Re: [RFC PATCH] accel/tcg: add tracepoints for cpu_loop_exit_atomic

[Richard Henderson <richard.henderson@linaro.org>]

On 10/4/24 06:52, Alex Bennée wrote:
> We try to avoid using cpu_loop_exit_atomic as it brings in an all-core
> sync point. However on some cpu/kernel/benchmark combinations it is
> starting to show up in the performance profile. To make it easier to
> see whats going on add tracepoints for the slow path so we can see
> what is triggering the wait.
> 
> It seems for a modern CPU it can be quite a bit, for example:
> 
> ./qemu-system-aarch64 \
>             -machine type=virt,virtualization=on,pflash0=rom,pflash1=efivars,gic-version=max \
>             -smp 4 \
>             -accel tcg \
>             -device virtio-net-pci,netdev=unet \
>             -device virtio-scsi-pci \
>             -device scsi-hd,drive=hd \
>             -netdev user,id=unet,hostfwd=tcp::2222-:22 \
>             -blockdev driver=raw,node-name=hd,file.driver=host_device,file.filename=/dev/zen-ssd2/trixie-arm64,discard=unmap \
>             -serialmon:stdio \
>             -blockdev node-name=rom,driver=file,filename=(pwd)/pc-bios/edk2-aarch64-code.fd,read-only=true \
>             -blockdev node-name=efivars,driver=file,filename=$HOME/images/qemu-arm64-efivars \
>             -m 8192 \
>             -object memory-backend-memfd,id=mem,size=8G,share=on \
>             -kernel /home/alex/lsrc/linux.git/builds/arm64/arch/arm64/boot/Image -append "root=/dev/sda2 console=ttyAMA0 systemd.unit=benchmark-stress-ng.service" \
>             -display none -dtrace:load_atom\*_fallback,trace:store_atom\*_fallback
> 
> With:
> 
>    -cpu neoverse-v1,pauth-impdef=on => 2203343
> 
> With:
> 
>    -cpu cortex-a76 => 0
> 
> Signed-off-by: Alex Bennée<alex.bennee@linaro.org>
> Cc: Pierrick Bouvier<pierrick.bouvier@linaro.org>
> ---
>   accel/tcg/user-exec.c          |  2 +-
>   accel/tcg/ldst_atomicity.c.inc |  9 +++++++++
>   accel/tcg/trace-events         | 12 ++++++++++++
>   3 files changed, 22 insertions(+), 1 deletion(-)

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>

r~