Re: [RFC patch 00/15] Tracer Timestamping

[Mathieu Desnoyers]

* Nicolas Pitre (nico@xxxxxxx) wrote:
> On Mon, 20 Oct 2008, Mathieu Desnoyers wrote:
> 
> > * Peter Zijlstra (a.p.zijlstra@xxxxxxxxx) wrote:
> > > Have you looked at the existing 32->63 extention code in
> > > include/linux/cnt32_to_63.h and considered unifying it?
> > > 
> > 
> > Yep, I felt this code was dangerous on SMP given it could suffer from
> > the following type of race due to lack of proper barriers :
> 
> You are wrong.
> 
> > CPU    A                                 B
> >        read hw cnt low
> >        read __m_cnt_hi
> >                                          read hw cnt low
> >        (wrap detected)
> >        write __m_cnt_hi (incremented)
> >                                          read __m_cnt_hi
> >                                          (wrap detected)
> >                                          write __m_cnt_hi (incremented)
> > 
> > we therefore increment the high bits twice in the given race.
> 
> No.  Either you do the _same_ incrementation twice effectively writing 
> the _same_ high value twice, or you don't have simultaneous wrap 
> detections.  Simulate it on paper with real numbers if you are not 
> convinced.
> 

Hi Nicolas,

Yup, you are right. However, the case where one CPU sees the clock source
a little bit off-sync (late) still poses a problem. Example follows :

CPU    A                                 B
       read __m_cnt_hi (0x80000000)
       read hw cnt low (0x00000001)
       (wrap detected :
        (s32)(0x80000000 ^ 0x1) < 0)
       write __m_cnt_hi = 0x00000001
       return 0x0000000100000001
                                            read __m_cnt_hi (0x00000001)
                                     (late) read hw cnt low (0xFFFFFFFA)
                                            (wrap detected :
                                             (s32)(0x00000001 ^ 0xFFFFFFFA) < 0)
                                            write __m_cnt_hi = 0x80000001
                                            return 0x80000001FFFFFFFA
                                                                  (time jumps)
       read __m_cnt_hi (0x80000001)
       read hw cnt low (0x00000020)
       (wrap detected :
        (s32)(0x80000001 ^ 0x20) < 0)
       write __m_cnt_hi = 0x00000002
       return 0x0000000200000020
                             (time jumps again)

A similar situation can be generated by out-of-order hi/low bits reads.

> > On UP, the same race could happen if the code is called with preemption
> > enabled.
> 
> Wrong again.
> 
> > I don't think the "volatile" statement would necessarily make sure the
> > compiler and CPU would do the __m_cnt_hi read before the hw cnt low
> > read. A real memory barrier to order mmio reads wrt memory reads (or
> > instruction sync barrier if the value is taken from the cpu registers)
> > would be required to insure such order.
> 
> That's easy enough to fix, right?
> 

For memory read vs cycle counter, something like the "data_barrier(x)"
on powerpc would probably be enough when available. We would need
something that orders memory reads with the rest of instruction
execution. On other architectures, smp_rmb() + get_cycles_barrier() (the
latter being an instruction sync) would be required.

For memory read vs mmap io read, a smp_rmb() should be enough.

> > I also felt it would be more solid to have per-cpu structures to keep
> > track of 32->64 bits TSC updates, given the TSCs can always be slightly
> > out-of-sync :
> 
> If the low part is a per CPU value then the high part has to be a per 
> CPU value too.  There only needs to be a per-CPU variant of the same 
> algorithm where the only difference is that __m_cnt_hi would be per CPU.
> 
> If the low part is global then __m_cnt_hi has to be global, even on SMP.
> 

Hrm, given the performance impact of barriers that would need to be
added to insure correct read order of hi and low bits, and also
considering the risk that a "synchronized" timestamp counter off by a
few cycles poses in terms of time jumping forward, I don't see why we
would ever want to use a global __m_cnt_hi ? I would therefore recommend
to always use a per-cpu __m_cnt_hi.

Also, a small nit, to make sure cnt32_to_63 never gets scheduled out
with a stale old __m_cnt_hi value, its comments should probably say that
it has to be always used with preemption off.

Mathieu

> 
> Nicolas

-- 
Mathieu Desnoyers
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