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

Jens Axboe
2023-01-30T21:57:10Z

On 1/30/23 2:55 PM, Jens Axboe wrote:
> On 1/30/23 2:33 PM, Jens Axboe wrote:
>> On 1/30/23 4:14 AM, David Howells wrote:
>>> Hi Jens,
>>>
>>> Could you consider pulling this patchset into the block tree? I think that
>>> Al's fears wrt to pinned pages being removed from page tables causing deadlock
>>> have been answered. Granted, there is still the issue of how to handle
>>> vmsplice and a bunch of other places to fix, not least skbuff handling.
>>>
>>> I also have patches to fix cifs in a separate branch that I would also like to
>>> push in this merge window - and that requires the first two patches from this
>>> series also, so would it be possible for you to merge at least those two
>>> rather than manually applying them?
>>
>> I've pulled this into a separate branch, but based on the block branch,
>> for-6.3/iov-extract. It's added to for-next as well.
>
> 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...

--
Jens Axboe