From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id AC6EFC47404 for ; Wed, 9 Oct 2019 17:08:46 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 78F3B20B7C for ; Wed, 9 Oct 2019 17:08:46 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1731873AbfJIRIp convert rfc822-to-8bit (ORCPT ); Wed, 9 Oct 2019 13:08:45 -0400 Received: from mx1.redhat.com ([209.132.183.28]:52340 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1730546AbfJIRIo (ORCPT ); Wed, 9 Oct 2019 13:08:44 -0400 Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.phx2.redhat.com [10.5.11.12]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id B4AE49B28D; Wed, 9 Oct 2019 17:08:43 +0000 (UTC) Received: from [10.18.17.163] (dhcp-17-163.bos.redhat.com [10.18.17.163]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 833A660C05; Wed, 9 Oct 2019 17:08:29 +0000 (UTC) Subject: Re: [PATCH v11 0/6] mm / virtio: Provide support for unused page reporting To: Alexander Duyck , Alexander Duyck Cc: LKML , linux-mm , David Hildenbrand , virtio-dev@lists.oasis-open.org, kvm list , "Michael S. Tsirkin" , Dave Hansen , Matthew Wilcox , Michal Hocko , Andrew Morton , Mel Gorman , Vlastimil Babka , Oscar Salvador , Yang Zhang , Pankaj Gupta , Konrad Rzeszutek Wilk , Rik van Riel , lcapitulino@redhat.com, "Wang, Wei W" , Andrea Arcangeli , Paolo Bonzini , Dan Williams References: <20191001152441.27008.99285.stgit@localhost.localdomain> <7233498c-2f64-d661-4981-707b59c78fd5@redhat.com> <1ea1a4e11617291062db81f65745b9c95fd0bb30.camel@linux.intel.com> <8bd303a6-6e50-b2dc-19ab-4c3f176c4b02@redhat.com> <0a16b11e-ec3b-7196-5b7f-e7395876cf28@redhat.com> <7fc13837-546c-9c4a-1456-753df199e171@redhat.com> <5b6e0b6df46c03bfac906313071ac0362d43c432.camel@linux.intel.com> <9bd52b8e-fa9e-a5ad-de39-660684757cdb@redhat.com> <4b6a9bda0a19c20b04338fd1d9b4f96086480355.camel@linux.intel.com> From: Nitesh Narayan Lal Openpgp: preference=signencrypt Autocrypt: addr=nitesh@redhat.com; prefer-encrypt=mutual; keydata= mQINBFl4pQoBEADT/nXR2JOfsCjDgYmE2qonSGjkM1g8S6p9UWD+bf7YEAYYYzZsLtbilFTe z4nL4AV6VJmC7dBIlTi3Mj2eymD/2dkKP6UXlliWkq67feVg1KG+4UIp89lFW7v5Y8Muw3Fm uQbFvxyhN8n3tmhRe+ScWsndSBDxYOZgkbCSIfNPdZrHcnOLfA7xMJZeRCjqUpwhIjxQdFA7 n0s0KZ2cHIsemtBM8b2WXSQG9CjqAJHVkDhrBWKThDRF7k80oiJdEQlTEiVhaEDURXq+2XmG jpCnvRQDb28EJSsQlNEAzwzHMeplddfB0vCg9fRk/kOBMDBtGsTvNT9OYUZD+7jaf0gvBvBB lbKmmMMX7uJB+ejY7bnw6ePNrVPErWyfHzR5WYrIFUtgoR3LigKnw5apzc7UIV9G8uiIcZEn C+QJCK43jgnkPcSmwVPztcrkbC84g1K5v2Dxh9amXKLBA1/i+CAY8JWMTepsFohIFMXNLj+B RJoOcR4HGYXZ6CAJa3Glu3mCmYqHTOKwezJTAvmsCLd3W7WxOGF8BbBjVaPjcZfavOvkin0u DaFvhAmrzN6lL0msY17JCZo046z8oAqkyvEflFbC0S1R/POzehKrzQ1RFRD3/YzzlhmIowkM BpTqNBeHEzQAlIhQuyu1ugmQtfsYYq6FPmWMRfFPes/4JUU/PQARAQABtCVOaXRlc2ggTmFy YXlhbiBMYWwgPG5pbGFsQHJlZGhhdC5jb20+iQI9BBMBCAAnBQJZeKUKAhsjBQkJZgGABQsJ CAcCBhUICQoLAgQWAgMBAh4BAheAAAoJEKOGQNwGMqM56lEP/A2KMs/pu0URcVk/kqVwcBhU SnvB8DP3lDWDnmVrAkFEOnPX7GTbactQ41wF/xwjwmEmTzLrMRZpkqz2y9mV0hWHjqoXbOCS 6RwK3ri5e2ThIPoGxFLt6TrMHgCRwm8YuOSJ97o+uohCTN8pmQ86KMUrDNwMqRkeTRW9wWIQ EdDqW44VwelnyPwcmWHBNNb1Kd8j3xKlHtnS45vc6WuoKxYRBTQOwI/5uFpDZtZ1a5kq9Ak/ MOPDDZpd84rqd+IvgMw5z4a5QlkvOTpScD21G3gjmtTEtyfahltyDK/5i8IaQC3YiXJCrqxE r7/4JMZeOYiKpE9iZMtS90t4wBgbVTqAGH1nE/ifZVAUcCtycD0f3egX9CHe45Ad4fsF3edQ ESa5tZAogiA4Hc/yQpnnf43a3aQ67XPOJXxS0Qptzu4vfF9h7kTKYWSrVesOU3QKYbjEAf95 NewF9FhAlYqYrwIwnuAZ8TdXVDYt7Z3z506//sf6zoRwYIDA8RDqFGRuPMXUsoUnf/KKPrtR ceLcSUP/JCNiYbf1/QtW8S6Ca/4qJFXQHp0knqJPGmwuFHsarSdpvZQ9qpxD3FnuPyo64S2N Dfq8TAeifNp2pAmPY2PAHQ3nOmKgMG8Gn5QiORvMUGzSz8Lo31LW58NdBKbh6bci5+t/HE0H pnyVf5xhNC/FuQINBFl4pQoBEACr+MgxWHUP76oNNYjRiNDhaIVtnPRqxiZ9v4H5FPxJy9UD