Le 02/09/2026 à 07:06, Aboorva Devarajan a écrit :
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.
One of them was added by commit 09a9d3a8499d ("powerpc: introduce
arch_enter_from_user_mode"), the other one by commit 893082ac769b
("powerpc: Prepare for IRQ entry exit"). Would be good to mention it and
explain how it went wrong.
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.
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")
Not sure it is the correct commit. Should probably be one of the ones
that introduced the wrong call.
Signed-off-by: Aboorva Devarajan <[email protected]>
---
Verified on a pseries LPAR (CONFIG_VIRT_CPU_ACCOUNTING_NATIVE=y),
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.
+ */
Don't polute the code with such a comment. This is to be explained in
the commit message. First a comment shall tell what code does, not what
it doesn't do. And the comment has no added value, when you see
account_cpu_user_entry() is called you already know that.
Here you feel the need to add such comment, but if you had implemented
this function yourself and done it right at the first time, would you
have felt the need for this comment ?
Have a look at https://docs.kernel.org/process/coding-style.html#commenting
account_cpu_user_entry();
account_stolen_time();
base-commit: fb442a6673ff1046bf67754957d95880fdb394b5
