Re: ext4-block-reservation.patch

[Eric Sandeen <sandeen@redhat.com>]

Aneesh Kumar K.V wrote:
> Hi,
> 
> In block reservation code while rebalancing the free blocks why are we not 
> looking at the reservation slots that have no free blocks left.  Rebalancing
> the free blocks equally across all the reservation slots will make sure
> we have less chances of failure later when we try to reserve blocks. 
> 
> 
> I understand that we consider the CPU slot on which reservation failed while
> rebalancing. But what is preventing considering other CPU slot that might have
> zero blocks left ?
> 
> 
> 
> 
> +void ext4_rebalance_reservation(struct ext4_reservation_slot *rs, __u64 free)
> +{
> +       int i, used_slots = 0;
> +       __u64 chunk;
> +
> +       /* let's know what slots have been used */
> +       for (i = 0; i < NR_CPUS; i++)

BTW... I think you really want:

  +       for_each_possible_cpu(i) {

in this and other similar places.

NR_CPUS is a config-time option that may be much more than your actual
count of runtime possible CPUs... on ia64 it's 512 by default, for
example.  That's a lot of pointlessness on a 2, 4 or 8 cpu box :)

I can whip up a proper patch for current code to send (again)...

-Eric


> +               if (rs[i].rs_reserved || i == smp_processor_id())
> +                       used_slots++;
> +
> +       /* chunk is a number of block every used
> +        * slot will get. make sure it isn't 0 */
> +       chunk = free + used_slots - 1;
> +       do_div(chunk, used_slots);
> +
> +       for (i = 0; i < NR_CPUS; i++) {
> +               if (free < chunk)
> +                       chunk = free;
> +               if (rs[i].rs_reserved || i == smp_processor_id()) {
> +                       rs[i].rs_reserved = chunk;
> +                       free -= chunk;
> +                       BUG_ON(free < 0);
> +               }
> +       }
> +       BUG_ON(free);
> +}
> 
> 
> -aneesh
> -
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