Bqr54rifr1E+K+yYNPt/Po43vVL2cAyfyI/LVLlhiY4yH6T1n+Di/hSkkviCaf13gczuvgz4 KVYLwojU8+naJUsiCJw01MjO3pg9GQ+47HgsnRjCdNmmHiUQqksMIfd8k3reO9SUNlEmDDNB XuSzkHjE5y/R/6p8uXaVpiKPfHoULjNRWaFc3d2JGmxJpBdpYnajoz61m7XJlgwl/B5Ql/6B dHGaX3VHxOZsfRfugwYF9CkrPbyO5PK7yJ5vaiWre7aQ9bmCtXAomvF1q3/qRwZp77k6i9R3 tWfXjZDOQokw0u6d6DYJ0Vkfcwheg2i/Mf/epQl7Pf846G3PgSnyVK6cRwerBl5a68w7xqVU 4KgAh0DePjtDcbcXsKRT9D63cfyfrNE+ea4i0SVik6+N4nAj1HbzWHTk2KIxTsJXypibOKFX 2VykltxutR1sUfZBYMkfU4PogE7NjVEU7KtuCOSAkYzIWrZNEQrxYkxHLJsWruhSYNRsqVBy KvY6JAsq/i5yhVd5JKKU8wIOgSwC9P6mXYRgwPyfg15GZpnw+Fpey4bCDkT5fMOaCcS+vSU1 UaFmC4Ogzpe2BW2DOaPU5Ik99zUFNn6cRmOOXArrryjFlLT5oSOe4IposgWzdwARAQABiQIl BBgBCAAPBQJZeKUKAhsMBQkJZgGAAAoJEKOGQNwGMqM5ELoP/jj9d9gF1Al4+9bngUlYohYu 0sxyZo9IZ7Yb7cHuJzOMqfgoP4tydP4QCuyd9Q2OHHL5AL4VFNb8SvqAxxYSPuDJTI3JZwI7 d8JTPKwpulMSUaJE8ZH9n8A/+sdC3CAD4QafVBcCcbFe1jifHmQRdDrvHV9Es14QVAOTZhnJ vweENyHEIxkpLsyUUDuVypIo6y/Cws+EBCWt27BJi9GH/EOTB0wb+2ghCs/i3h8a+bi+bS7L FCCm/AxIqxRurh2UySn0P/2+2eZvneJ1/uTgfxnjeSlwQJ1BWzMAdAHQO1/lnbyZgEZEtUZJ x9d9ASekTtJjBMKJXAw7GbB2dAA/QmbA+Q+Xuamzm/1imigz6L6sOt2n/X/SSc33w8RJUyor SvAIoG/zU2Y76pKTgbpQqMDmkmNYFMLcAukpvC4ki3Sf086TdMgkjqtnpTkEElMSFJC8npXv 3QnGGOIfFug/qs8z03DLPBz9VYS26jiiN7QIJVpeeEdN/LKnaz5LO+h5kNAyj44qdF2T2AiF HxnZnxO5JNP5uISQH3FjxxGxJkdJ8jKzZV7aT37sC+Rp0o3KNc+GXTR+GSVq87Xfuhx0LRST NK9ZhT0+qkiN7npFLtNtbzwqaqceq3XhafmCiw8xrtzCnlB/C4SiBr/93Ip4kihXJ0EuHSLn VujM7c/b4pps Organization: Red Hat Inc, Message-ID: <90ab549c-2155-7a57-5dc9-f5a785049e8c@redhat.com> Date: Wed, 9 Oct 2019 13:08:28 -0400 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.8.0 MIME-Version: 1.0 In-Reply-To: <4b6a9bda0a19c20b04338fd1d9b4f96086480355.camel@linux.intel.com> Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8BIT Content-Language: en-US X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.39]); Wed, 09 Oct 2019 17:08:44 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 10/9/19 12:50 PM, Alexander Duyck wrote: > On Wed, 2019-10-09 at 12:25 -0400, Nitesh Narayan Lal wrote: >> On 10/7/19 1:20 PM, Alexander Duyck wrote: >>> On Mon, Oct 7, 2019 at 10:07 AM Nitesh Narayan Lal wrote: >>>> On 10/7/19 12:27 PM, Alexander Duyck wrote: >>>>> On Mon, 2019-10-07 at 12:19 -0400, Nitesh Narayan Lal wrote: >>>>>> On 10/7/19 11:33 AM, Alexander Duyck wrote: >>>>>>> On Mon, 2019-10-07 at 08:29 -0400, Nitesh Narayan Lal wrote: >>>>>>>> On 10/2/19 10:25 AM, Alexander Duyck wrote: >>> >>> >>>>>>>> page_reporting.c change: >>>>>>>> @@ -101,8 +101,12 @@ static void scan_zone_bitmap(struct page_reporting_config >>>>>>>> *phconf, >>>>>>>> /* Process only if the page is still online */ >>>>>>>> page = pfn_to_online_page((setbit << PAGE_REPORTING_MIN_ORDER) + >>>>>>>> zone->base_pfn); >>>>>>>> - if (!page) >>>>>>>> + if (!page || !PageBuddy(page)) { >>>>>>>> + clear_bit(setbit, zone->bitmap); >>>>>>>> + atomic_dec(&zone->free_pages); >>>>>>>> continue; >>>>>>>> + } >>>>>>>> >>>>>>> I suspect the zone->free_pages is going to be expensive for you to deal >>>>>>> with. It is a global atomic value and is going to have the cacheline >>>>>>> bouncing that it is contained in. As a result thinks like setting the >>>>>>> bitmap with be more expensive as every tome a CPU increments free_pages it >>>>>>> will likely have to take the cache line containing the bitmap pointer as >>>>>>> well. >>>>>> I see I will have to explore this more. I am wondering if there is a way to >>>>>> measure this If its effect is not visible in will-it-scale/page_fault1. If >>>>>> there is a noticeable amount of degradation, I will have to address this. >>>>> If nothing else you might look at seeing if you can split