Re: [PATCH 09/15] swiotlb: make the swiotlb_init interface more useful

[Nathan Chancellor <nathan@kernel.org>]

Hi Christoph,

On Mon, Apr 04, 2022 at 07:05:53AM +0200, Christoph Hellwig wrote:
> Pass a bool to pass if swiotlb needs to be enabled based on the
> addressing needs and replace the verbose argument with a set of
> flags, including one to force enable bounce buffering.
> 
> Note that this patch removes the possibility to force xen-swiotlb
> use using swiotlb=force on the command line on x86 (arm and arm64
> never supported that), but this interface will be restored shortly.
> 
> Signed-off-by: Christoph Hellwig <hch@lst.de>

I bisected a performance regression in WSL2 to this change as commit
c6af2aa9ffc9 ("swiotlb: make the swiotlb_init interface more useful") in
mainline (bisect log below). I initially noticed it because accessing the
Windows filesystem through the /mnt/c mount is about 40x slower if I am doing
my math right based on the benchmarks below.

Before:

$ uname -r; and hyperfine "ls -l /mnt/c/Users/natec/Downloads"
5.18.0-rc3-microsoft-standard-WSL2-00008-ga3e230926708
Benchmark 1: ls -l /mnt/c/Users/natec/Downloads
  Time (mean ± σ):     564.5 ms ±  24.1 ms    [User: 2.5 ms, System: 130.3 ms]
  Range (min … max):   510.2 ms … 588.0 ms    10 runs

After

$ uname -r; and hyperfine "ls -l /mnt/c/Users/natec/Downloads"
5.18.0-rc3-microsoft-standard-WSL2-00009-gc6af2aa9ffc9
Benchmark 1: ls -l /mnt/c/Users/natec/Downloads
  Time (mean ± σ):     23.282 s ±  1.220 s    [User: 0.013 s, System: 0.101 s]
  Range (min … max):   21.793 s … 25.317 s    10 runs

I do see 'swiotlb=force' on the cmdline:

$ cat /proc/cmdline
initrd=\initrd.img panic=-1 nr_cpus=8 swiotlb=force earlycon=uart8250,io,0x3f8,115200 console=hvc0 debug pty.legacy_count=0

/mnt/c appears to be a 9p mount, not sure if that is relevant here:

$ mount &| grep /mnt/c
drvfs on /mnt/c type 9p (rw,noatime,dirsync,aname=drvfs;path=C:\;uid=1000;gid=1000;symlinkroot=/mnt/,mmap,access=client,msize=262144,trans=virtio)

If there is any other information I can provide, please let me know.

Cheers,
Nathan

# bad: [700170bf6b4d773e328fa54ebb70ba444007c702] Merge tag 'nfs-for-5.19-1' of git://git.linux-nfs.org/projects/anna/linux-nfs
# good: [4b0986a3613c92f4ec1bdc7f60ec66fea135991f] Linux 5.18
git bisect start '700170bf6b4d773e328fa54ebb70ba444007c702' 'v5.18'
# good: [86c87bea6b42100c67418af690919c44de6ede6e] Merge tag 'devicetree-for-5.19' of git://git.kernel.org/pub/scm/linux/kernel/git/robh/linux
git bisect good 86c87bea6b42100c67418af690919c44de6ede6e
# bad: [ae862183285cbb2ef9032770d98ffa9becffe9d5] Merge tag 'arm-dt-5.19' of git://git.kernel.org/pub/scm/linux/kernel/git/soc/soc
git bisect bad ae862183285cbb2ef9032770d98ffa9becffe9d5
# good: [2518f226c60d8e04d18ba4295500a5b0b8ac7659] Merge tag 'drm-next-2022-05-25' of git://anongit.freedesktop.org/drm/drm
git bisect good 2518f226c60d8e04d18ba4295500a5b0b8ac7659
# bad: [babf0bb978e3c9fce6c4eba6b744c8754fd43d8e] Merge tag 'xfs-5.19-for-linus' of git://git.kernel.org/pub/scm/fs/xfs/xfs-linux
git bisect bad babf0bb978e3c9fce6c4eba6b744c8754fd43d8e
# good: [beed983621fbdfd291e6e3a0cdc4d10517e60af8] ASoC: Intel: avs: Machine board registration
git bisect good beed983621fbdfd291e6e3a0cdc4d10517e60af8
# good: [fbe86daca0ba878b04fa241b85e26e54d17d4229] Merge tag 'scsi-misc' of git://git.kernel.org/pub/scm/linux/kernel/git/jejb/scsi
git bisect good fbe86daca0ba878b04fa241b85e26e54d17d4229
# good: [166afc45ed5523298541fd0297f9ad585cc2708c] Merge tag 'reflink-speedups-5.19_2022-04-28' of git://git.kernel.org/pub/scm/linux/kernel/git/djwong/xfs-linux into xfs-5.19-for-next
git bisect good 166afc45ed5523298541fd0297f9ad585cc2708c
# bad: [e375780b631a5fc2a61a3b4fa12429255361a31e] Merge tag 'fsnotify_for_v5.19-rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/jack/linux-fs
git bisect bad e375780b631a5fc2a61a3b4fa12429255361a31e
# bad: [4a37f3dd9a83186cb88d44808ab35b78375082c9] dma-direct: don't over-decrypt memory
git bisect bad 4a37f3dd9a83186cb88d44808ab35b78375082c9
# bad: [742519538e6b07250c8085bbff4bd358bc03bf16] swiotlb: pass a gfp_mask argument to swiotlb_init_late
git bisect bad 742519538e6b07250c8085bbff4bd358bc03bf16
# good: [9bbe7a7fc126e3d14fefa4b035854aba080926d9] arm/xen: don't check for xen_initial_domain() in xen_create_contiguous_region
git bisect good 9bbe7a7fc126e3d14fefa4b035854aba080926d9
# good: [a3e230926708125205ffd06d3dc2175a8263ae7e] x86: centralize setting SWIOTLB_FORCE when guest memory encryption is enabled
git bisect good a3e230926708125205ffd06d3dc2175a8263ae7e
# bad: [8ba2ed1be90fc210126f68186564707478552c95] swiotlb: add a SWIOTLB_ANY flag to lift the low memory restriction
git bisect bad 8ba2ed1be90fc210126f68186564707478552c95
# bad: [c6af2aa9ffc9763826607bc2664ef3ea4475ed18] swiotlb: make the swiotlb_init interface more useful
git bisect bad c6af2aa9ffc9763826607bc2664ef3ea4475ed18
# first bad commit: [c6af2aa9ffc9763826607bc2664ef3ea4475ed18] swiotlb: make the swiotlb_init interface more useful