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

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Message-ID: <88d50843-9aa6-7930-433d-9b488857dc14@redhat.com>
Date:   Tue, 31 Jan 2023 14:48:47 +0100
From:   David Hildenbrand <david@...hat.com>
To:     David Howells <dhowells@...hat.com>
Cc:     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)

On 31.01.23 14:41, David Howells wrote:
> 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)?

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