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SubjectRe: [RFC PATCH v1] powerpc/accounting: do not account system time on transition to user.
Christophe Leroy's on February 9, 2019 12:40 am:
> 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@c-s.fr>

This looks like a clever little optimization, although I don't know
this time accounting code very well.

> ---
> But surprisingly, this patch degrades the null_syscall selftest by 20% on the 8xx. Any idea of the reason ?

I don't know microarchitecture of any of those CPUs I'm afraid.
On the 64s CPUs, mftb is what hurts.

>
> 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
>
>

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