Hi Aboorva.
On 9/2/26 10:36 AM, Aboorva Devarajan wrote:
Since the switch to generic entry, an interrupt taken from user mode
accounts user time twice: once in arch_interrupt_enter_prepare() and
again in arch_enter_from_user_mode(), which irqentry_enter() invokes
for the same interrupt:
arch_interrupt_enter_prepare()
account_cpu_user_entry()
irqentry_enter()
arch_enter_from_user_mode()
account_cpu_user_entry()
account_cpu_user_entry() accumulates the time spent in user mode
since the last return to user space, so the second call charges the
same interval again.
With CONFIG_VIRT_CPU_ACCOUNTING_NATIVE=y this roughly doubles the
reported user time of any workload that takes interrupts. On a
pseries LPAR, ps/top show ~200% CPU for a single-threaded CPU-bound
loop, and time(1) reports user time about twice the elapsed time.
Could you please run mpstat with 50% or less loading workload like stress-ng
and document the difference in changelog.
Remove the accounting from arch_interrupt_enter_prepare() and rely on
arch_enter_from_user_mode(), which runs for both syscalls and
interrupts. The duplicate account_stolen_time() call is removed the
same way.
Fixes: bee25f97ad24 ("powerpc: Enable GENERIC_ENTRY feature")
Signed-off-by: Aboorva Devarajan <[email protected]>
---
Verified on a pseries LPAR (CONFIG_VIRT_CPU_ACCOUNTING_NATIVE=y),
Usual default is VIRT_CPU_ACCOUNTING_GEN, selected by NO_HZ_FULL.
Most distros usually enable NO_HZ_FULL=y.
At hindsight, I don't see the issue dependency on it. But better to be sure.
7.3.0-rc1, single-threaded CPU-bound loop:
Before:
$ python3 -c 'while True: pass' &
$ sleep 3; ps -p $! -o pid,etime,time,pcpu
PID ELAPSED TIME %CPU
4980 00:03 00:00:06 210
After:
$ python3 -c 'while True: pass' &
$ sleep 3; ps -p $! -o pid,etime,time,pcpu
PID ELAPSED TIME %CPU
4951 00:03 00:00:03 105
arch/powerpc/include/asm/entry-common.h | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)
diff --git a/arch/powerpc/include/asm/entry-common.h
b/arch/powerpc/include/asm/entry-common.h
index c5adb5006361..984294e62568 100644
--- a/arch/powerpc/include/asm/entry-common.h
+++ b/arch/powerpc/include/asm/entry-common.h
@@ -222,8 +222,6 @@ static inline void arch_interrupt_enter_prepare(struct
pt_regs *regs)
if (user_mode(regs)) {
kuap_lock();
- account_cpu_user_entry();
- account_stolen_time();
} else {
kuap_save_and_lock(regs);
/*
@@ -426,6 +424,10 @@ static __always_inline void
arch_enter_from_user_mode(struct pt_regs *regs)
#endif
kuap_assert_locked();
booke_restore_dbcr0();
+ /*
+ * User and stolen time is accounted here for every entry from
+ * user mode. The interrupt prepare hooks must not account again.
+ */
account_cpu_user_entry();
account_stolen_time();
base-commit: fb442a6673ff1046bf67754957d95880fdb394b5
