This series converts all variants of SNDRV_PCM_IOCTL_SYNC_PTR to
user_access_begin/user_access_end() in order to reduce the CPU load
measured in function snd_pcm_ioctl.
With the current implementation, "perf top" reports a high load in
snd_pcm_iotcl(). Most calls to that function are SNDRV_PCM_IOCTL_SYNC_PTR.
14.20% test_perf [.] engine_main
==> 12.86% [kernel] [k] snd_pcm_ioctl
11.91% [kernel] [k] finish_task_switch.isra.0
4.15% [kernel] [k] snd_pcm_group_unlock_irq.part.0
4.07% libc.so.6 [.] __ioctl_time64
3.58% libasound.so.2.0.0 [.] __snd_pcm_mmap_begin_generic
3.37% [kernel] [k] sys_ioctl
2.96% libasound.so.2.0.0 [.] snd_pcm_hw_avail_update
2.73% libasound.so.2.0.0 [.] __snd_pcm_mmap_begin
2.58% [kernel] [k] system_call_exception
1.93% libasound.so.2.0.0 [.] sync_ptr1
1.85% libasound.so.2.0.0 [.] snd_pcm_unlock
1.84% libasound.so.2.0.0 [.] snd_pcm_mmap_begin
1.83% libasound.so.2.0.0 [.] bad_pcm_state
1.68% libasound.so.2.0.0 [.] snd_pcm_mmap_avail
1.67% libasound.so.2.0.0 [.] snd_pcm_avail_update
A tentative was done with going via intermediaire structs on stack to
replace the multiple get_user() and put_user() with copy_from_user()
and copy_to_user(). But copy_from_user() calls _copy_from_user() and
copy_to_user() calls _copy_to_user(). Both then call __copy_tofrom_user().
In total it is 16.4% so it is worse than before.
14.47% test_perf [.] engine_main
12.00% [kernel] [k] finish_task_switch.isra.0
==> 8.37% [kernel] [k] snd_pcm_ioctl
5.44% libc.so.6 [.] __ioctl_time64
5.03% [kernel] [k] snd_pcm_group_unlock_irq.part.0
==> 4.86% [kernel] [k] __copy_tofrom_user
4.62% [kernel] [k] sys_ioctl
3.22% [kernel] [k] system_call_exception
2.42% libasound.so.2.0.0 [.] snd_pcm_mmap_begin
2.31% [kernel] [k] fdget
2.23% libasound.so.2.0.0 [.] __snd_pcm_mmap_begin_generic
2.19% [kernel] [k] syscall_exit_prepare
1.92% libasound.so.2.0.0 [.] snd_pcm_mmap_avail
1.86% libasound.so.2.0.0 [.] __snd_pcm_mmap_begin
1.68% libasound.so.2.0.0 [.] snd_pcm_hw_avail_update
==> 1.67% [kernel] [k] _copy_from_user
1.66% libasound.so.2.0.0 [.] bad_pcm_state
==> 1.53% [kernel] [k] _copy_to_user
1.40% libasound.so.2.0.0 [.] sync_ptr1
With this series which uses unsafe_put_user() and unsafe_get_user(),
the load is significantly reduced:
17.46% test_perf [.] engine_main
9.14% [kernel] [k] finish_task_switch.isra.0
==> 4.92% [kernel] [k] snd_pcm_ioctl
3.99% [kernel] [k] snd_pcm_group_unlock_irq.part.0
3.71% libc.so.6 [.] __ioctl_time64
3.61% libasound.so.2.0.0 [.] __snd_pcm_mmap_begin_generic
2.72% libasound.so.2.0.0 [.] sync_ptr1
2.65% [kernel] [k] system_call_exception
2.46% [kernel] [k] sys_ioctl
2.43% [kernel] [k] __rseq_handle_notify_resume
2.34% [kernel] [k] do_epoll_wait
2.30% libasound.so.2.0.0 [.] __snd_pcm_mmap_commit
2.14% libasound.so.2.0.0 [.] __snd_pcm_avail
2.04% libasound.so.2.0.0 [.] snd_pcm_hw_avail_update
1.89% libasound.so.2.0.0 [.] snd_pcm_lock
1.84% libasound.so.2.0.0 [.] snd_pcm_mmap_avail
1.76% libasound.so.2.0.0 [.] __snd_pcm_avail_update
1.61% libasound.so.2.0.0 [.] bad_pcm_state
1.60% libasound.so.2.0.0 [.] __snd_pcm_mmap_begin
1.49% libasound.so.2.0.0 [.] query_status_data
Since RFC:
- Added a cover letter to summarize some of the measurements done on and around
the RFC
- Fixed relevant checkpatch feedback
- Split last patch in two
Christophe Leroy (4):
ALSA: pcm: refactor copy from/to user in SNDRV_PCM_IOCTL_SYNC_PTR
ALSA: pcm: Convert SNDRV_PCM_IOCTL_SYNC_PTR to
user_access_begin/user_access_end()
ALSA: pcm: Replace [audio_]tstamp_[n]sec by struct __snd_timespec in
struct snd_pcm_mmap_status32
ALSA: pcm: Convert snd_pcm_sync_ptr() to
user_access_begin/user_access_end()
sound/core/pcm_compat.c | 14 +-----
sound/core/pcm_native.c | 96 +++++++++++++++++++++++++----------------
2 files changed, 62 insertions(+), 48 deletions(-)
--
2.47.0
