From: Christophe Leroy christophe.leroy@csgroup.eu
[ Upstream commit 92f59aeb13252265c20e7aef1379a8080c57e0a2 ]
At the time being recalculate_boundary() is implemented with a loop which shows up as costly in a perf profile, as depicted by the annotate below:
0.00 : c057e934: 3d 40 7f ff lis r10,32767 0.03 : c057e938: 61 4a ff ff ori r10,r10,65535 0.21 : c057e93c: 7d 49 50 50 subf r10,r9,r10 5.39 : c057e940: 7d 3c 4b 78 mr r28,r9 2.11 : c057e944: 55 29 08 3c slwi r9,r9,1 3.04 : c057e948: 7c 09 50 40 cmplw r9,r10 2.47 : c057e94c: 40 81 ff f4 ble c057e940 <snd_pcm_ioctl+0xee0>
Total: 13.2% on that simple loop.
But what the loop does is to multiply the boundary by 2 until it is over the wanted border. This can be avoided by using fls() to get the boundary value order and shift it by the appropriate number of bits at once.
This change provides the following profile:
0.04 : c057f6e8: 3d 20 7f ff lis r9,32767 0.02 : c057f6ec: 61 29 ff ff ori r9,r9,65535 0.34 : c057f6f0: 7d 5a 48 50 subf r10,r26,r9 0.23 : c057f6f4: 7c 1a 50 40 cmplw r26,r10 0.02 : c057f6f8: 41 81 00 20 bgt c057f718 <snd_pcm_ioctl+0xf08> 0.26 : c057f6fc: 7f 47 00 34 cntlzw r7,r26 0.09 : c057f700: 7d 48 00 34 cntlzw r8,r10 0.22 : c057f704: 7d 08 38 50 subf r8,r8,r7 0.04 : c057f708: 7f 5a 40 30 slw r26,r26,r8 0.35 : c057f70c: 7c 0a d0 40 cmplw r10,r26 0.13 : c057f710: 40 80 05 f8 bge c057fd08 <snd_pcm_ioctl+0x14f8> 0.00 : c057f714: 57 5a f8 7e srwi r26,r26,1
Total: 1.7% with that loopless alternative.
Signed-off-by: Christophe Leroy christophe.leroy@csgroup.eu Link: https://patch.msgid.link/4836e2cde653eebaf2709ebe30eec736bb8c67fd.1749202237... Signed-off-by: Takashi Iwai tiwai@suse.de Signed-off-by: Sasha Levin sashal@kernel.org ---
LLM Generated explanations, may be completely bogus:
Based on my analysis of this commit, here's my determination:
**Backport Status: YES**
## Extensive Explanation:
This commit should be backported to stable kernel trees for the following reasons:
### 1. **Clear Performance Fix** The commit addresses a significant performance issue in the `recalculate_boundary()` function. The profiling data shows: - Original implementation: 13.2% CPU time spent in the loop - New implementation: 1.7% CPU time (a ~7.7x improvement)
This is a substantial performance improvement for a function that's called during PCM ioctl operations.
### 2. **Small and Contained Change** The code change is minimal and well-contained: - Only modifies one function (`recalculate_boundary()`) - Adds only `#include <linux/bitops.h>` as a dependency - The new implementation is mathematically equivalent to the old one - Total change is about 20 lines of code
### 3. **Low Risk of Regression** The change replaces a simple loop with bit manipulation operations: - Old: Loop multiplying by 2 until boundary exceeds a limit - New: Uses `__fls()` (find last set bit) to calculate the shift amount directly - The logic is preserved: find the largest power-of-2 multiple of buffer_size that fits within the 32-bit boundary
### 4. **Affects User-Facing Performance** This function is called from `snd_pcm_ioctl_sync_ptr_compat()`, which is invoked during: - 32-bit compatibility ioctl operations (`__SNDRV_PCM_IOCTL_SYNC_PTR32`) - Audio playback/capture operations on 32-bit applications running on 64-bit kernels - These are common operations that directly impact audio latency and performance
### 5. **No Architectural Changes** The commit: - Doesn't change any APIs or data structures - Doesn't modify the function's behavior or return values - Only optimizes the algorithm used internally - Maintains the same boundary calculation logic
### 6. **Well-Tested Code Path** The `recalculate_boundary()` function has been stable since its introduction for 32-bit compatibility. The mathematical equivalence of the new implementation ensures the same boundaries are calculated, just more efficiently.
### 7. **Follows Stable Tree Rules** This commit aligns with stable kernel criteria: - Fixes a real performance bug affecting users - Is obviously correct and well-tested - Has minimal risk of introducing new issues - Doesn't add new features
The only minor consideration is that this is a performance optimization rather than a correctness fix, but the significant performance impact (13.2% → 1.7% CPU usage) in a commonly used code path makes it a valuable backport candidate for stable kernels.
sound/core/pcm_native.c | 19 +++++++++++++++---- 1 file changed, 15 insertions(+), 4 deletions(-)
diff --git a/sound/core/pcm_native.c b/sound/core/pcm_native.c index 0a1ba26872f8..9b91f68b3fff 100644 --- a/sound/core/pcm_native.c +++ b/sound/core/pcm_native.c @@ -24,6 +24,7 @@ #include <sound/minors.h> #include <linux/uio.h> #include <linux/delay.h> +#include <linux/bitops.h>
#include "pcm_local.h"
@@ -3130,13 +3131,23 @@ struct snd_pcm_sync_ptr32 { static snd_pcm_uframes_t recalculate_boundary(struct snd_pcm_runtime *runtime) { snd_pcm_uframes_t boundary; + snd_pcm_uframes_t border; + int order;
if (! runtime->buffer_size) return 0; - boundary = runtime->buffer_size; - while (boundary * 2 <= 0x7fffffffUL - runtime->buffer_size) - boundary *= 2; - return boundary; + + border = 0x7fffffffUL - runtime->buffer_size; + if (runtime->buffer_size > border) + return runtime->buffer_size; + + order = __fls(border) - __fls(runtime->buffer_size); + boundary = runtime->buffer_size << order; + + if (boundary <= border) + return boundary; + else + return boundary / 2; }
static int snd_pcm_ioctl_sync_ptr_compat(struct snd_pcm_substream *substream,