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

[John Hubbard <jhubbard@nvidia.com>]

On 1/30/23 13:57, Jens Axboe wrote:
>> This does cause about a 2.7% regression for me, using O_DIRECT on a raw
>> block device. Looking at a perf diff, here's the top:
>>
>>                 +2.71%  [kernel.vmlinux]  [k] mod_node_page_state
>>                 +2.22%  [kernel.vmlinux]  [k] iov_iter_extract_pages
>>
>> and these two are gone:
>>
>>       2.14%             [kernel.vmlinux]  [k] __iov_iter_get_pages_alloc
>>       1.53%             [kernel.vmlinux]  [k] iov_iter_get_pages
>>
>> rest is mostly in the noise, but mod_node_page_state() sticks out like
>> a sore thumb. They seem to be caused by the node stat accounting done
>> in gup.c for FOLL_PIN.
> 
> Confirmed just disabling the node_stat bits in mm/gup.c and now the
> performance is back to the same levels as before.
> 
> An almost 3% regression is a bit hard to swallow...

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.

At this point, it's a good time to consider if there is any
way to speed it up. But I wanted to confirm that you're absolutely
right: the measurement sounds about right, and that's also the
hotspot that we say, too.
  

thanks,
-- 
John Hubbard
NVIDIA