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

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Message-ID: <f392399b-a4c4-2251-e12b-e89fff351c4d@kernel.dk>
Date:   Mon, 30 Jan 2023 15:15:42 -0700
From:   Jens Axboe <axboe@...nel.dk>
To:     David Howells <dhowells@...hat.com>,
        John Hubbard <jhubbard@...dia.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 3:12 PM, David Howells wrote:
> John Hubbard <jhubbard@...dia.com> wrote:
> 
>> 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.
> 
> percpu counters maybe - add them up at the point of viewing?

They are percpu, see my last email. But for every 108 changes (on
my system), they will do two atomic_long_adds(). So not very
useful for anything but low frequency modifications.

-- 
Jens Axboe