On 2026/09/04 08:28 AM, Aboorva Devarajan wrote:
> Since the switch to generic entry, an interrupt 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() /* first */
> irqentry_enter()
> arch_enter_from_user_mode()
> account_cpu_user_entry() /* second */
>
> The second call charges the same interval again, because
> account_cpu_user_entry() accumulates the time spent in user mode
> since the last return to user space.
>
> The two calls come from the GENERIC_ENTRY preparation series,
> where each step was a no-op on its own. Commit 09a9d3a8499d
> ("powerpc: introduce arch_enter_from_user_mode") added the hook
> with the user-time accounting in it, but nothing called it yet.
> Commit 893082ac769b ("powerpc: Prepare for IRQ entry exit")
> copied interrupt_enter_prepare() verbatim into entry-common.h as
> arch_interrupt_enter_prepare(); that copy was equally unused, as
> handlers still called interrupt_enter_prepare().
>
> Commit bee25f97ad24 ("powerpc: Enable GENERIC_ENTRY feature")
> made both live. On the syscall side it did the full conversion:
> system_call_exception() now accounts once through the hook via
> syscall_enter_from_user_mode(), rather than calling
> account_cpu_user_entry() directly. On the interrupt side it
> switched the handler macros to arch_interrupt_enter_prepare()
> followed by irqentry_enter(), which also runs the hook, but the
> accounting in arch_interrupt_enter_prepare() was not removed to
> match. The double accounting starts with that commit.
>
> With CONFIG_VIRT_CPU_ACCOUNTING_NATIVE=y this roughly doubles the
> reported user time of any workload that takes interrupts. The
> other accounting modes compile account_cpu_user_entry() to an
> empty stub, so they are not affected.
>
> Remove the accounting from arch_interrupt_enter_prepare() and rely
> on arch_enter_from_user_mode(), which already runs for both
> syscalls and interrupts. The duplicate account_stolen_time() call
> is removed the same way.
>
> On a pseries LPAR a busy loop reports 6s user time in 3s elapsed
> (~210% CPU) before the fix, and 3s (~105% CPU) after it:
>
> $ python3 -c 'while True: pass' &
> $ sleep 3; ps -p $! -o etime,time,pcpu
>
> ELAPSED TIME %CPU
> Before 00:03 00:00:06 210
> After 00:03 00:00:03 105
>
> A 50% load reports ~70% usr / 30% idle before the fix, and
> ~49% usr / 51% idle after it:
>
> $ taskset -c 6 stress-ng --cpu 1 --cpu-load 50 &
> $ mpstat -P 6 1
>
> CPU %usr %idle
> Before 6 69.74 30.26
> After 6 48.51 50.50
>
> Fixes: bee25f97ad24 ("powerpc: Enable GENERIC_ENTRY feature")
> Reviewed-by: Mukesh Kumar Chaurasiya (IBM) <[email protected]>
> Signed-off-by: Aboorva Devarajan <[email protected]>
The fix is minimal and correct: removing the two lines from
arch_interrupt_enter_prepare() leaves arch_enter_from_user_mode() as the single
accounting site for both syscalls and interrupts, which matches the generic
entry design intent. The benchmarks in the commit message are convincing.
Reviewed-by: Amit Machhiwal <[email protected]>
Thanks,
Amit
> ---
> Changes in v2:
> - Explain which commits introduced each accounting call and how
> they overlapped in commit message.
> - Drop the code comment.
>
> v1:
> https://lore.kernel.org/linuxppc-dev/[email protected]/
>
> arch/powerpc/include/asm/entry-common.h | 2 --
> 1 file changed, 2 deletions(-)
>
> diff --git a/arch/powerpc/include/asm/entry-common.h
> b/arch/powerpc/include/asm/entry-common.h
> index c5adb5006361..000fa925e26a 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);
> /*
>
> base-commit: fb442a6673ff1046bf67754957d95880fdb394b5
> --
> 2.54.0
>
