Message-ID: <88d50843-9aa6-7930-433d-9b488857dc14@redhat.com>
Date:   Tue, 31 Jan 2023 14:48:47 +0100
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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>
Cc:     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
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 <3791872.1675172490@warthog.procyon.org.uk>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
In-Reply-To: <3791872.1675172490@warthog.procyon.org.uk>
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On 31.01.23 14:41, David Howells wrote:
> 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 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)?

As it's really just "how many pinning events" vs. "how many unpinning 
events", I assume it's only for debugging.

For example, if you pin the same page twice it would not get accounted 
as "a single page is pinned".

-- 
Thanks,

David / dhildenb