On Thu, Nov 17, 2016 at 03:00:14PM -0500, Chris Metcalf wrote: > On 11/17/2016 4:53 AM, Peter Zijlstra wrote: > >On Wed, Nov 16, 2016 at 03:16:59PM -0500, Chris Metcalf wrote: > >>PeterZ (cc'ed) then improved it to use __int128 math via > >>mul_u64_u32_shr(), but that doesn't help tile; we only do one multiply > >>instead of two, but the multiply is handled by an out-of-line call to > >>__multi3, and the sched_clock() function ends up about 2.5x slower as > >>a result. > >Well, only if you set CONFIG_ARCH_SUPPORTS_INT128, otherwise it reduces > >to 2 32x23->64 multiplications, of which one if conditional on there > >actually being bits set in the high word of the u64 argument. > > I didn't notice that. It took me down an interesting rathole. > > Obviously the branch optimization won't help on cycle counter values, > since we blow out of the low 32 bits in the first few seconds of > uptime. So the conditional test won't help, but the 32x32 > multiply optimizations should.
Now, I don't quite remember things, but isn't it the idea to convert cycle deltas and accumulate in ns? That way you most always convert small values.
