Re: help needed pls. scheduler(kernel 2.6) + hyperthreaded related questions?

[Arun Srinivas]

few more trivial Q's (bear with me  I'm a newbie to kernel world):

1) As I said I have a process that spawns 2 threads(thread A and B).I am 
trying to measure the exact time @ which they are being scheduled.For this I 
am using the rdtsc() (when threads A and B come)  in enqueue_task()..where 
they are being inserted into the priority array.
Is this a correct way of measuring?

2) also in task_struct.....is "tgid" the id of my process and each of 
threads hav a unique pid??

3) I saw frm the kernel docs tht realtime tasks hav priority 0 to 99. So 
using setscheduler means do I have to enforce a priority in one of these 
ranges to make my threads as soft/hard realtime task.

thanks in advance for your patience.

> From: Nick Piggin <nickpiggin@xxxxxxxxxxxx>
> To: Arun Srinivas <getarunsri@xxxxxxxxxxx>
> CC: linux-kernel@xxxxxxxxxxxxxxx
> Subject: Re: help needed pls. scheduler(kernel 2.6) + hyperthreaded related 
> questions?
> Date: Wed, 23 Mar 2005 18:37:16 +1100
>
> Arun Srinivas wrote:
> > If the SMT (apart from SMP) support is enabled in the  .config file, does 
> > the kernel recogonize the 2 logical processor as 2 logical or 2 physical 
> > processors?
>
> You shouldn't be able to select SMT if SMP is not enabled.
> If SMT and SMP is selected, then the scheduler will recognise
> the 2 processors as logical ones.
>
> > Also, as the hyperthreaded processor may schedule 2 threads in the 2 
> > logical cpu's, and it may not necessarily be form the same process i.e., 
> > the 2 thread it schedules may be from the same or from the different 
> > process.
>
> Yes.
>
> > So, is there any way I can tell the scheduler (assuming I make the 
> > scheduler recogonize my 2 threads..i.e., it knows their pid) to schedule 
> > always my 2 threads @ the same time? How do I go abt it?
>
> Use sched_setaffinity to force each thread onto the particular
> CPU. Use sched_setscheduler to acquire a realtime scheduling
> policy. Then use mutexes to synchronise your threads so they
> run the desired code segment at the same time.

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