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From:   David Howells <dhowells@redhat.com>
In-Reply-To: <040ed7a7-3f4d-dab7-5a49-1cd9933c5445@redhat.com>
References: <040ed7a7-3f4d-dab7-5a49-1cd9933c5445@redhat.com> <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> <f392399b-a4c4-2251-e12b-e89fff351c4d@kernel.dk>
To:     David Hildenbrand <david@redhat.com>
Cc:     dhowells@redhat.com, Jens Axboe <axboe@kernel.dk>,
        John Hubbard <jhubbard@nvidia.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Christoph Hellwig <hch@infradead.org>,
        Matthew Wilcox <willy@infradead.org>, Jan Kara <jack@suse.cz>,
        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
Subject: Re: [GIT PULL] iov_iter: Improve page extraction (pin or just list)
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Date:   Tue, 31 Jan 2023 13:41:30 +0000
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David Hildenbrand <david@redhat.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 mech=
anism
> 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 ca=
nnot
> really defer it any longer ... but I'm curious if it would be of any hel=
p to
> only have a single PINNED counter that goes into both directions (inc/de=
c on
> pin/release), to reduce the flushing.
> =

> Of course, once we pin/release more than ~108 pages in one go or we swit=
ch
> 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 actu=
ally
have users for them (control groups spring to mind)?

David