Re: [PATCH 04/15] powerpc/time: Prepare to stop elapsing in dynticks-idle
From: Shrikanth Hegde
Date: Wed Feb 25 2026 - 08:40:36 EST
On 2/25/26 4:44 PM, Christophe Leroy (CS GROUP) wrote:
Hi Hegde,
Le 25/02/2026 à 11:34, Shrikanth Hegde a écrit :
Hi Christophe.
On 2/25/26 3:15 PM, Christophe Leroy (CS GROUP) wrote:
Hope it is more explicit now.
Got it. The main concern was around with additional computation that sched_clock,
not any additional paths per se.
yes, that would be possible,
How about we do below? This adds only one subtraction.
This achieves the same outcome.
It adds a bit more than just a substration. It adds a call to an extern fonction.
I think we should make it always inline and move it to time.h
00000164 <my_account_cpu_user_entry>:
164: 94 21 ff f0 stwu r1,-16(r1)
168: 7c 08 02 a6 mflr r0
16c: 90 01 00 14 stw r0,20(r1)
170: 93 e1 00 0c stw r31,12(r1)
174: 7f ec 42 e6 mftb r31
178: 48 00 00 01 bl 178 <my_account_cpu_user_entry+0x14>
178: R_PPC_REL24 get_boot_tb
17c: 81 02 00 08 lwz r8,8(r2)
180: 81 22 00 28 lwz r9,40(r2)
184: 7c 84 f8 50 subf r4,r4,r31
188: 7d 29 40 50 subf r9,r9,r8
18c: 7d 29 22 14 add r9,r9,r4
190: 90 82 00 24 stw r4,36(r2)
194: 91 22 00 08 stw r9,8(r2)
198: 80 01 00 14 lwz r0,20(r1)
19c: 83 e1 00 0c lwz r31,12(r1)
1a0: 7c 08 03 a6 mtlr r0
1a4: 38 21 00 10 addi r1,r1,16
1a8: 4e 80 00 20 blr
000001ac <my_account_cpu_user_exit>:
1ac: 94 21 ff f0 stwu r1,-16(r1)
1b0: 7c 08 02 a6 mflr r0
1b4: 90 01 00 14 stw r0,20(r1)
1b8: 93 e1 00 0c stw r31,12(r1)
1bc: 7f ec 42 e6 mftb r31
1c0: 48 00 00 01 bl 1c0 <my_account_cpu_user_exit+0x14>
1c0: R_PPC_REL24 get_boot_tb
1c4: 81 02 00 0c lwz r8,12(r2)
1c8: 81 22 00 24 lwz r9,36(r2)
1cc: 7c 84 f8 50 subf r4,r4,r31
1d0: 7d 29 40 50 subf r9,r9,r8
1d4: 7d 29 22 14 add r9,r9,r4
1d8: 90 82 00 28 stw r4,40(r2)
1dc: 91 22 00 0c stw r9,12(r2)
1e0: 80 01 00 14 lwz r0,20(r1)
1e4: 83 e1 00 0c lwz r31,12(r1)
1e8: 7c 08 03 a6 mtlr r0
1ec: 38 21 00 10 addi r1,r1,16
1f0: 4e 80 00 20 blr
I really still can't see the point of this substraction.
At one place we do
tb1 = mftb1;
acct->utime += (tb1 - acct->starttime_user);
acct->starttime = tb1;
At the other place we do
tb2 = mftb2;
acct->stime += (tb2 - acct->starttime);
acct->starttime_user = tb2;
So at the end we have
acct->utime += mftb1 - mftb2;
acct->stime += mftb2 - mftb1;
You want to change to
tb1 = mftb1 - boot_tb;
tb2 = mftb2 - boot_tb;
At the end we would get
acct->utime += mftb1 - boot_tb - mftb2 + boot_tb = mftb1 - mftb2;
acct->stime += mftb2 - boot_tb - mftb1 + boot_tb = mftb2 - mftb1;
So what's the point in doing such a useless substract that disappears at the end ? What am I missing ?
I had similar thought, but I saw this data below when i do exec on the system.
This was the stats seen on PowerNV system with 144 CPUs.
Nothing is running on the system after boot. So it is mostly idle.
======== With the series applied ===
cat /proc/stat | head
cpu 1494 0 135607576 9628633227 16876 142 63 0 0 0
cpu0 0 0 8 67807311 0 2 40 0 0 0
cpu1 0 0 6 67807349 0 0 0 0 0 0
cat /proc/uptime
48.32 96286332.82 << Note this value is too huge. Also system value is also huge.
========= without the series(tip/master) ===============
cat /proc/stat | head
cpu 2003 0 67866261 859414 15923 249 66 0 0 0
cpu0 5 0 23 5595 461 2 38 0 0 0
cpu1 0 0 9 6092 21 0 3 0 0 0
cat /proc/uptime
61.29 8594.82 << This is right. 144*61 = 8784.
But note, the system time reported. i.e 67866261. It is too huge again. And very close to actual mftb value
rather than the diff. i.e we have paths were tb1 is not done. tb2 is effectively mftb - 0
========= with proposed fix of mftb - boot_tb ===============
cat /proc/stat | head
cpu 5187 0 10996 2025690 16566 765 184 0 0 0
cpu0 9 0 28 14096 65 6 108 0 0 0
cpu1 4 0 15 14277 0 0 2 0 0 0
cat /proc/uptime
142.97 20257.42 << Looks correct, since 142*144 is close to 20448
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Now lets go to CONFIG_VIRT_CPU_ACCOUNTING_GEN=y
cat /proc/stat | head
cpu 1804 0 3003 791760 15695 0 0 0 0 0
cpu0 22 0 46 5535 0 0 0 0 0 0
cpu1 0 0 7 5637 0 0 0 0 0 0
cat /proc/uptime
56.49 7918.05 << Looks correct. close 56*144
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