Le 02/01/2026 à 15:09, Ryan Roberts a écrit :

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On 02/01/2026 13:39, Jason A. Donenfeld wrote:

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Hi Ryan,

On Fri, Jan 2, 2026 at 2:12 PM Ryan Roberts [off-list ref] wrote:

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context. Given the function is just a handful of operations and doesn't

How many? What's this looking like in terms of assembly?

25 instructions on arm64:

31 instructions on powerpc:

00000000 <prandom_u32_state>:
   0:    7c 69 1b 78     mr      r9,r3
   4:    80 63 00 00     lwz     r3,0(r3)
   8:    80 89 00 08     lwz     r4,8(r9)
   c:    81 69 00 04     lwz     r11,4(r9)
  10:    80 a9 00 0c     lwz     r5,12(r9)
  14:    54 67 30 32     slwi    r7,r3,6
  18:    7c e7 1a 78     xor     r7,r7,r3
  1c:    55 66 10 3a     slwi    r6,r11,2
  20:    54 88 68 24     slwi    r8,r4,13
  24:    54 63 90 18     rlwinm  r3,r3,18,0,12
  28:    7d 6b 32 78     xor     r11,r11,r6
  2c:    7d 08 22 78     xor     r8,r8,r4
  30:    54 aa 18 38     slwi    r10,r5,3
  34:    54 e7 9b 7e     srwi    r7,r7,13
  38:    7c e7 1a 78     xor     r7,r7,r3
  3c:    51 66 2e fe     rlwimi  r6,r11,5,27,31
  40:    54 84 38 28     rlwinm  r4,r4,7,0,20
  44:    7d 4a 2a 78     xor     r10,r10,r5
  48:    55 08 5d 7e     srwi    r8,r8,21
  4c:    7d 08 22 78     xor     r8,r8,r4
  50:    7c e3 32 78     xor     r3,r7,r6
  54:    54 a5 68 16     rlwinm  r5,r5,13,0,11
  58:    55 4a a3 3e     srwi    r10,r10,12
  5c:    7d 4a 2a 78     xor     r10,r10,r5
  60:    7c 63 42 78     xor     r3,r3,r8
  64:    90 e9 00 00     stw     r7,0(r9)
  68:    90 c9 00 04     stw     r6,4(r9)
  6c:    91 09 00 08     stw     r8,8(r9)
  70:    91 49 00 0c     stw     r10,12(r9)
  74:    7c 63 52 78     xor     r3,r3,r10
  78:    4e 80 00 20     blr

Among those, 8 instructions are for reading/writing the state in stack. They of
course disappear when inlining.

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It'd also be
nice to have some brief analysis of other call sites to have
confirmation this isn't blowing up other users.

I compiled defconfig before and after this patch on arm64 and compared the text
sizes:

$ ./scripts/bloat-o-meter -t vmlinux.before vmlinux.after
add/remove: 3/4 grow/shrink: 4/1 up/down: 836/-128 (708)
Function                                     old     new   delta
prandom_seed_full_state                      364     932    +568
pick_next_task_fair                         1940    2036     +96
bpf_user_rnd_u32                             104     196     +92
prandom_bytes_state                          204     260     +56
e843419@0f2b_00012d69_e34                      -       8      +8
e843419@0db7_00010ec3_23ec                     -       8      +8
e843419@02cb_00003767_25c                      -       8      +8
bpf_prog_select_runtime                      448     444      -4
e843419@0aa3_0000cfd1_1580                     8       -      -8
e843419@0aa2_0000cfba_147c                     8       -      -8
e843419@075f_00008d8c_184                      8       -      -8
prandom_u32_state                            100       -    -100
Total: Before=19078072, After=19078780, chg +0.00%

So 708 bytes more after inlining. The main cost is prandom_seed_full_state(),
which calls prandom_u32_state() 10 times (via prandom_warmup()). I expect we
could turn that into a loop to reduce ~450 bytes overall.

