[RFC for-next v2 0/4] enable pcpu bio caching for IRQ I/O

[Pavel Begunkov]

This series implements bio pcpu caching for normal / IRQ-driven I/O
extending REQ_ALLOC_CACHE currently limited to iopoll. The allocation side
still only works from non-irq context, which is the reason it's not enabled
by default, but turning it on for other users (e.g. filesystems) is
as a matter of passing a flag.

t/io_uring with an Optane SSD setup showed +7% for batches of 32 requests
and +4.3% for batches of 8.

IRQ, 128/32/32, cache off
IOPS=59.08M, BW=28.84GiB/s, IOS/call=31/31
IOPS=59.30M, BW=28.96GiB/s, IOS/call=32/32
IOPS=59.97M, BW=29.28GiB/s, IOS/call=31/31
IOPS=59.92M, BW=29.26GiB/s, IOS/call=32/32
IOPS=59.81M, BW=29.20GiB/s, IOS/call=32/31

IRQ, 128/32/32, cache on
IOPS=64.05M, BW=31.27GiB/s, IOS/call=32/31
IOPS=64.22M, BW=31.36GiB/s, IOS/call=32/32
IOPS=64.04M, BW=31.27GiB/s, IOS/call=31/31
IOPS=63.16M, BW=30.84GiB/s, IOS/call=32/32

IRQ, 32/8/8, cache off
IOPS=50.60M, BW=24.71GiB/s, IOS/call=7/8
IOPS=50.22M, BW=24.52GiB/s, IOS/call=8/7
IOPS=49.54M, BW=24.19GiB/s, IOS/call=8/8
IOPS=50.07M, BW=24.45GiB/s, IOS/call=7/7
IOPS=50.46M, BW=24.64GiB/s, IOS/call=8/8

IRQ, 32/8/8, cache on
IOPS=51.39M, BW=25.09GiB/s, IOS/call=8/7
IOPS=52.52M, BW=25.64GiB/s, IOS/call=7/8
IOPS=52.57M, BW=25.67GiB/s, IOS/call=8/8
IOPS=52.58M, BW=25.67GiB/s, IOS/call=8/7
IOPS=52.61M, BW=25.69GiB/s, IOS/call=8/8

The main part is in patch 3. Would be great to take patch 1 separately
for 6.1 for extra safety.

v2: fix botched splicing threshold checks

Pavel Begunkov (4):
  bio: safeguard REQ_ALLOC_CACHE bio put
  bio: split pcpu cache part of bio_put into a helper
  block/bio: add pcpu caching for non-polling bio_put
  io_uring/rw: enable bio caches for IRQ rw

 block/bio.c   | 94 ++++++++++++++++++++++++++++++++++++++++-----------
 io_uring/rw.c |  3 +-
 2 files changed, 76 insertions(+), 21 deletions(-)

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2.38.0