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

[Arun Srinivas]

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?

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.

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?

Pls. help.Thanks in Advance.

> 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 10:16:20 +1100
>
> Arun Srinivas wrote:
> > Pls. help me.  I went through the sched.c for kernel 2.6 and saw that it 
> > supports
> > hyperthreading.I would be glad if someone could answer this 
> > question....(if
> > am not wrong a HT processor has 2 architectural states and one execution
> > unit...i.e., two pipeline streams)
> >
> > 1)when there are 2 processes a parent and child(created by fork()) do they
> > get scheduled @ the same time...ie., when the parent process is put into 
> > one
> > pipeline, do the child also gets scheduled the same time?
>
> No.
>
> > 2) what abt in the case of threads(I read tht as opposed to 
> > kernel2.4,where
> > threads are treated as processes) ..kernel 2.6 treats threads as threads.
> > So, when two paired threads get into execution are they always scheduled 
> > at
> > the same time?
>
> No.
>
> > Also, it would be helpful if someone could suggest which part of sched.c
> > shud i look into to find out how threads are scheduled for a normal
> > processor and for a hyperthreaded processor
>
> It is pretty tricky. Basically processes on different CPUs are
> scheduled completely independently of one another. The only time
> when they may get moved from one CPU to another is with
> load_balance, load_balance_newidle, active_load_balance,
> try_to_wake_up, sched_exec, wake_up_new_task.

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