With following change the increase of prandom_seed_full_state() remains
reasonnable and performance wise it is a lot better as it avoids the read/write
of the state via the stack

diff --git a/lib/random32.c b/lib/random32.c
index 24e7acd9343f6..28a5b109c9018 100644
--- a/lib/random32.c
+++ b/lib/random32.c

@@ -94,17 +94,11 @@ EXPORT_SYMBOL(prandom_bytes_state);

 static void prandom_warmup(struct rnd_state *state)
 {
+    int i;
+
     /* Calling RNG ten times to satisfy recurrence condition */
-    prandom_u32_state(state);
-    prandom_u32_state(state);
-    prandom_u32_state(state);
-    prandom_u32_state(state);
-    prandom_u32_state(state);
-    prandom_u32_state(state);
-    prandom_u32_state(state);
-    prandom_u32_state(state);
-    prandom_u32_state(state);
-    prandom_u32_state(state);
+    for (i = 0; i < 10; i++)
+        prandom_u32_state(state);
 }

 void prandom_seed_full_state(struct rnd_state __percpu *pcpu_state)

The loop is:

 248:    38 e0 00 0a     li      r7,10
 24c:    7c e9 03 a6     mtctr   r7
 250:    55 05 30 32     slwi    r5,r8,6
 254:    55 46 68 24     slwi    r6,r10,13
 258:    55 27 18 38     slwi    r7,r9,3
 25c:    7c a5 42 78     xor     r5,r5,r8
 260:    7c c6 52 78     xor     r6,r6,r10
 264:    7c e7 4a 78     xor     r7,r7,r9
 268:    54 8b 10 3a     slwi    r11,r4,2
 26c:    7d 60 22 78     xor     r0,r11,r4
 270:    54 a5 9b 7e     srwi    r5,r5,13
 274:    55 08 90 18     rlwinm  r8,r8,18,0,12
 278:    54 c6 5d 7e     srwi    r6,r6,21
 27c:    55 4a 38 28     rlwinm  r10,r10,7,0,20
 280:    54 e7 a3 3e     srwi    r7,r7,12
 284:    55 29 68 16     rlwinm  r9,r9,13,0,11
 288:    7d 64 5b 78     mr      r4,r11
 28c:    7c a8 42 78     xor     r8,r5,r8
 290:    7c ca 52 78     xor     r10,r6,r10
 294:    7c e9 4a 78     xor     r9,r7,r9
 298:    50 04 2e fe     rlwimi  r4,r0,5,27,31
 29c:    42 00 ff b4     bdnz    250 <prandom_seed_full_state+0x7c>

Which replaces the 10 calls to prandom_u32_state()

  fc:    91 3f 00 0c     stw     r9,12(r31)
 100:    7f e3 fb 78     mr      r3,r31
 104:    48 00 00 01     bl      104 <prandom_seed_full_state+0x88>
            104: R_PPC_REL24    prandom_u32_state
 108:    7f e3 fb 78     mr      r3,r31
 10c:    48 00 00 01     bl      10c <prandom_seed_full_state+0x90>
            10c: R_PPC_REL24    prandom_u32_state
 110:    7f e3 fb 78     mr      r3,r31
 114:    48 00 00 01     bl      114 <prandom_seed_full_state+0x98>
            114: R_PPC_REL24    prandom_u32_state
 118:    7f e3 fb 78     mr      r3,r31
 11c:    48 00 00 01     bl      11c <prandom_seed_full_state+0xa0>
            11c: R_PPC_REL24    prandom_u32_state
 120:    7f e3 fb 78     mr      r3,r31
 124:    48 00 00 01     bl      124 <prandom_seed_full_state+0xa8>
            124: R_PPC_REL24    prandom_u32_state
 128:    7f e3 fb 78     mr      r3,r31
 12c:    48 00 00 01     bl      12c <prandom_seed_full_state+0xb0>
            12c: R_PPC_REL24    prandom_u32_state
 130:    7f e3 fb 78     mr      r3,r31
 134:    48 00 00 01     bl      134 <prandom_seed_full_state+0xb8>
            134: R_PPC_REL24    prandom_u32_state
 138:    7f e3 fb 78     mr      r3,r31
 13c:    48 00 00 01     bl      13c <prandom_seed_full_state+0xc0>
            13c: R_PPC_REL24    prandom_u32_state
 140:    7f e3 fb 78     mr      r3,r31
 144:    48 00 00 01     bl      144 <prandom_seed_full_state+0xc8>
            144: R_PPC_REL24    prandom_u32_state
 148:    80 01 00 24     lwz     r0,36(r1)
 14c:    7f e3 fb 78     mr      r3,r31
 150:    83 e1 00 1c     lwz     r31,28(r1)
 154:    7c 08 03 a6     mtlr    r0
 158:    38 21 00 20     addi    r1,r1,32
 15c:    48 00 00 00     b       15c <prandom_seed_full_state+0xe0>
            15c: R_PPC_REL24    prandom_u32_state


So approx the same number of instructions in size, while better performance.

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I'm not really sure if 708 is good or bad...

That's in the noise compared to the overall size of vmlinux, but if we change it
to a loop we also reduce pressure on the cache.