[RFC PATCH v1] powerpc/accounting: do not account system time on transition to user.

[Christophe Leroy]

Time spent in kernel mode don't need to be accounted on transition
to user space. As far as the time spent in user is known, it
is possible to calculate the time spent in kernel by substracting
the time spent in user.

To do so, this patch modifies vtime_delta() to substract the
time spent in user since the last call to vtime_delta().

This patch gives a 2% improvment of null_syscall() selftest on a 83xx.

Signed-off-by: Christophe Leroy <christophe.leroy@xxxxxx>
---
But surprisingly, this patch degrades the null_syscall selftest by 20% on the 8xx. Any idea of the reason ?

 arch/powerpc/include/asm/accounting.h | 1 +
 arch/powerpc/include/asm/ppc_asm.h    | 8 +-------
 arch/powerpc/kernel/asm-offsets.c     | 8 ++------
 arch/powerpc/kernel/time.c            | 4 +++-
 4 files changed, 7 insertions(+), 14 deletions(-)

diff --git a/arch/powerpc/include/asm/accounting.h b/arch/powerpc/include/asm/accounting.h
index c607c5d835cc..2f1ff5f9fd7a 100644
--- a/arch/powerpc/include/asm/accounting.h
+++ b/arch/powerpc/include/asm/accounting.h
@@ -27,6 +27,7 @@ struct cpu_accounting_data {
 	/* Internal counters */
 	unsigned long starttime;	/* TB value snapshot */
 	unsigned long starttime_user;	/* TB value on exit to usermode */
+	unsigned long utime_asm;
 #ifdef CONFIG_ARCH_HAS_SCALED_CPUTIME
 	unsigned long startspurr;	/* SPURR value snapshot */
 	unsigned long utime_sspurr;	/* ->user_time when ->startspurr set */
diff --git a/arch/powerpc/include/asm/ppc_asm.h b/arch/powerpc/include/asm/ppc_asm.h
index e0637730a8e7..be17d570d484 100644
--- a/arch/powerpc/include/asm/ppc_asm.h
+++ b/arch/powerpc/include/asm/ppc_asm.h
@@ -28,9 +28,8 @@
 #define ACCOUNT_STOLEN_TIME
 #else
 #define ACCOUNT_CPU_USER_ENTRY(ptr, ra, rb)				\
-	MFTB(ra);			/* get timebase */		\
 	PPC_LL	rb, ACCOUNT_STARTTIME_USER(ptr);			\
-	PPC_STL	ra, ACCOUNT_STARTTIME(ptr);				\
+	MFTB(ra);			/* get timebase */		\
 	subf	rb,rb,ra;		/* subtract start value */	\
 	PPC_LL	ra, ACCOUNT_USER_TIME(ptr);				\
 	add	ra,ra,rb;		/* add on to user time */	\
@@ -38,12 +37,7 @@
 
 #define ACCOUNT_CPU_USER_EXIT(ptr, ra, rb)				\
 	MFTB(ra);			/* get timebase */		\
-	PPC_LL	rb, ACCOUNT_STARTTIME(ptr);				\
 	PPC_STL	ra, ACCOUNT_STARTTIME_USER(ptr);			\
-	subf	rb,rb,ra;		/* subtract start value */	\
-	PPC_LL	ra, ACCOUNT_SYSTEM_TIME(ptr);				\
-	add	ra,ra,rb;		/* add on to system time */	\
-	PPC_STL	ra, ACCOUNT_SYSTEM_TIME(ptr)
 
 #ifdef CONFIG_PPC_SPLPAR
 #define ACCOUNT_STOLEN_TIME						\
diff --git a/arch/powerpc/kernel/asm-offsets.c b/arch/powerpc/kernel/asm-offsets.c
index 7a1b93c5af63..f2ba7735f56f 100644
--- a/arch/powerpc/kernel/asm-offsets.c
+++ b/arch/powerpc/kernel/asm-offsets.c
@@ -260,19 +260,15 @@ int main(void)
 	OFFSET(PACAHWCPUID, paca_struct, hw_cpu_id);
 	OFFSET(PACAKEXECSTATE, paca_struct, kexec_state);
 	OFFSET(PACA_DSCR_DEFAULT, paca_struct, dscr_default);
-	OFFSET(ACCOUNT_STARTTIME, paca_struct, accounting.starttime);
 	OFFSET(ACCOUNT_STARTTIME_USER, paca_struct, accounting.starttime_user);
-	OFFSET(ACCOUNT_USER_TIME, paca_struct, accounting.utime);
-	OFFSET(ACCOUNT_SYSTEM_TIME, paca_struct, accounting.stime);
+	OFFSET(ACCOUNT_USER_TIME, paca_struct, accounting.utime_asm);
 	OFFSET(PACA_TRAP_SAVE, paca_struct, trap_save);
 	OFFSET(PACA_NAPSTATELOST, paca_struct, nap_state_lost);
 	OFFSET(PACA_SPRG_VDSO, paca_struct, sprg_vdso);
 #else /* CONFIG_PPC64 */
 #ifdef CONFIG_VIRT_CPU_ACCOUNTING_NATIVE
-	OFFSET(ACCOUNT_STARTTIME, thread_info, accounting.starttime);
 	OFFSET(ACCOUNT_STARTTIME_USER, thread_info, accounting.starttime_user);
-	OFFSET(ACCOUNT_USER_TIME, thread_info, accounting.utime);
-	OFFSET(ACCOUNT_SYSTEM_TIME, thread_info, accounting.stime);
+	OFFSET(ACCOUNT_USER_TIME, thread_info, accounting.utime_asm);
 #endif
 #endif /* CONFIG_PPC64 */
 
diff --git a/arch/powerpc/kernel/time.c b/arch/powerpc/kernel/time.c
index bc0503ef9c9c..79420643b45f 100644
--- a/arch/powerpc/kernel/time.c
+++ b/arch/powerpc/kernel/time.c
@@ -331,8 +331,10 @@ static unsigned long vtime_delta(struct task_struct *tsk,
 	WARN_ON_ONCE(!irqs_disabled());
 
 	now = mftb();
-	stime = now - acct->starttime;
+	stime = now - acct->starttime - acct->utime_asm;
 	acct->starttime = now;
+	acct->utime += acct->utime_asm;
+	acct->utime_asm = 0;
 
 	*stime_scaled = vtime_delta_scaled(acct, now, stime);
 
-- 
2.13.3