The s390 clock has a higher granularity than nanoseconds. 1 nanosec equals 4.096 in s390 cputime_t. Therefore we leak a remainder while flushing the cputime through cputime_to_nsecs().
For more precision, make sure we keep that remainder on cputime accumulators for later accounting. Reported-by: Martin Schwidefsky <[email protected]> Cc: Benjamin Herrenschmidt <[email protected]> Cc: Paul Mackerras <[email protected]> Cc: Michael Ellerman <[email protected]> Cc: Heiko Carstens <[email protected]> Cc: Martin Schwidefsky <[email protected]> Cc: Tony Luck <[email protected]> Cc: Fenghua Yu <[email protected]> Cc: Peter Zijlstra <[email protected]> Cc: Rik van Riel <[email protected]> Cc: Thomas Gleixner <[email protected]> Cc: Ingo Molnar <[email protected]> Cc: Stanislaw Gruszka <[email protected]> Cc: Wanpeng Li <[email protected]> Signed-off-by: Frederic Weisbecker <[email protected]> --- arch/s390/include/asm/cputime.h | 4 ++++ arch/s390/kernel/vtime.c | 33 +++++++++++++++++++++------------ 2 files changed, 25 insertions(+), 12 deletions(-) diff --git a/arch/s390/include/asm/cputime.h b/arch/s390/include/asm/cputime.h index d1c407d..86b5e3f 100644 --- a/arch/s390/include/asm/cputime.h +++ b/arch/s390/include/asm/cputime.h @@ -33,6 +33,10 @@ static inline unsigned int cputime_to_usecs(const cputime_t cputime) return (__force unsigned long long) cputime >> 12; } +static inline cputime_t nsecs_to_cputime(u64 cputime) +{ + return (__force cputime_t) cputime * CPUTIME_PER_USEC * NSEC_PER_USEC; +} u64 arch_cpu_idle_time(int cpu); diff --git a/arch/s390/kernel/vtime.c b/arch/s390/kernel/vtime.c index b4a3e9e..922f959 100644 --- a/arch/s390/kernel/vtime.c +++ b/arch/s390/kernel/vtime.c @@ -94,8 +94,14 @@ static inline u64 update_tsk_timer(unsigned long *tsk_vtime, u64 new) { u64 delta; - delta = new - *tsk_vtime; - *tsk_vtime = new; + /* + * Since nsecs is less granular than cputime_t in s390, + * the conversion to nsecs is rounded. Make sure we don't + * lose the remainder. + */ + delta = cputime_to_nsecs(new - *tsk_vtime); + *tsk_vtime += nsecs_to_cputime(delta); + return delta; } @@ -124,7 +130,8 @@ static void account_system_index_scaled(struct task_struct *p, */ static int do_account_vtime(struct task_struct *tsk) { - u64 timer, clock, user, guest, system, hardirq, softirq, steal; + u64 timer, clock, delta; + u64 user, guest, system, hardirq, softirq, steal; timer = S390_lowcore.last_update_timer; clock = S390_lowcore.last_update_clock; @@ -161,18 +168,19 @@ static int do_account_vtime(struct task_struct *tsk) READ_ONCE(S390_lowcore.hardirq_timer)); softirq = update_tsk_timer(&tsk->thread.softirq_timer, READ_ONCE(S390_lowcore.softirq_timer)); - S390_lowcore.steal_timer += - clock - user - guest - system - hardirq - softirq; + + delta = nsecs_to_cputime(user + guest + system + hardirq + softirq); + S390_lowcore.steal_timer += clock - delta; /* Push account value */ if (user) { - account_user_time(tsk, cputime_to_nsecs(user)); - tsk->utimescaled += cputime_to_nsecs(scale_vtime(user)); + account_user_time(tsk, user); + tsk->utimescaled += scale_vtime(user); } if (guest) { - account_guest_time(tsk, cputime_to_nsecs(guest)); - tsk->utimescaled += cputime_to_nsecs(scale_vtime(guest)); + account_guest_time(tsk, guest); + tsk->utimescaled += scale_vtime(guest); } if (system) @@ -187,11 +195,12 @@ static int do_account_vtime(struct task_struct *tsk) steal = S390_lowcore.steal_timer; if ((s64) steal > 0) { - S390_lowcore.steal_timer = 0; - account_steal_time(cputime_to_nsecs(steal)); + u64 nsecs = cputime_to_nsecs(steal); + S390_lowcore.steal_timer -= nsecs_to_cputime(nsecs); + account_steal_time(nsecs); } - return virt_timer_forward(user + guest + system + hardirq + softirq); + return virt_timer_forward(delta); } void vtime_task_switch(struct task_struct *prev) -- 2.7.4
