RE: Interrupt Handling and Scheduler

"Sonawane, Rahul (Rahul)" <sonawane@avaya.com> · Thu, 24 Oct 2002 16:22:23 +0800

It was for Linux. All the interrupt run 
in the context of the currently running process. Now say process A is in wait 
queue (for some IO )  and B is currently running. Now B is interrupted and the ISR runs in the context of 
process B.  the wake_up_process  function if A was waiting on it, 
which will set the resched_flag =1 
if
        if (goodness(B, A) > 
goodness(B, B)) B->need_resched = 1; 

Now the flag is checked in  
ret_from_int and a context switch is made then A will start executing. ISR uses 
the process's kernel stack during excecution ,this means that the ISR runs the 
context for the process. Check this url for more info http://www.oreilly.com/catalog/linuxkernel/chapter/ch10.html
Thank 
you,

Rahul

[Sonawane, Rahul 
(Rahul)]  -----Original Message-----
From: 
Anjaneyulu [mailto:anjaneyulu@mistralsoftware.com]
Sent: Thursday, 
October 24, 2002 11:30 AM
To: Sonawane, Rahul 
(Rahul)
Subject: Re: Interrupt Handling and 
Scheduler

  Hi,
  The explanation given by you is for RT-Linux OR 
  linux!!!! I am interested in non-preemptive linux kernel.
  You have explained wrt Timer interrupt. Is it the 
  same behavior with other external interrupts( Ex: PCI device)????

  In the timer interrupt itself, What does "ISR 
  executes in the current process context" mean??? What is the counter's initial 
  value???

  If the interrupts are coming at very high 
  frequency, I would like to know whether the process pending in kernel 
  will ever get the chance to execute.
  i.e. when schedule is called does it guarentee 
  that the process will get the chance.

  Regards,
  Anj

    ----- Original Message ----- 
    From: 
    Sonawane, Rahul 
    (Rahul) 
    To: Anjaneyulu 
    Cc: kernelnewbies@nl.linux.org 

    Sent: Thursday, October 24, 2002 10:58 
    AM
    Subject: RE: Interrupt Handling and 
    Scheduler

      When an interrupt happens say timer 
      interrupt now the ISR will execute in the context of the current process. 
      It will decrement the current process counter by 
      the
      number of clock ticks. If the 
      counter is zero then resched_flag = 1. Now when returning from the  
      interrupt in ret_from_int()  the resched_flag is checked 
      .
       If the resched_flag = 1 , then a 
      call is made to the schedule()  which will do a context 
      switch.
      Have a made myself 
      understood.

      Regards,
      Rahul

       -----Original 
      Message-----
From: Anjaneyulu 
      [mailto:anjaneyulu@mistralsoftware.com]
Sent: Thursday, October 
      24, 2002 9:55 AM
To: 
      kernelnewbies@nl.linux.org
Subject: Interrupt Handling and 
      Scheduler

      Hi,
      I have a doubt about the interrupt handling 
      and the scheduling.
      When an ISR is being executed, that 
      particular interrupt is disable. 
      Suppose the ISR is waking up a process using 
      wake_up_process().
      Before that particular process is able to 
      execute any statement, if another interrupt comes and hence ISR will again 
      execute.

      My question is if the interrupts are coming 
      at very high frequency, the process will never get the chance to execute. 
      Is it right???

      Please tell me the scheduler reaction to an 
      interrupt. What happens to the current process????

      Thanks in advance.

      Regards,
      Anj