linux-kernel - [PATCH v2 0/4] ALSA: pcm: Convert SNDRV_PCM_IOCTL_SYNC_PTR to user_access_begin/user_access

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Message-ID: <cover.1749828169.git.christophe.leroy@csgroup.eu>
Date: Fri, 13 Jun 2025 17:37:07 +0200
From: Christophe Leroy <christophe.leroy@...roup.eu>
To: Jaroslav Kysela <perex@...ex.cz>,
	Takashi Iwai <tiwai@...e.com>
Cc: Christophe Leroy <christophe.leroy@...roup.eu>,
	linux-kernel@...r.kernel.org,
	linuxppc-dev@...ts.ozlabs.org,
	linux-sound@...r.kernel.org,
	Herve Codina <herve.codina@...tlin.com>
Subject: [PATCH v2 0/4] ALSA: pcm: Convert SNDRV_PCM_IOCTL_SYNC_PTR to user_access_begin/user_access_end()

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