On Mon, 1 Dec 2014 13:24:52 +0100 Heiko Carstens <[email protected]> wrote:
> On Fri, Nov 28, 2014 at 07:23:36PM +0100, Frederic Weisbecker wrote: > > This will be needed for the conversion of kernel stat to nsecs. > > > > Cc: Benjamin Herrenschmidt <[email protected]> > > Cc: Heiko Carstens <[email protected]> > > Cc: Ingo Molnar <[email protected]> > > Cc: Martin Schwidefsky <[email protected]> > > Cc: Oleg Nesterov <[email protected]> > > Cc: Paul Mackerras <[email protected]> > > Cc: Peter Zijlstra <[email protected]> > > Cc: Rik van Riel <[email protected]> > > Cc: Thomas Gleixner <[email protected]> > > Cc: Tony Luck <[email protected]> > > Cc: Wu Fengguang <[email protected]> > > Signed-off-by: Frederic Weisbecker <[email protected]> > > --- > > arch/s390/include/asm/cputime.h | 5 +++++ > > 1 file changed, 5 insertions(+) > > > > diff --git a/arch/s390/include/asm/cputime.h > > b/arch/s390/include/asm/cputime.h > > index 820b38a..75ba96f 100644 > > --- a/arch/s390/include/asm/cputime.h > > +++ b/arch/s390/include/asm/cputime.h > > @@ -59,6 +59,11 @@ static inline cputime64_t jiffies64_to_cputime64(const > > u64 jif) > > return (__force cputime64_t)(jif * (CPUTIME_PER_SEC / HZ)); > > } > > > > +static inline u64 cputime64_to_nsecs(cputime64_t cputime) > > +{ > > + return (__force u64)cputime * CPUTIME_PER_USEC * NSEC_PER_USEC; > > +} > > + > > This is incorrect. You probably wanted to write something like > > return (__force u64)cputime / CPUTIME_PER_USEC * NSEC_PER_USEC; ? > > However we would still lose a lot of precision. > The correct algorithm to convert from cputime to nanoseconds can be found in > tod_to_ns() - see arch/s390/include/asm/timex.h > > And if you see that rather complex algorithm, I doubt we want to have the > changes you propose. We need to have that calculation three times for each > irq (user, system and steal time) and would still have worse precision than > we have right now. Not talking about the additional wasted cpu cycles... > > But I guess Martin wanted to comment on your patches anyway ;) The function that gets called most often is the accounting code for irq_enter and irq_exit. Both are mapped to vtime_account_irq_enter, with the correct implementation for the cputime_to_nsec the function gets 15 instructions longer. The relevant code sequence Upstream code: 10592e: e3 10 02 e8 00 04 lg %r1,744 105934: b2 09 f0 a0 stpt 160(%r15) 105938: e3 30 f0 a0 00 04 lg %r3,160(%r15) 10593e: e3 10 02 d8 00 08 ag %r1,728 105944: e3 30 02 e8 00 24 stg %r3,744 10594a: b9 09 00 13 sgr %r1,%r3 10594e: e3 10 02 d8 00 24 stg %r1,728 105954: b9 04 00 41 lgr %r4,%r1 105958: e3 40 c0 68 00 09 sg %r4,104(%r12) 10595e: e3 30 02 e0 00 04 lg %r3,736 105964: b9 09 00 34 sgr %r3,%r4 105968: b9 04 00 54 lgr %r5,%r4 10596c: e3 30 02 e0 00 24 stg %r3,736 105972: a7 39 00 00 lghi %r3,0 105976: e3 10 c0 68 00 24 stg %r1,104(%r12) 10597c: b9 04 00 b4 lgr %r11,%r4 105980: c0 e5 00 03 78 4c brasl %r14,174a18 <account_system_time Patched code: 105a3e: e3 50 02 e8 00 04 lg %r5,744 105a44: b2 09 f0 a0 stpt 160(%r15) 105a48: b9 04 00 15 lgr %r1,%r5 105a4c: e3 50 f0 a0 00 04 lg %r5,160(%r15) 105a52: e3 50 02 e8 00 24 stg %r5,744 105a58: e3 10 02 d8 00 08 ag %r1,728 105a5e: b9 e9 50 51 sgrk %r5,%r1,%r5 105a62: e3 00 02 e0 00 04 lg %r0,736 105a68: e3 50 02 d8 00 24 stg %r5,728 105a6e: b9 04 00 15 lgr %r1,%r5 105a72: e3 10 a0 68 00 09 sg %r1,104(%r10) 105a78: b9 04 00 e1 lgr %r14,%r1 105a7c: b9 04 00 81 lgr %r8,%r1 105a80: eb 11 00 20 00 0c srlg %r1,%r1,32 105a86: ec 3e 20 bf 00 55 risbg %r3,%r14,32,191,0 105a8c: eb 91 00 02 00 0d sllg %r9,%r1,2 105a92: eb c3 00 07 00 0d sllg %r12,%r3,7 105a98: eb b1 00 07 00 0d sllg %r11,%r1,7 105a9e: eb 43 00 02 00 0d sllg %r4,%r3,2 105aa4: b9 09 00 b9 sgr %r11,%r9 105aa8: b9 e9 40 4c sgrk %r4,%r12,%r4 105aac: b9 08 00 43 agr %r4,%r3 105ab0: b9 08 00 1b agr %r1,%r11 105ab4: b9 e9 e0 30 sgrk %r3,%r0,%r14 105ab8: eb 11 00 17 00 0d sllg %r1,%r1,23 105abe: e3 30 02 e0 00 24 stg %r3,736 105ac4: eb 44 00 09 00 0c srlg %r4,%r4,9 105aca: e3 50 a0 68 00 24 stg %r5,104(%r10) 105ad0: b9 08 00 41 agr %r4,%r1 105ad4: b9 04 00 54 lgr %r5,%r4 105ad8: a7 39 00 00 lghi %r3,0 105adc: c0 e5 00 03 78 02 brasl %r14,174ae0 <account_system_time The function is called two times for each interrupt and accounts the system time only, that makes 2 * 15 instructions more for each interrupt, while loosing a small amount of precision. Imho not good. The idea of cputime_t was to allow an architecture to define its preferred format, for s390 this is a pure CPU timer delta. We do not loose *any* precision as long as the CPU timer works correctly. From my point of view this is a change for the worse. On the positive side, there are some nice improvements in the patch series. We will definitely pick up some of the patches. -- blue skies, Martin. "Reality continues to ruin my life." - Calvin. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
