Re: Regression in v5.0-rc1 with autosuspend hrtimers

From: Ladislav Michl
Date: Wed Jan 09 2019 - 11:07:42 EST

On Wed, Jan 09, 2019 at 03:12:25PM +0100, Vincent Guittot wrote:
> Please keep all thread list when replying :-)

Ahh, sorry for that...
[snip]
> On Wed, 9 Jan 2019 at 14:33, Ladislav Michl <ladis@xxxxxxxxxxxxxx> wrote:
> > I agree, but it doea all the magic correctly, so you won't need
> > to touch that code is ktime_t changes (I know, unlikely..)
>
> But the current implementation doesn't care of any changes in ktime_t
> as it uses raw ns

Fair enough, so let's go back to:
diff --git a/drivers/base/power/runtime.c b/drivers/base/power/runtime.c
index 70624695b6d5..e61ee9b47a26 100644
--- a/drivers/base/power/runtime.c
+++ b/drivers/base/power/runtime.c
@@ -121,7 +121,7 @@ static void pm_runtime_cancel_pending(struct device *dev)
* Compute the autosuspend-delay expiration time based on the device's
* power.last_busy time. If the delay has already expired or is disabled
* (negative) or the power.use_autosuspend flag isn't set, return 0.
- * Otherwise return the expiration time in jiffies (adjusted to be nonzero).
+ * Otherwise return the expiration time in nanoseconds (adjusted to be nonzero).
*
* This function may be called either with or without dev->power.lock held.
* Either way it can be racy, since power.last_busy may be updated at any time.
@@ -129,24 +129,21 @@ static void pm_runtime_cancel_pending(struct device *dev)
u64 pm_runtime_autosuspend_expiration(struct device *dev)
{
int autosuspend_delay;
- u64 last_busy, expires = 0;
- u64 now = ktime_to_ns(ktime_get());
+ u64 expires;

if (!dev->power.use_autosuspend)
- goto out;
+ return 0;

autosuspend_delay = READ_ONCE(dev->power.autosuspend_delay);
if (autosuspend_delay < 0)
- goto out;
-
- last_busy = READ_ONCE(dev->power.last_busy);
+ return 0;

- expires = last_busy + autosuspend_delay * NSEC_PER_MSEC;
- if (expires <= now)
- expires = 0; /* Already expired. */
+ expires = READ_ONCE(dev->power.last_busy);
+ expires += NSEC_PER_MSEC * (u64)autosuspend_delay;
+ if (expires > ktime_to_ns(ktime_get()))
+ return expires; /* Expires in the future */

- out:
- return expires;
+ return 0;
}
EXPORT_SYMBOL_GPL(pm_runtime_autosuspend_expiration);

Which gives for arm:
00000480 <pm_runtime_autosuspend_expiration>:
480: e92d41f0 push {r4, r5, r6, r7, r8, lr}
484: e5d030d1 ldrb r3, [r0, #209] ; 0xd1
488: e3130004 tst r3, #4
48c: 0a00000c beq 4c4 <pm_runtime_autosuspend_expiration+0x44>
490: e59030e4 ldr r3, [r0, #228] ; 0xe4
494: e3530000 cmp r3, #0
498: ba000009 blt 4c4 <pm_runtime_autosuspend_expiration+0x44>
49c: e28050e8 add r5, r0, #232 ; 0xe8
4a0: e8950030 ldm r5, {r4, r5}
4a4: e59f2030 ldr r2, [pc, #48] ; 4dc <pm_runtime_autosuspend_expiration+0x5c>
4a8: e0e54392 smlal r4, r5, r2, r3
4ac: ebfffffe bl 0 <ktime_get>
4b0: e1550001 cmp r5, r1
4b4: 01540000 cmpeq r4, r0
4b8: e1a06004 mov r6, r4
4bc: e1a07005 mov r7, r5
4c0: 8a000001 bhi 4cc <pm_runtime_autosuspend_expiration+0x4c>
4c4: e3a06000 mov r6, #0
4c8: e3a07000 mov r7, #0
4cc: e1a00006 mov r0, r6
4d0: e1a01007 mov r1, r7
4d4: e8bd41f0 pop {r4, r5, r6, r7, r8, lr}
4d8: e12fff1e bx lr
4dc: 000f4240 .word 0x000f4240

