svn commit: r237434 - in head/lib/libc: amd64/sys gen i386/sys include sys

[Konstantin Belousov]

On Sat, Jun 23, 2012 at 03:17:57PM +0200, Marius Strobl wrote:
> On Fri, Jun 22, 2012 at 10:48:17AM +0300, Konstantin Belousov wrote:
> > On Fri, Jun 22, 2012 at 09:34:56AM +0200, Marius Strobl wrote:
> > > On Fri, Jun 22, 2012 at 07:13:31AM +0000, Konstantin Belousov wrote:
> > > > Author: kib
> > > > Date: Fri Jun 22 07:13:30 2012
> > > > New Revision: 237434
> > > > URL: http://svn.freebsd.org/changeset/base/237434
> > > > 
> > > > Log:
> > > >   Use struct vdso_timehands data to implement fast gettimeofday(2) and
> > > >   clock_gettime(2) functions if supported. The speedup seen in
> > > >   microbenchmarks is in range 4x-7x depending on the hardware.
> > > >   
> > > >   Only amd64 and i386 architectures are supported. Libc uses rdtsc and
> > > >   kernel data to calculate current time, if enabled by kernel.
> > > 
> > > I don't know much about x86 CPUs but is my understanding correct
> > > that TSCs are not synchronized in any way across CPUs, i.e.
> > > reading it on different CPUs may result in time going backwards
> > > etc., which is okay for this application though?
> > 
> > Generally speaking, tsc state among different CPU after boot is not
> > synchronized, you are right.
> > 
> > Kernel has somewhat doubtful test which verifies whether the after-boot
> > state of tsc looks good. If the test fails, TSC is not enabled by
> > default as timecounter, and then usermode follows kernel policy and
> > falls back to slow syscall. So we err on the safe side.
> > I tested this on Core i7 2xxx, where the test (usually) passes.
> 
> Okay, so for x86 the TSCs are not used as timecounters by either
> the kernel or userland in the SMP case if they don't appear to
> be synchronized, correct?
Correct as for now.  But this is bug and not a feature. The tscs shall
be synchronized, or skew tables calculated instead of refusing to use it.

> 
> > 
> > While you are there. do you have comments about sparc64 TICK counter ?
> > On SMP, the counter of BSP is used by IPI. Is it unavoidable ?
> 
> The TICK counters are per-core and not synchronized by the hardware.
> We synchronize APs with the BSP on bring-up but they drift over time
> and the initial synchronization might not be perfect in the first
> place. At least in the past, drifting TICK counters caused all sorts
> of issues and strange behavior in FreeBSD when used as timecounter
> in the SMP case. If my understanding of the above is right, as is
> this still rules them out as timecounters for userland.
> Linux has some complex code (based on equivalent code origining in
> their ia64 port) for constantly synchronizing the TICK counters.
> In order to avoid that complexity and overhead, what I do in
> FreeBSD in the SMP case is to (ab)use counters (either intended
> for that purpose or bus cycle counters probably intended for
> debugging the hardware during development) available in the
> various host-to-foo bridges so it doesn't matter which CPU they
> are read by. This works just fine except for pre-PCI-Express
> based USIIIi machines, where the bus cycle counters are broken.
> That's where the TICK counter is always read from the BSP
> using an IPI in the SMP case. The latter is done as sched_bind(9)
> isn't possible with td_critnest > 1 according to information
> from jhb@ and mav at .
> So apart from introducing code to constantly synchronize the
> TICK counters, using the timecounters on the host busses also
> seems to be the only viable solution for userland. The latter
> should be doable but is long-winded as besides duplicating
> portions of the corresponding device drivers in userland, it
> probably also means to get some additional infrastructure
> like being able to memory map registers for devices on the
> nexus(4) level in place ...

Understand. I do plan eventually to map HPET counters page into usermode
on x86.

Also, as I noted above, some code to synchronize per-package counters
would be useful for x86, so it might be developed with multi-arch
usage in mind.
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