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

John Hubbard
2023-01-31T17:54:57Z

On 1/31/23 04:28, Jan Kara wrote:
> On Tue 31-01-23 09:32:27, David Hildenbrand wrote:
>> On 30.01.23 23:15, Jens Axboe wrote:
>>> On 1/30/23 3:12 PM, David Howells wrote:
>>>> John Hubbard <jhubbard@nvidia.com> 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?
>
> Yes, AFAIK it is just a debug mechanism for helping to find out issues with
> page pinning conversions. So I think we can put this behind some debugging
> ifdef. John?
>

Yes, just for debugging. I wrote a little note just now in response to
the patch about how we ended up here: "yes, it's time to hide these
behind an ifdef".

thanks,
--
John Hubbard
NVIDIA