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

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Date:   Mon, 30 Jan 2023 14:02:52 -0800
From:   John Hubbard <jhubbard@...dia.com>
To:     Jens Axboe <axboe@...nel.dk>, 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 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