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

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Message-ID: <e8480b18-08af-d101-a721-50d213893492@kernel.dk>
Date:   Mon, 30 Jan 2023 14:57:04 -0700
From:   Jens Axboe <axboe@...nel.dk>
To:     David Howells <dhowells@...hat.com>
Cc:     Al Viro <viro@...iv.linux.org.uk>,
        Christoph Hellwig <hch@...radead.org>,
        Matthew Wilcox <willy@...radead.org>, Jan Kara <jack@...e.cz>,
        David Hildenbrand <david@...hat.com>,
        John Hubbard <jhubbard@...dia.com>,
        Jason Gunthorpe <jgg@...dia.com>,
        Logan Gunthorpe <logang@...tatee.com>,
        Jeff Layton <jlayton@...nel.org>, linux-block@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [GIT PULL] iov_iter: Improve page extraction (pin or just list)

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