up the >>>>> structures so that the bitmap and nr_bits is in a different region >>>>> somewhere since those are read-mostly values. >>>> ok, I will try to understand the issue and your suggestion. >>>> Thank you for bringing this up. >>>> >>>>> Also you are now updating the bitmap and free_pages both inside and >>>>> outside of the zone lock so that will likely have some impact. >>>> So as per your previous suggestion, I have made the bitmap structure >>>> object as a rcu protected pointer. So we are safe from that side. >>>> The other downside which I can think of is a race where one page >>>> trying to increment free_pages and other trying to decrements it. >>>> However, being an atomic variable that should not be a problem. >>>> Did I miss anything? >>> I'm not so much worried about a race as the cache line bouncing >>> effect. Basically your notifier combined within this hinting thread >>> will likely result in more time spent by the thread that holds the >>> lock since it will be trying to access the bitmap to set the bit and >>> the free_pages to report the bit, but at the same time you will have >>> this thread clearing bits and decrementing the free_pages values. >>> >>> One thing you could consider in your worker thread would be to do >>> reallocate and replace the bitmap every time you plan to walk it. By >>> doing that you would avoid the cacheline bouncing on the bitmap since >>> you would only have to read it, and you would no longer have another >>> thread dirtying it. You could essentially reset the free_pages at the >>> same time you replace the bitmap. It would need to all happen with the >>> zone lock held though when you swap it out. >> If I am not mistaken then from what you are suggesting, I will have to hold >> the zone lock for the entire duration of swap & scan which would be costly if >> the bitmap is large, isn't? Also, we might end up missing free pages that are >> getting >> freed while we are scanning. > You would only need to hold the zone lock when you swap the bitmap. Once > it is swapped you wouldn't need to worry about the locking again for > bitmap access since your worker thread would be the only one holding the > current bitmap. Think of it as a batch clearing of the bits. I see. > > You already end up missing pages freed while scanning since you are doing > it linearly. I was referring to free pages for whom bits will not be set while we are doing the batch clearing of the bits. > >> As far as free_pages count is concerned, I am thinking if I should >> replace it with zone->free_area[REPORTING_ORDER].nr_free which is already there >> (I still need to explore this in a bit more depth). >> >>> - Alex > So there ends up being two ways you could use nr_free. One is to track it > the way I did with the number of reported pages being tracked, however > that requires reducing the count when reported pages are pulled from the > free_area and identifying reported pages vs unreported ones. > > The other option would be to look at converting nr_free into a pair of > free running counters, one tracking frees, and another tracking > allocations. Then you just need to record a snapshot of the nr_free values > when you do something like the bitmap swap, and then you would be able to > track churn, but it wouldn't give you an exact count of unreported pages > since it is possible to just alloc/free a single page multiple times to > make it look like you have freed a number of pages even though you really > haven't. Yeah possibly. I will think about it a little bit more to see what is the best way to do it. -- Thanks Nitesh