Re: [PATCH v2] tile: avoid using clocksource_cyc2ns with absolute cycle count

[Chris Metcalf]

On 11/18/2016 5:34 AM, Peter Zijlstra wrote:
> 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.

I would think you would also unnecessarily accumulate small errors.

The x86 sched_clock() seems to purely scale the current TSC value,
so what tile is doing is consistent with that, at least.

--
Chris Metcalf, Mellanox Technologies
http://www.mellanox.com