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

[David Hildenbrand]

On 30.01.23 23:15, Jens Axboe wrote:
> On 1/30/23 3:12 PM, David Howells wrote:
> > John Hubbard <jhubbard@xxxxxxxxxx> wrote:
> >
> > > This is something that we say when adding pin_user_pages_fast(),
> > > yes. I doubt that I can quickly find the email thread, but we
> > > measured it and weren't immediately able to come up with a way
> > > to make it faster.
> >
> > 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 ...

--
Thanks,

David / dhildenb