Re: General Question on Scheduler

[J <jeebu19@yahoo.com>]

On Mon, Mar 04, 2002 at 11:05:03PM -0800, William Lee Irwin III wrote:
> 
> > 2. When does the scheduler get control so that it can perform the task
> > switch. i.e is the scheduler called from the timer interrupt or something
> > like that?
> 
> The scheduler gets control during explicit calls to schedule() and other
> functions in sched.c


...during return to user mode (interrupts, exceptions, syscalls)
figure 4.5 page 134 Understanding the Linux Kernel

and and also with voluntary yield.

 
> > 4. Does the linux kernel use the facilities provided by the processor ( I
> > think x86 has facilities for task switch ) for the task switching?
> 
> TSS's are essentially required for SMP operation. However, Linux uses
> one TSS per cpu.


I think this answers it -- arch/i386/kernel/process.c

/*
 *      switch_to(x,yn) should switch tasks from x to y.
 *
 * We fsave/fwait so that an exception goes off at the right time
 * (as a call from the fsave or fwait in effect) rather than to
 * the wrong process. Lazy FP saving no longer makes any sense
 * with modern CPU's, and this simplifies a lot of things (SMP
 * and UP become the same).
 *
 * NOTE! We used to use the x86 hardware context switching. The
 * reason for not using it any more becomes apparent when you
 * try to recover gracefully from saved state that is no longer
 * valid (stale segment register values in particular). With the
 * hardware task-switch, there is no way to fix up bad state in
 * a reasonable manner.
 *
 * The fact that Intel documents the hardware task-switching to
 * be slow is a fairly red herring - this code is not noticeably
 * faster. However, there _is_ some room for improvement here,
 * so the performance issues may eventually be a valid point.
 * More important, however, is the fact that this allows us much
 * more flexibility.
 */
void __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
{

Regards,
Sourav

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