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

[Aboorva Devarajan <aboorvad@linux.ibm.com>]

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.

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 <aboorvad@linux.ibm.com>
---
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.
+	 */
 	account_cpu_user_entry();
 	account_stolen_time();
 

base-commit: fb442a6673ff1046bf67754957d95880fdb394b5
-- 
2.54.0