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

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Message-ID: <088e40fd-3fc7-77dd-a3de-0a2b097d3717@kernel.dk>
Date:   Mon, 30 Jan 2023 15:11:39 -0700
From:   Jens Axboe <axboe@...nel.dk>
To:     John Hubbard <jhubbard@...dia.com>,
        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>,
        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 3:02?PM, John Hubbard wrote:
> 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.

>From spending all of 5 minutes on this, it must be due to exceeding the
pcp stat_threashold, as we then end up doing two atomic_long_adds().
Looking at proc, looks like it's 108. And with this test, then we're
hitting that slow path ~80k/second. Uhm...

-- 
Jens Axboe