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Wed, 26 Oct 2022 10:42:26 +0000 (GMT) Received: from [9.43.91.80] (unknown [9.43.91.80]) by d06av26.portsmouth.uk.ibm.com (Postfix) with ESMTP; Wed, 26 Oct 2022 10:42:26 +0000 (GMT) Message-ID: Date: Wed, 26 Oct 2022 16:12:25 +0530 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.4.0 Subject: Re: [PATCH] mm/vmscan: respect cpuset policy during page demotion Content-Language: en-US To: Michal Hocko , Feng Tang Cc: Andrew Morton , Johannes Weiner , Tejun Heo , Zefan Li , Waiman Long , "Huang, Ying" , "linux-mm@kvack.org" , "cgroups@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "Hansen, Dave" , "Chen, Tim C" , "Yin, Fengwei" References: <20221026074343.6517-1-feng.tang@intel.com> From: Aneesh Kumar K V In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-TM-AS-GCONF: 00 X-Proofpoint-GUID: B8Yy4Vryoz1bfCOlBhP62fdAHQVDl50e X-Proofpoint-ORIG-GUID: gsWuB0QGRMpc6iSOccX_MWZSRDyoKLb1 X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.205,Aquarius:18.0.895,Hydra:6.0.545,FMLib:17.11.122.1 definitions=2022-10-26_05,2022-10-26_01,2022-06-22_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 phishscore=0 impostorscore=0 mlxlogscore=999 lowpriorityscore=0 spamscore=0 adultscore=0 bulkscore=0 suspectscore=0 priorityscore=1501 mlxscore=0 malwarescore=0 clxscore=1015 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2209130000 definitions=main-2210260057 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 10/26/22 2:49 PM, Michal Hocko wrote: > On Wed 26-10-22 16:00:13, Feng Tang wrote: >> On Wed, Oct 26, 2022 at 03:49:48PM +0800, Aneesh Kumar K V wrote: >>> On 10/26/22 1:13 PM, Feng Tang wrote: >>>> In page reclaim path, memory could be demoted from faster memory tier >>>> to slower memory tier. Currently, there is no check about cpuset's >>>> memory policy, that even if the target demotion node is not allowd >>>> by cpuset, the demotion will still happen, which breaks the cpuset >>>> semantics. >>>> >>>> So add cpuset policy check in the demotion path and skip demotion >>>> if the demotion targets are not allowed by cpuset. >>>> >>> >>> What about the vma policy or the task memory policy? Shouldn't we respect >>> those memory policy restrictions while demoting the page? >> >> Good question! We have some basic patches to consider memory policy >> in demotion path too, which are still under test, and will be posted >> soon. And the basic idea is similar to this patch. > > For that you need to consult each vma and it's owning task(s) and that > to me sounds like something to be done in folio_check_references. > Relying on memcg to get a cpuset cgroup is really ugly and not really > 100% correct. Memory controller might be disabled and then you do not > have your association anymore. > I was looking at this recently and I am wondering whether we should worry about VM_SHARE vmas. ie, page_to_policy() can just reverse lookup just one VMA and fetch the policy right? if it VM_SHARE it will be a shared policy we can find using vma->vm_file? For non anonymous and anon vma not having any policy set it is owning task vma->vm_mm->owner task policy ? We don't worry about multiple tasks that can be possibly sharing that page right? > This all can get quite expensive so the primary question is, does the > existing behavior generates any real issues or is this more of an > correctness exercise? I mean it certainly is not great to demote to an > incompatible numa node but are there any reasonable configurations when > the demotion target node is explicitly excluded from memory > policy/cpuset? I guess vma policy is important. Applications want to make sure that they don't have variable performance and they go to lengths to avoid that by using MEM_BIND. So if they access the memory they always know access is satisfied from a specific set of NUMA nodes. Swapin can cause performance impact but then all continued access will be from a specific NUMA nodes. With slow memory demotion that is not going to be the case. Large in-memory database applications are observed to be sensitive to these access latencies. -aneesh