4194304         - free -  (2.0G)
   34088960   33554432  da0p4  4C8GCA72swp16  (16G)
   67643392  401217536  da0p3  4C8GCA72zfs  (191G)
  468860928       1160         - free -  (580K)

=>        40  2000409184    ada0  GPT  (954G)
          40      409600  ada0p1  (null)  (200M)
      409640  1740636160  ada0p2  FBSDmacchroot  (830G)
  1741045800    58720256  ada0p3  FBSDmacchswp0  (28G)
  1799766056   176160768  ada0p4  FBSDmacchswp1  (84G)
  1975926824    24482400          - free -  (12G)

=>       40  937703008    nda0  GPT  (447G)
         40     532480  nda0p1  CA72opt0EFI  (260M)
     532520       2008          - free -  (1.0M)
     534528  117440512  nda0p2  CA72opt0swp56  (56G)
  117975040   16777216          - free -  (8.0G)
  134752256  134217728  nda0p4  CA72opt0swp64  (64G)
  268969984  668731392  nda0p3  CA72opt0zfs  (319G)
  937701376       1672          - free -  (836K)

The system running was that on /dev/ada0p2 (FBSDmacchroot,
which is UFS instead of ZFS).

The [usb{usbus2}] process eventually got stuck-busy, no
more I/O:

CPU 0:  0.0% user,  0.0% nice,  100% system,  0.0% interrupt,  0.0% idle
CPU 1:  0.0% user,  0.0% nice,  0.0% system,  0.0% interrupt,  100% idle
CPU 2:  0.0% user,  0.0% nice,  0.0% system,  0.0% interrupt,  100% idle
CPU 3:  0.4% user,  0.0% nice,  0.0% system,  0.0% interrupt, 99.6% idle

  PID USERNAME    PRI NICE     SIZE       RES STATE    C   TIME     CPU COMMAND
   15 root        -72    -       0B   262144B CPU0     0   8:51  99.95% [usb{usbus2}]

 1295 root         -8    0  20108Ki    8092Ki q->bq_   2   0:04   0.00% zfs recv -Fdv zpnew{receive_writer_thre}
 1295 root         48    0  20108Ki    8092Ki piperd   2   0:22   0.00% zfs recv -Fdv zpnew{zfs}
 1294 root         -8    0  17544Ki    7740Ki q->bq_   2   0:01   0.00% zfs send -R zpold@for-copy{send_reader_thread}
 1294 root         -8    0  17544Ki    7740Ki q->bq_   0   0:00   0.00% zfs send -R zpold@for-copy{send_merge_thread}
 1294 root         -8    0  17544Ki    7740Ki hdr->b   2   0:00   0.00% zfs send -R zpold@for-copy{send_traverse_threa}
 1294 root         52    0  17544Ki    7740Ki range-   3   0:20   0.00% zfs send -R zpold@for-copy{zfs}

 1036 root         -8    -       0B    1488Ki t->zth   0   0:00   0.00% [zfskern{z_checkpoint_discar}]
 1036 root         -8    -       0B    1488Ki t->zth   1   0:00   0.00% [zfskern{z_livelist_condense}]
 1036 root         -8    -       0B    1488Ki t->zth   2   0:00   0.00% [zfskern{z_livelist_destroy}]
 1036 root         -8    -       0B    1488Ki t->zth   1   0:00   0.00% [zfskern{z_indirect_condense}]
 1036 root         -8    -       0B    1488Ki mmp->m   3   0:00   0.00% [zfskern{mmp_thread_enter}]
 1036 root         -8    -       0B    1488Ki tx->tx   1   0:00   0.00% [zfskern{txg_thread_enter}]
 1036 root         -8    -       0B    1488Ki tx->tx   2   0:00   0.00% [zfskern{txg_thread_enter}]

I was unable to ^c or ^z the process where I
typed the command. I eventually stopped the
system with "shutdown -p now" from a ssh
session (that had already been in place).

When I retried after rebooting and scrubbing (no
problems found), the problem did not repeat.

I do not have more information nor a way to repeat
the problem on demand, unfortunately.

Details of the vintage of the system software and
such:

# ~/fbsd-based-on-what-freebsd-main.sh 
FreeBSD FBSDmacch 14.0-CURRENT FreeBSD 14.0-CURRENT mm-src-n245445-def0058cc690 GENERIC-NODBG  arm64 aarch64 1400005 1400005
def0058cc690 (HEAD -> mm-src) mm-src snapshot for mm's patched build in git context.
merge-base: 7381bbee29df959e88ec59866cf2878263e7f3b2
merge-base: CommitDate: 2021-03-12 20:29:42 +0000
7381bbee29df (freebsd/main, freebsd/HEAD, pure-src, main) cam: Run all XPT_ASYNC ccbs in a dedicated thread
n245444 (--first-parent --count for merge-base)

The system was a MACCHIATObin Double Shot.

===
Mark Millard
marklmi at yahoo.com
( dsl-only.net went
away in early 2018-Mar)

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