On 12/15/2009 07:53 PM, [email protected] wrote:
> Hi,Stephane.
>
> I have been tracing your work in perfmon2/3,and doing some similar work in my
> thesis.
>
> I extended the task_struct with pmu-related context and added support to
> save/restore it in __switch_to,all under linux 2.6.30.
> Then I encounter a problem, when the following code is running with counting
> disabled.
> ...
> {
> wrmsrl(pmu_reg->counter_addr,pmu_reg->counter);
> rdmsrl(pmu_reg->counter_addr,co);
> }
> so far what really confusing is that quite often values of pmu_reg->counter
> and co
are not equal，and the later one would suddenly become really
large（e.g. the high 8 bits of counter register are ff），i.e. the value
written into counter register is wrong
Are you aware of the sign-extension on PMC writes? Here's a quote from
the Intel manual, volume 3B section 30.2.1.1:
"The bit width of an IA32_PMCx MSR is reported using the
CPUID.0AH:EAX[23:16]. Bits beyond the width of the programmable
counter are undefined, and are ignored when written to. On write
operations, the lower-order 32 bits of each MSR may be written with
any value, and the high-order bits are sign-extended according to the
value of bit 31. The bit width for read operations is reported using
CPUID."
> Then, I modified the code to the following version, like
> {
> again:
> wrmsrl(pmu_reg->counter_addr,pmu_reg->counter & PMUC_MASK);
> rdmsrl(pmu_reg->counter_addr,co);
> if(pmu_reg->counter!=co)
> goto again;
> }
>
> This time, the system runs into a state of unhalted busy with no reaction and
> no oops printing.
> And why? I know you shoule be familiar with these issues, so could any help?
There's no oops because nothing really failed -- it's an infinite loop.
This could work if PMUC_MASK is 0x7fffffff, but you also need to use the
mask in the comparison.
Josh
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