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[2003:d8:2f0a:ca00:f74f:2017:1617:3ec3]) by smtp.gmail.com with ESMTPSA id j5-20020a05600c1c0500b003dc4aae4739sm13771171wms.27.2023.01.31.05.48.48 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 31 Jan 2023 05:48:48 -0800 (PST) Message-ID: <88d50843-9aa6-7930-433d-9b488857dc14@redhat.com> Date: Tue, 31 Jan 2023 14:48:47 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.6.0 Subject: Re: [GIT PULL] iov_iter: Improve page extraction (pin or just list) To: David Howells Cc: Jens Axboe , John Hubbard , Al Viro , Christoph Hellwig , Matthew Wilcox , Jan Kara , Jason Gunthorpe , Logan Gunthorpe , Jeff Layton , linux-block@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <040ed7a7-3f4d-dab7-5a49-1cd9933c5445@redhat.com> <3351099.1675077249@warthog.procyon.org.uk> <3520518.1675116740@warthog.procyon.org.uk> <3791872.1675172490@warthog.procyon.org.uk> From: David Hildenbrand Organization: Red Hat In-Reply-To: <3791872.1675172490@warthog.procyon.org.uk> X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: 223461A001D X-Rspam-User: X-Stat-Signature: hkpxerq1xp7nfpcq7m5mjrm7qteio4tm X-HE-Tag: 1675172934-566235 X-HE-Meta: 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 J7s2olOz F56iGCvzl1s2vWQ/YmxJ8vSFWiQaEIl/yz5ufUe8A1CcOGvO/vXaZr+YYkbqtnqp4teQorCrP8uHq4RrrQedy7veW0LHwiAx/Rs55kdTiqGXSGDi8jz95IvL9pCXLmBwB9XwzKo5F1VP6oEHYOOapyA3Rw1BeRGdWnRdTaUnGYJQWzWsTTb1GeOvO4yQGS1/fTCrr2NRqRf+QrM1zWuvIHIlQdEpUMd9qe8b8mlrpTBWKW1qI1EYxac3btOJa5KhaieQsB0FBdiXEC6hR/bM58uP2Eziv5lI79ehxNbepe54LPmywMAFCUdZgxYcLe/+TGffOMR6YvFs+vEkh764BsG5UUf8buACFAHcn+zp0DbI8ah+tBU6KEWNvjaJeiGh0gU1K4n3YnUTbwXSQvW6HbeL0aJwuMkh9TlnCiXPdmV+k2dY0YZ+69YkHlMskVWXmJ+kQ8tO9qH1XNzwa1dDI8kDff6BJV+EZoqOB X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On 31.01.23 14:41, David Howells wrote: > David Hildenbrand 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