Re: Counting CPU_CLK_UNHALTED on an idle system

[Stephane Eranian <eranian@hpl.hp.com>]

John,

On PIII, CPU_CLK_UNHALTED is broken and keeps on counting while in HALT state.

On Fri, Jul 13, 2007 at 02:33:09PM +0200, John Sigler wrote:
> [ Note: cross-posted to linux-rt-users and oprofile-list ]
> 
> Hello,
> 
> The x86 Performance Monitoring Event "CPU_CLK_UNHALTED" is defined as:
> Number of cycles during which the processor is not halted.
> 
> IIUC, when the CPU is in the HALT state, CPU_CLK_UNHALTED should not 
> increment, right? When Linux has nothing to do, it executes HLT.
> 
> http://lxr.linux.no/source/arch/i386/kernel/process.c#L101
> 
> I set up OProfile to sample EIP every 200000 CPU_CLK_UNHALTED cycles on 
> an idle system. I was expecting a very small number of samples, since 
> CPU_CLK_UNHALTED should not increment (most of the time).
> 
> I ran the daemon 5 minutes on a idle system
> 
> # opreport -l
> CPU: PIII, speed 1266.7 MHz (estimated)
> Counted CPU_CLK_UNHALTED events (clocks processor is not halted) with a 
> unit mask of 0x00 (No unit mask) count 200000
> samples    %      symbol name
> 1868484  99.4914  cpu_idle
>     4560   0.2428  (oprofiled)
>      727   0.0387  ip_route_input
>      457   0.0243  acpi_ev_asynch_execute_gpe_method
>      454   0.0242  ip_route_output_slow
>      286   0.0152  dma_free_coherent
>      280   0.0149  ip_route_input_slow
>      273   0.0145  acpi_ex_opcode_6A_0T_1R
>      220   0.0117  __acpi_nmi_disable
>      130   0.0069  Dta1xxRxProcessStatusFlags
>       94   0.0050  Dta1xxTxProcessStatusFlags
>       93   0.0050  (bash)
>       85   0.0045  Dta1xxIsr
>       79   0.0042  intel_machine_check
>       56   0.0030  sys_olduname
>       51   0.0027  Dta1xxPeriodicIntHandler
> 
> cpu_idle breakdown:
> 
>    c0100e70 1        5.4e-05
>    c0100e71 1        5.4e-05
>    c0100e78 1        5.4e-05
>    c0100e88 1        5.4e-05
>    c0100e8a 1        5.4e-05
>    c0100e97 4        2.1e-04
>    c0100ea8 1868475  99.9995
> 
> cpu_idle objdump:
> 
> c0100ea6:       fb                      sti
> c0100ea7:       f4                      hlt
> c0100ea8:       eb 0b                   jmp    c0100eb5
> 
> (If an interrupt is used to resume execution after a HLT instruction, 
> the saved instruction pointer points to the instruction following the 
> HLT instruction.)
> 
> oprofiled collected 1878300 samples. In other words, the system received 
> 1878300 NMIs. (5 * 60 * 1.2667e9) / 1878300 = 202316 cycles between NMI
> 
> Seems like CPU_CLK_UNHALTED increments every cycle.
> 
> Perhaps HLT does not put my CPU in the HALT state?
> 
> Did I miss something?
> 
> Regards.
> 
> 
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-- 

-Stephane

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