Re: [PATCH v3 1/1] riscv: __asm_copy_to-from_user: Optimize unaligned memory access and pipeline stall

[Guenter Roeck]

Hi,

On Wed, Jun 23, 2021 at 09:40:39PM +0900, Akira Tsukamoto wrote:
> This patch will reduce cpu usage dramatically in kernel space especially
> for application which use sys-call with large buffer size, such as network
> applications. The main reason behind this is that every unaligned memory
> access will raise exceptions and switch between s-mode and m-mode causing
> large overhead.
> 
> First copy in bytes until reaches the first word aligned boundary in
> destination memory address. This is the preparation before the bulk
> aligned word copy.
> 
> The destination address is aligned now, but oftentimes the source address
> is not in an aligned boundary. To reduce the unaligned memory access, it
> reads the data from source in aligned boundaries, which will cause the
> data to have an offset, and then combines the data in the next iteration
> by fixing offset with shifting before writing to destination. The majority
> of the improving copy speed comes from this shift copy.
> 
> In the lucky situation that the both source and destination address are on
> the aligned boundary, perform load and store with register size to copy the
> data. Without the unrolling, it will reduce the speed since the next store
> instruction for the same register using from the load will stall the
> pipeline.
> 
> At last, copying the remainder in one byte at a time.
> 
> Signed-off-by: Akira Tsukamoto <akira.tsukamoto@xxxxxxxxx>

This patch causes all riscv32 qemu emulations to stall during boot.
The log suggests that something in kernel/user communication may be wrong.

Bad case:

Starting syslogd: OK
Starting klogd: OK
/etc/init.d/S02sysctl: line 68: syntax error: EOF in backquote substitution
/etc/init.d/S20urandom: line 1: syntax error: unterminated quoted string
Starting network: /bin/sh: syntax error: unterminated quoted string
sed: unmatched '/'
/bin/sh: syntax error: unterminated quoted string
FAIL
/etc/init.d/S55runtest: line 48: syntax error: EOF in backquote substitution

Good case (this patch reverted):

Starting syslogd: OK
Starting klogd: OK
Running sysctl: OK
Saving random seed: [   12.277714] random: dd: uninitialized urandom read (512 bytes read)
OK
Starting network: [   12.949529] e1000: eth0 NIC Link is Up 1000 Mbps Full Duplex, Flow Control: RX
[   12.951170] IPv6: ADDRCONF(NETDEV_CHANGE): eth0: link becomes ready
udhcpc: started, v1.33.0
udhcpc: sending discover
udhcpc: sending select for 10.0.2.15
udhcpc: lease of 10.0.2.15 obtained, lease time 86400
deleting routers
adding dns 10.0.2.3
OK
Found console ttyS0

Reverting this patch fixes the problem. Bisect log attached.

Guenter

---
# bad: [50be9417e23af5a8ac860d998e1e3f06b8fd79d7] Merge tag 'io_uring-5.14-2021-07-09' of git://git.kernel.dk/linux-block
# good: [f55966571d5eb2876a11e48e798b4592fa1ffbb7] Merge tag 'drm-next-2021-07-08-1' of git://anongit.freedesktop.org/drm/drm
git bisect start 'HEAD' 'f55966571d5e'
# good: [7a400bf28334fc7734639db3566394e1fc80670c] Merge tag 'for-linus-5.14-rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/rw/ubifs
git bisect good 7a400bf28334fc7734639db3566394e1fc80670c
# bad: [d8dc121eeab9abfbc510097f8db83e87560f753b] Merge branch 'linus' of git://git.kernel.org/pub/scm/linux/kernel/git/herbert/crypto-2.6
git bisect bad d8dc121eeab9abfbc510097f8db83e87560f753b
# bad: [7761e36bc7222d1221242c5f195ee0fd40caea40] riscv: Fix PTDUMP output now BPF region moved back to module region
git bisect bad 7761e36bc7222d1221242c5f195ee0fd40caea40
# good: [5def4429aefe65b494816d9ba8ae7f971d522251] riscv: mm: Use better bitmap_zalloc()
git bisect good 5def4429aefe65b494816d9ba8ae7f971d522251
# good: [47513f243b452a5e21180dcf3d6ac1c57e1781a6] riscv: Enable KFENCE for riscv64
git bisect good 47513f243b452a5e21180dcf3d6ac1c57e1781a6
# good: [01112e5e20f5298a81639806cd0a3c587aade467] Merge branch 'riscv-wx-mappings' into for-next
git bisect good 01112e5e20f5298a81639806cd0a3c587aade467
# good: [70eee556b678d1e4cd4ea6742a577b596963fa25] riscv: ptrace: add argn syntax
git bisect good 70eee556b678d1e4cd4ea6742a577b596963fa25
# bad: [ca6eaaa210deec0e41cbfc380bf89cf079203569] riscv: __asm_copy_to-from_user: Optimize unaligned memory access and pipeline stall
git bisect bad ca6eaaa210deec0e41cbfc380bf89cf079203569
# good: [31da94c25aea835ceac00575a9fd206c5a833fed] riscv: add VMAP_STACK overflow detection
git bisect good 31da94c25aea835ceac00575a9fd206c5a833fed
# first bad commit: [ca6eaaa210deec0e41cbfc380bf89cf079203569] riscv: __asm_copy_to-from_user: Optimize unaligned memory access and pipeline stall