The introduction of the graph lock is causing blk_get_geometry, a hot
function used in the I/O path, to create a coroutine for the call to
bdrv_co_refresh_total_sectors.
In theory the call to bdrv_co_refresh_total_sectors should only matter
in the rare case of host CD-ROM devices, whose size changes when a medium
is added or removed. However, the call is actually keyed by a field in
BlockDriver, drv->has_variable_length, and the field is true in the common
case of the raw driver! This is because the host CD-ROM is usually
layered below the raw driver.
So, this series starts by moving has_variable_length from BlockDriver to
BlockLimits. This is patches 1-4, which also include a fix for a small
latent bug (patch 3).
The second half of the series then cleans up the functions to retrieve
the BlockDriverState's size (patches 5-7) to limit the amount of duplicated
code introduced by the hand-written wrappers of patch 8. The final result
is that blk_get_geometry will not anymore create a coroutine.
This series applies to qemu.git, or to the block-next branch if commit
d8fbf9aa85ae ("block/export: Fix graph locking in blk_get_geometry()
call", 2023-03-27) is cherry picked. Commit d8fbf9aa85ae is also where
bdrv_co_get_geometry() was introduced and with it the performance
regression. It is quite a recent change, and therefore this is
probably a regression in 8.0 that had not been detected yet (except by
Stefan who talked to Kevin and me about it yesterday). I'm not sure how
we can avoid the regression, if not by disabling completely the graph lock
(!) or applying this large series.
I'm throwing this out before disappearing for a couple days for Easter;
I have only tested it with qemu-iotests and "make check-unit".
Thanks,
Paolo
Paolo Bonzini (8):
block: move has_variable_length to BlockLimits
block: remove has_variable_length from filters
block: refresh bs->total_sectors on reopen
block: remove has_variable_length from BlockDriver
migration/block: replace uses of blk_nb_sectors that do not check
result
block-backend: inline bdrv_co_get_geometry
block-backend: ignore inserted state in blk_co_nb_sectors
block, block-backend: write some hot coroutine wrappers by hand
block.c | 35 ++++++++++++++++++--------
block/block-backend.c | 42 ++++++++++++++++++++++++-------
block/copy-on-read.c | 1 -
block/file-posix.c | 12 ++++++---
block/file-win32.c | 2 +-
block/filter-compress.c | 1 -
block/io.c | 4 +++
block/preallocate.c | 1 -
block/raw-format.c | 3 ++-
block/replication.c | 1 -
include/block/block-io.h | 5 +---
include/block/block_int-common.h | 10 ++++++--
include/sysemu/block-backend-io.h | 5 ++--
migration/block.c | 5 ++--
14 files changed, 85 insertions(+), 42 deletions(-)
--
2.39.2
