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Sun, 21 Dec 2025 04:38:30 -0800 (PST) Date: Sun, 21 Dec 2025 12:38:30 +0000 From: Wei Yang To: Vernon Yang Cc: Wei Yang , "David Hildenbrand (Red Hat)" , akpm@linux-foundation.org, lorenzo.stoakes@oracle.com, ziy@nvidia.com, baohua@kernel.org, lance.yang@linux.dev, linux-mm@kvack.org, linux-kernel@vger.kernel.org, Vernon Yang Subject: Re: [PATCH 3/4] mm: khugepaged: move mm to list tail when MADV_COLD/MADV_FREE Message-ID: <20251221123830.zr2szhudd6pdq7h5@master> Reply-To: Wei Yang References: <20251215090419.174418-1-yanglincheng@kylinos.cn> <20251215090419.174418-4-yanglincheng@kylinos.cn> <3c75d915-5d7f-4e80-975f-4479393e7139@kernel.org> <6e8684a5-1f71-4be6-8805-9b047a2bcb78@kernel.org> <20251221021044.2r5fhepiyyhvuo7h@master> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: User-Agent: NeoMutt/20170113 (1.7.2) On Sun, Dec 21, 2025 at 12:25:44PM +0800, Vernon Yang wrote: >On Sun, Dec 21, 2025 at 02:10:44AM +0000, Wei Yang wrote: >> On Fri, Dec 19, 2025 at 09:58:17AM +0100, David Hildenbrand (Red Hat) wrote: >> >On 12/19/25 06:29, Vernon Yang wrote: >> >> On Thu, Dec 18, 2025 at 10:31:58AM +0100, David Hildenbrand (Red Hat) wrote: >> >> > On 12/15/25 10:04, Vernon Yang wrote: >> >> > > For example, create three task: hot1 -> cold -> hot2. After all three >> >> > > task are created, each allocate memory 128MB. the hot1/hot2 task >> >> > > continuously access 128 MB memory, while the cold task only accesses >> >> > > its memory briefly andthen call madvise(MADV_COLD). However, khugepaged >> >> > > still prioritizes scanning the cold task and only scans the hot2 task >> >> > > after completing the scan of the cold task. >> >> > > >> >> > > So if the user has explicitly informed us via MADV_COLD/FREE that this >> >> > > memory is cold or will be freed, it is appropriate for khugepaged to >> >> > > scan it only at the latest possible moment, thereby avoiding unnecessary >> >> > > scan and collapse operations to reducing CPU wastage. >> >> > > >> >> > > Here are the performance test results: >> >> > > (Throughput bigger is better, other smaller is better) >> >> > > >> >> > > Testing on x86_64 machine: >> >> > > >> >> > > | task hot2 | without patch | with patch | delta | >> >> > > |---------------------|---------------|---------------|---------| >> >> > > | total accesses time | 3.14 sec | 2.92 sec | -7.01% | >> >> > > | cycles per access | 4.91 | 2.07 | -57.84% | >> >> > > | Throughput | 104.38 M/sec | 112.12 M/sec | +7.42% | >> >> > > | dTLB-load-misses | 288966432 | 1292908 | -99.55% | >> >> > > >> >> > > Testing on qemu-system-x86_64 -enable-kvm: >> >> > > >> >> > > | task hot2 | without patch | with patch | delta | >> >> > > |---------------------|---------------|---------------|---------| >> >> > > | total accesses time | 3.35 sec | 2.96 sec | -11.64% | >> >> > > | cycles per access | 7.23 | 2.12 | -70.68% | >> >> > > | Throughput | 97.88 M/sec | 110.76 M/sec | +13.16% | >> >> > > | dTLB-load-misses | 237406497 | 3189194 | -98.66% | >> >> > >> >> > Again, I also don't like that because you make assumptions on a full process >> >> > based on some part of it's address space. >> >> > >> >> > E.g., if a library issues a MADV_COLD on some part of the memory the library >> >> > manages, why should the remaining part of the process suffer as well? >> >> >> >> Yes, you make a good point, thanks! >> >> >> >> > This seems to be an heuristic focused on some specific workloads, no? >> >> >> >> Right. >> >> >> >> Could we use the VM_NOHUGEPAGE flag to indicate that this region should >> >> not be collapsed, so that khugepaged can simply skip this VMA during >> >> scanning? This way, it won't affect the remaining part of the task's >> >> memory regions. >> > >> >I thought we would skip these regions already properly in khugeapged, or >> >maybe I misunderstood your question. >> > >> >> I think we should, but seems we didn't do this for anonymous memory during >> khugepaged. >> >> We check the vma with thp_vma_allowable_order() during scan. >> >> * For anonymous memory during khugepaged, if we always enable 2M collapse, >> we will scan this vma. Even VM_NOHUGEPAGE is set. >> >> * For other cases, it looks good since __thp_vma_allowable_order() will skip >> this vma with vma_thp_disabled(). > >Hi David, Wei, > >The khugepaged has already checked the VM_NOHUGEPAGE flag for anonymous >memory during scan, as below: > >khugepaged_scan_mm_slot() > thp_vma_allowable_order() > thp_vma_allowable_orders() Oops, you are right. It only bypass __thp_vma_allowable_order() if orders is 0. > __thp_vma_allowable_orders() > vma_thp_disabled() { > if (vm_flags & VM_NOHUGEPAGE) > return true; > } > >REAL ISSUE: when madvise(MADV_COLD)，not set VM_NOHUGEPAGE flag to vma, >so the khugepaged will continue scan this vma. > >I set VM_NOHUGEPAGE flag to vma when madvise(MADV_COLD), the test has >been successful. I will send it in the next version. > >-- >Thanks, >Vernon -- Wei Yang Help you, Help me