io_uring: Add support for napi_busy_poll

The sqpoll thread can be used for performing the napi busy poll in a
similar way that it does io polling for file systems supporting direct
access bypassing the page cache.

The other way that io_uring can be used for napi busy poll is by
calling io_uring_enter() to get events.

If the user specify a timeout value, it is distributed between polling
and sleeping by using the systemwide setting
/proc/sys/net/core/busy_poll.

The changes have been tested with this program:
https://github.com/lano1106/io_uring_udp_ping

and the result is:
Without sqpoll:
NAPI busy loop disabled:
rtt min/avg/max/mdev = 40.631/42.050/58.667/1.547 us
NAPI busy loop enabled:
rtt min/avg/max/mdev = 30.619/31.753/61.433/1.456 us

With sqpoll:
NAPI busy loop disabled:
rtt min/avg/max/mdev = 42.087/44.438/59.508/1.533 us
NAPI busy loop enabled:
rtt min/avg/max/mdev = 35.779/37.347/52.201/0.924 us

v2:
 * Evaluate list_empty(&ctx->napi_list) outside io_napi_busy_loop() to keep
   __io_sq_thread() execution as fast as possible
 * In io_cqring_wait(), move up the sig block to avoid needless computation
   if the block exits the function
 * In io_cqring_wait(), protect ctx->napi_list from race condition by
   splicing it into a local list
 * In io_cqring_wait(), allow busy polling when uts is missing
 * Fix kernel test robot issues
v3:
 * Fix do_div() type mismatch warning
 * Reduce uring_lock contention by creating a spinlock for protecting
   napi_list
 * Support correctly MULTISHOT poll requests
v4:
 * Put back benchmark result in commit text
v5:
 * Protect napi_list from concurrent access from io_workers threads

Olivier Langlois (2):
  io_uring: minor io_cqring_wait() optimization
  io_uring: Add support for napi_busy_poll

 fs/io_uring.c | 248 ++++++++++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 239 insertions(+), 9 deletions(-)

-- 
2.35.1