On Wed, 2026-09-02 at 12:37 +0530, Shrikanth Hegde wrote:
Hi Shrikanth,
> 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.
Sure, will add it in v2.
1. On the same pseries LPAR, taskset -c 6 stress-ng --cpu 1 --cpu-load 50:
Without the patch, the reported %usr is inflated, while with the patch
it is close to the expected 50%.
Without Patch:
06:43:09 AM CPU %usr** %nice %sys %iowait %irq %soft %steal
%guest %gnice %idle
06:43:10 AM 6 69.74 0.00 0.00 0.00 0.00 0.00 0.00
0.00 0.00 30.26
With Patch:
06:29:33 AM CPU %usr** %nice %sys %iowait %irq %soft %steal
%guest %gnice %idle
06:29:34 AM 6 48.51 0.00 0.00 0.00 0.99 0.00 0.00
0.00 0.00 50.50
2. Single-threaded CPU-bound workload:
taskset -c 100 python3 -c 'while True: pass' (executed for ~3 seconds)
Without patch:
TIME ELAPSED %CPU
00:00:06 00:02 209
With patch:
TIME ELAPSED %CPU
00:00:03 00:03 104
>
> > 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.
It is only observed with CONFIG_VIRT_CPU_ACCOUNTING_NATIVE.
account_cpu_user_entry() updates utime only with NATIVE accounting.
With CONFIG_VIRT_CPU_ACCOUNTING_GEN or tick-based accounting, it is
an empty stub and therefore a no-op.
So kernels using CONFIG_VIRT_CPU_ACCOUNTING_GEN or tick-based accounting
are not affected.
> > 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
Thanks,
Aboorva
