Hi Thomas, Thanks for the reviews.
On Wed, Mar 06, 2013 at 03:09:26PM +0100, Thomas Gleixner wrote: > On Wed, 6 Mar 2013, Feng Tang wrote: > > > Current clocksource_cyc2ns() has a implicit limit that the (cycles * mult) > > can not exceed 64 bits limit. Jason Gunthorpe proposed a way to > > handle this big cycles case, and this patch put the handling into > > clocksource_cyc2ns() so that it could be used unconditionally. > > Could be used if it wouldn't break the world and some more. Exactly. One excuse I can think of is usually the clocksource_cyc2ns() will be called for cycles less than 600 seconds, based on which the "mult" and "shift" are calculated for a clocksource. > > > Suggested-by: Jason Gunthorpe <[email protected]> > > Signed-off-by: Feng Tang <[email protected]> > > --- > > include/linux/clocksource.h | 11 ++++++++++- > > 1 file changed, 10 insertions(+), 1 deletion(-) > > > > diff --git a/include/linux/clocksource.h b/include/linux/clocksource.h > > index aa7032c..1ecc872 100644 > > --- a/include/linux/clocksource.h > > +++ b/include/linux/clocksource.h > > @@ -274,7 +274,16 @@ static inline u32 clocksource_hz2mult(u32 hz, u32 > > shift_constant) > > */ > > static inline s64 clocksource_cyc2ns(cycle_t cycles, u32 mult, u32 shift) > > { > > - return ((u64) cycles * mult) >> shift; > > + u64 max = ULLONG_MAX / mult; > > This breaks everything which does not have a 64/32bit divide > instruction. And you can't replace it with do_div() as that would > impose massive overhead on those architectures in the fast path. I thought about this once. And in my v2 patch, I used some code like + /* + * The system suspended time and the delta cycles may be very + * long, so we can't call clocksource_cyc2ns() directly with + * clocksource's default mult and shift to avoid overflow. + */ + max_cycles = 1ULL << (63 - (ilog2(mult) + 1)); + while (cycle_delta > max_cycles) { + max_cycles <<= 1; + mult >>= 1; + shift--; + } + trying to avoid expensieve maths. But as Jason pointed, there is some accuracy lost. > > The max value can be precalculated and stored in the timekeeper > struct. We really do not want expensive calculations in the fast path. Yeah, just like the max_idle_ns. > > + s64 nsec = 0; > > + > > + /* The (mult * cycles) may overflow 64 bits, so add a max check */ > > + if (cycles > max) { > > + nsec = ((max * mult) >> shift) * (cycles / max); > > This breaks everything which does not have a 64/64bit divide instruction. > > > + cycles %= max; > > Ditto. > > As this is the slow path on resume you can use the 64bit functions > declared in math64.h. And you want to put the slow path out of line. So should I leave the clocksource_cyl2ns() untouched and only add these do_div() 64 bit operation inside the timekeeping_resume() slow path? Thanks, Feng -- 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/
