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Forum Topic - Getting CPU ID: (5 Items)

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sang lee

07/11/2016 4:19 PM

post116529

Hi

Is there a way to get the CPU ID using the QNX API?

I found how to tie to a specific CPU, but I couldnt find any documentation on getting the CPU number of the current 
thread

Will Miles

07/12/2016 5:13 PM

post116537

Re: Getting CPU ID

Hi,

Just to be clear - you're looking for the equivalent of Linux's getcpu(2) or glibc's sched_getcpu(3) ?    I had a 
similar goal in an implementation of ntp's nanosecond kernel for an SMP system.   I couldn't find a nicely packaged 
solution.   You might be able to get what you want by running the DCMD_PROC_TIDSTATUS command on one's own thread and 
checking the last_cpu field.   I needed something safe to run even in an ISR (so I could capture timestamps with the 
lowest possible latencies), so I never investigated that solution further.

-Will

sang lee

07/12/2016 7:18 PM

post116538

Re: Getting CPU ID

Hi Will

Yes, that is what I needed!

I ended up using a platform specific solution, reading one of system registers indicating the CPU ID

Artem Kashkanov

09/15/2016 4:42 AM

post116793

Re: Getting CPU ID

> Hi Will
> 
> Yes, that is what I needed!
> 
> I ended up using a platform specific solution, reading one of system registers
>  indicating the CPU ID

Can you notice, what register already contain cpu number you read? 
I just found how to read Lapic Id and have to transform it into cpu id, using smpinfo from syspage - they are generally 
not the same.

Will Miles

09/15/2016 10:30 AM

post116795

Re: Getting CPU ID

Hi Artem,

On x86 as of the QNX version I'm using, it's stored in the task register, which you can retrieve via the STR instruction
.   I believe the full expression is:

cpunum = ( TR / 8 ) - 32

where TR is the value in the task register.

I haven't personally investigated any other architectures yet.

-Will

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