Re: [PATCH V3] block/throttle: Add IO throttled information in blkio.throttle

[Vivek Goyal <vgoyal@redhat.com>]

On Thu, Oct 18, 2012 at 04:24:04PM -0700, Tejun Heo wrote:
> Hello, Vivek.
> 
> On Wed, Oct 17, 2012 at 09:49:45AM -0400, Vivek Goyal wrote:
> > Can you explain a bit more. Whe do you mean by "total number queued". I
> > think throttle.io_queued will total number of bios queued in the cgroup
> > at the time of query.
> 
> Instead of exposing the number that blk-throttle currently has
> deferred, we can expose the number of bios that have been sent to
> blk-throttle and the number of bios which left blk-throttle, both
> monotically increasing and the difference indicating the number being
> deferred.

Ok, I see it now. So we currently already maintain the number of IOs
dispatched from blk-throttle  in throttle.io_serviced. Now you are 
suggesting that maintain another counter which keeps track of total
number IOs submitted to blk-throttle, say throttle.io_submitted? I think
using throttle.io_queued will be little confusing because in CFQ we
already use blkio.io_queued to represent number of IOs currently queued
and it is not monotonically increasing value.

> That way we can stick to the usual stats facility.

So how does this help? Because it is a monotonically increasing value
we can use per cpu stats without extra locking? Or somthing else?

Thanks
Vivek