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

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Message-ID: <3791872.1675172490@warthog.procyon.org.uk>
Date:   Tue, 31 Jan 2023 13:41:30 +0000
From:   David Howells <dhowells@...hat.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     dhowells@...hat.com, Jens Axboe <axboe@...nel.dk>,
        John Hubbard <jhubbard@...dia.com>,
        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)

David Hildenbrand <david@...hat.com> wrote:

> >> 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 ...

What are the stats actually used for?  Is it just debugging, or do we actually
have users for them (control groups spring to mind)?

David