And x86:
0000000000000230 <pm_runtime_autosuspend_expiration.part.0>:
230: 8b 87 a4 01 00 00 mov 0x1a4(%rdi),%eax
236: 53 push %rbx
237: 85 c0 test %eax,%eax
239: 78 1e js 259 <pm_runtime_autosuspend_expiration.part.0+0x29>
23b: 48 63 d8 movslq %eax,%rbx
23e: 48 8b 97 a8 01 00 00 mov 0x1a8(%rdi),%rdx
245: 48 69 db 40 42 0f 00 imul $0xf4240,%rbx,%rbx
24c: 48 01 d3 add %rdx,%rbx
24f: e8 00 00 00 00 callq 254 <pm_runtime_autosuspend_expiration.part.0+0x24>
254: 48 39 c3 cmp %rax,%rbx
257: 77 02 ja 25b <pm_runtime_autosuspend_expiration.part.0+0x2b>
259: 31 db xor %ebx,%ebx
25b: 48 89 d8 mov %rbx,%rax
25e: 5b pop %rbx
25f: c3 retq

00000000000002a0 <pm_runtime_autosuspend_expiration>:
2a0: f6 87 91 01 00 00 04 testb $0x4,0x191(%rdi)
2a7: 74 02 je 2ab <pm_runtime_autosuspend_expiration+0xb>
2a9: eb 85 jmp 230 <pm_runtime_autosuspend_expiration.part.0>
2ab: 31 c0 xor %eax,%eax
2ad: c3 retq
2ae: 66 90 xchg %ax,%ax


[snip]
> > Well, main concern was not to call ktime_get at the beginning of function
> > as it is not too cheap.
>
> Doesn't the compiler optimize that to just before the test ?

No (compare with above). And it is also almost certain that someone will run
script and send "...do not needlesly initialize..." patch :)

gcc version 8.2.1 20181130 (OSELAS.Toolchain-2018.12.0 8-20181130)

00000110 <pm_runtime_autosuspend_expiration>:
110: e92d41f0 push {r4, r5, r6, r7, r8, lr}
114: e1a06000 mov r6, r0
118: ebfffffe bl 0 <ktime_get>
11c: e5d630d1 ldrb r3, [r6, #209] ; 0xd1
120: e3130004 tst r3, #4
124: 0a00000d beq 160 <pm_runtime_autosuspend_expiration+0x50>
128: e596c0e4 ldr ip, [r6, #228] ; 0xe4
12c: e35c0000 cmp ip, #0
130: ba00000a blt 160 <pm_runtime_autosuspend_expiration+0x50>
134: e28630e8 add r3, r6, #232 ; 0xe8
138: e893000c ldm r3, {r2, r3}
13c: e1a05001 mov r5, r1
140: e1a04000 mov r4, r0
144: e59f002c ldr r0, [pc, #44] ; 178 <pm_runtime_autosuspend_expiration+0x68>
148: e0010c90 mul r1, r0, ip
14c: e0926001 adds r6, r2, r1
150: e0a37fc1 adc r7, r3, r1, asr #31
154: e1550007 cmp r5, r7
158: 01540006 cmpeq r4, r6
15c: 3a000001 bcc 168 <pm_runtime_autosuspend_expiration+0x58>
160: e3a06000 mov r6, #0
164: e3a07000 mov r7, #0
168: e1a00006 mov r0, r6
16c: e1a01007 mov r1, r7
170: e8bd41f0 pop {r4, r5, r6, r7, r8, lr}
174: e12fff1e bx lr
178: 000f4240 .word 0x000f4240