Re: [GIT PULL] iov_iter: Improve page extraction (pin or just list)

[David Hildenbrand]

On 31.01.23 14:41, David Howells wrote:
> David Hildenbrand <david@xxxxxxxxxx> wrote:
>
> > > > percpu counters maybe - add them up at the point of viewing?
> > > They are percpu, see my last email. But for every 108 changes (on
> > > my system), they will do two atomic_long_adds(). So not very
> > > useful for anything but low frequency modifications.
> >
> > Can we just treat the whole acquired/released accounting as a debug mechanism
> > to detect missing releases and do it only for debug kernels?
> >
> >
> > The pcpu counter is an s8, so we have to flush on a regular basis and cannot
> > really defer it any longer ... but I'm curious if it would be of any help to
> > only have a single PINNED counter that goes into both directions (inc/dec on
> > pin/release), to reduce the flushing.
> >
> > Of course, once we pin/release more than ~108 pages in one go or we switch
> > CPUs frequently it won't be that much of a help ...
>
> What are the stats actually used for?  Is it just debugging, or do we actually
> have users for them (control groups spring to mind)?

As it's really just "how many pinning events" vs. "how many unpinning 
events", I assume it's only for debugging.

For example, if you pin the same page twice it would not get accounted 
as "a single page is pinned".

--
Thanks,

David / dhildenb