Message-ID: <f392399b-a4c4-2251-e12b-e89fff351c4d@kernel.dk>
Date:   Mon, 30 Jan 2023 15:15:42 -0700
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Subject: Re: [GIT PULL] iov_iter: Improve page extraction (pin or just list)
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To:     David Howells <dhowells@redhat.com>,
        John Hubbard <jhubbard@nvidia.com>
Cc:     Al Viro <viro@zeniv.linux.org.uk>,
        Christoph Hellwig <hch@infradead.org>,
        Matthew Wilcox <willy@infradead.org>, Jan Kara <jack@suse.cz>,
        David Hildenbrand <david@redhat.com>,
        Jason Gunthorpe <jgg@nvidia.com>,
        Logan Gunthorpe <logang@deltatee.com>,
        Jeff Layton <jlayton@kernel.org>, linux-block@vger.kernel.org,
        linux-fsdevel@vger.kernel.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
References: <e68c5cab-c3a6-1872-98fa-9f909f23be79@nvidia.com>
 <3351099.1675077249@warthog.procyon.org.uk>
 <fd0003a0-a133-3daf-891c-ba7deafad768@kernel.dk>
 <f57ee72f-38e9-6afa-182f-2794638eadcb@kernel.dk>
 <e8480b18-08af-d101-a721-50d213893492@kernel.dk>
 <3520518.1675116740@warthog.procyon.org.uk>
From:   Jens Axboe <axboe@kernel.dk>
In-Reply-To: <3520518.1675116740@warthog.procyon.org.uk>
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On 1/30/23 3:12 PM, David Howells wrote:
> John Hubbard <jhubbard@nvidia.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