Re: [PATCH] powerpc/entry: Fix double accounting of user time on interrupt entry
From: "Christophe Leroy (CS GROUP)" <chleroy@kernel.org>
Date: 2026-09-02 05:36:29
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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.
quoted hunk ↗ jump to hunk
Signed-off-by: Aboorva Devarajan <redacted> --- 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