Re: [PATCH] powerpc/entry: Fix double accounting of user time on interrupt entry
From: Shrikanth Hegde <hidden>
Date: 2026-09-02 07:07:58
Also in:
lkml
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 <redacted>
---
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.
quoted hunk ↗ jump to hunk
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