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

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Date:   Tue, 31 Jan 2023 09:32:27 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Jens Axboe <axboe@...nel.dk>, 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>,
        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 30.01.23 23:15, Jens Axboe wrote:
> 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.
> 

Can we just treat the whole acquired/released accounting as a debug 
mechanism to detect missing releases and do it only for debug kernels?


The pcpu counter is an s8, so we have to flush on a regular basis and 
cannot really defer it any longer ... but I'm curious if it would be of 
any help to only have a single PINNED counter that goes into both 
directions (inc/dec on pin/release), to reduce the flushing.

Of course, once we pin/release more than ~108 pages in one go or we 
switch CPUs frequently it won't be that much of a help ...

-- 
Thanks,

David / dhildenb