Re: [PATCH] powerpc/entry: Fix double accounting of user time on interrupt entry

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