Re: Question about request queues in I/O scheduling

[Max Kanushin]

Hello,

I was trying to use spin_trylock(q->queue_lock) on several queues when 
q->nr_sorted exceeds the per-defined number to block them and unlock the 
them later with spin_unlock(q->queue_lock), but I have faced the 
following problem: my system freezes when I am trying to test it by 
moving files around in my system. I thought it might be caused by 
blocking a queue of a system partition or swap, but I do not know how to 
check this as well.
Is it the right way to stop the queue/elevator? What are freezes might 
be caused by? Is there a simple way to find out which block device is a 
queue belong to?
I would really appreciate your reply.

Best regards,
  Max Kanushin.

On 04/29/2016 07:22 PM, Jeff Moyer wrote:
> Max Kanushin <rextuz@xxxxxxxxx> writes:
>
> > Thank you very much for the reply. My general idea is take control of all
> > request queues to block and unblock them manually depending on their load.
> > One of my steps is to find a length of a queue to decide if to block it.
> > Actually I think I've found the way. If I got it right, I can move from one
> > request to another within the list:
> > struct list_head queue_head;
> > So that I can calculate how many requests are there in the queue.
>
> Hi, Max,
>
> The queue_head is the dispatch list for I/O, so it does not represent
> all I/O queued for a request_queue.  The number of requests in the
> scheduler would be reflected by q->nr_sorted.  The number of dispatched
> requests is in an array, in_flight.
>
> Cheers,
> Jeff
>
>
>
> > Best regards,
> >    Max Kanushin.
> > On Apr 29, 2016 6:32 PM, "Jeff Moyer" <jmoyer@xxxxxxxxxx> wrote:
> >
> > > Max Kanushin <rextuz@xxxxxxxxx> writes:
> > >
> > > > Hello,
> > > >
> > > > I was searching for a way to find out the length of a request_queue
> > > > (that is defined as a structure in
> > > > linux/include/linux/blkdev.h). However I am new to the kernel
> > > > development and can't figure out where is the actual list of requests
> > > > to be processed by an elevator.
> > > > Is there a way to iterate requests in a queue or at least find a
> > > > number of them?
> > >
> > > Hi, Max,
> > >
> > > What exactly are you trying to accomplish?
> > >
> > > Cheers,
> > > Jeff