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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000002, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 12/30/2024 11:06 AM, David Rientjes wrote: > On Thu, 26 Dec 2024, Karim Manaouil wrote: > >> On Wed, Dec 18, 2024 at 07:56:19PM -0500, Gregory Price wrote: >>> On Tue, Dec 17, 2024 at 08:19:56PM -0800, David Rientjes wrote: >>>> ----->o----- >>>> Raghu noted the current promotion destination is node 0 by default. Wei >>>> noted we could get some page owner information to determine things like >>>> mempolicies or compute the distance between nodes and, if multiple nodes >>>> have the same distance, choose one of them just as we do for demotions. >>>> >>>> Gregory Price noted some downsides to using mempolicies for this based on >>>> per-task, per-vma, and cross socket policies, so using the kernel's >>>> memory tiering policies is probably the best way to go about it. >>>> >>> >>> Slightly elaborating here: >>> - In an async context, associating a page with a specific task is not >>> presently possible (that I know of). The most we know is the last >>> accessing CPU - maybe - in the page/folio struct. Right now this >>> is disabled in favor of a timestamp when tiering is enabled. >>> >>> a process with 2 tasks which have access to the page may not run >>> on the same socket, so we run the risk of migrating to a bad target. >>> Best effort here would suggest either socket is fine - since they're >>> both "fast nodes" - but this requires that we record the last >>> accessing CPU for a page at identification time. >>> >> >> This can be sovled with a two steps migration: first, you promote the >> page from CXL to a NUMA node, then you rely on NUMA balancing to >> further place the page into the right NUMA node. NUMA hint faults can >> still be enabled for pages allocated from NUMA nodes, but not for CXL. >> > > I think it would be a shame to promote to the wrong top-tier NUMA node and > rely on NUMA Balancing to fix it up with yet another migration :/ Agree here. Advantage of promotion is lost, considering the typical access time for CXL vs regular node we have currently. > > Since these cpuless memory nodes should have a promotion node associated > with them, which defaults to the latency given to us by the HMAT, can we > make that the default promotion target when memory is accessed? The > "normal mode" for NUMA Balancing could fix this up subsequent to the > promotion, but only if enabled. > > Raghu noted in the session that the current patch series only promotes to > node 0 but that choice is only for the RFC. I *assume* that every CXL > memory node will have a standard top-tier node to promote to *or* that we > stash that promotion node information at the time of demotion so memory > comes back to the same node it was demoted from. > > Either way, this feels like a solvable problem? How about sharing the hint between NUMAB mode=1 and kernel thread. For e.g., NUMAB mode=1 needs help on hot VMAs to scan. (which is supplied from kernel thread) whereas promotion target is kept at VMA level as a hint based on hint faults?? (Thinking loud here). Even top-tier node associated CXL might work, but need to think more here. PS: I had run my experiment with NUMAB mode=1 the benefit of kernel thread was intact. Thanks and Regards - Raghu