On 01.06.22 04:17, zhenwei pi wrote: > On 5/31/22 12:08, Jue Wang wrote: >> On Mon, May 30, 2022 at 8:49 AM Peter Xu <peterx@xxxxxxxxxx> wrote: >>> >>> On Mon, May 30, 2022 at 07:33:35PM +0800, zhenwei pi wrote: >>>> A VM uses RAM of 2M huge page. Once a MCE(@HVAy in [HVAx,HVAz)) occurs, the >>>> 2M([HVAx,HVAz)) of hypervisor becomes unaccessible, but the guest poisons 4K >>>> (@GPAy in [GPAx, GPAz)) only, it may hit another 511 MCE ([GPAx, GPAz) >>>> except GPAy). This is the worse case, so I want to add >>>> '__le32 corrupted_pages' in struct virtio_balloon_config, it is used in the >>>> next step: reporting 512 * 4K 'corrupted_pages' to the guest, the guest has >>>> a chance to isolate the other 511 pages ahead of time. And the guest >>>> actually loses 2M, fixing 512*4K seems to help significantly. >>> >>> It sounds hackish to teach a virtio device to assume one page will always >>> be poisoned in huge page granule. That's only a limitation to host kernel >>> not virtio itself. >>> >>> E.g. there're upstream effort ongoing with enabling doublemap on hugetlbfs >>> pages so hugetlb pages can be mapped in 4k with it. It provides potential >>> possibility to do page poisoning with huge pages in 4k too. When that'll >>> be ready the assumption can go away, and that does sound like a better >>> approach towards this problem. >> >> +1. >> >> A hypervisor should always strive to minimize the guest memory loss. >> >> The HugeTLB double mapping enlightened memory poisoning behavior (only >> poison 4K out of a 2MB huge page and 4K in guest) is a much better >> solution here. To be completely transparent, it's not _strictly_ >> required to poison the page (whatever the granularity it is) on the >> host side, as long as the following are true: >> >> 1. A hypervisor can emulate the _minimized_ (e.g., 4K) the poison to the guest. >> 2. The host page with the UC error is "isolated" (could be PG_HWPOISON >> or in some other way) and prevented from being reused by other >> processes. >> >> For #2, PG_HWPOISON and HugeTLB double mapping enlightened memory >> poisoning is a good solution. >> >>> >>>> >>>>> >>>>> I assume when talking about "the performance memory drops a lot", you >>>>> imply that this patch set can mitigate that performance drop? >>>>> >>>>> But why do you see a performance drop? Because we might lose some >>>>> possible THP candidates (in the host or the guest) and you want to plug >>>>> does holes? I assume you'll see a performance drop simply because >>>>> poisoning memory is expensive, including migrating pages around on CE. >>>>> >>>>> If you have some numbers to share, especially before/after this change, >>>>> that would be great. >>>>> >>>> >>>> The CE storm leads 2 problems I have even seen: >>>> 1, the memory bandwidth slows down to 10%~20%, and the cycles per >>>> instruction of CPU increases a lot. >>>> 2, the THR (/proc/interrupts) interrupts frequently, the CPU has to use a >>>> lot time to handle IRQ. >>> >>> Totally no good knowledge on CMCI, but if 2) is true then I'm wondering >>> whether it's necessary to handle the interrupts that frequently. When I >>> was reading the Intel CMCI vector handler I stumbled over this comment: >>> >>> /* >>> * The interrupt handler. This is called on every event. >>> * Just call the poller directly to log any events. >>> * This could in theory increase the threshold under high load, >>> * but doesn't for now. >>> */ >>> static void intel_threshold_interrupt(void) >>> >>> I think that matches with what I was thinking.. I mean for 2) not sure >>> whether it can be seen as a CMCI problem and potentially can be optimized >>> by adjust the cmci threshold dynamically. >> >> The CE storm caused performance drop is caused by the extra cycles >> spent by the ECC steps in memory controller, not in CMCI handling. >> This is observed in the Google fleet as well. A good solution is to >> monitor the CE rate closely in user space via /dev/mcelog and migrate >> all VMs to another host once the CE rate exceeds some threshold. >> >> CMCI is a _background_ interrupt that is not handled in the process >> execution context and its handler is setup to switch to poll (1 / 5 >> min) mode if there are more than ~ a dozen CEs reported via CMCI per >> second. >>> >>> -- >>> Peter Xu >>> > > Hi, Andrew, David, Naoya > > According to the suggestions, I'd give up the improvement of memory > failure on huge page in this series. > > Is it worth recovering corrupted pages for the guest kernel? I'd follow > your decision. Well, as I said, I am not sure if we really need/want this for a handful of 4k poisoned pages in a VM. As I suspected, doing so might primarily be interesting for some sort of de-fragmentation (allow again a higher order page to be placed at the affected PFNs), not because of the slight reduction of available memory. A simple VM reboot would get the job similarly done. As the poisoning refcount code is already a bit shaky as I learned recently in the context of memory offlining, I do wonder if we really want to expose the unpoisoning code outside of debugfs (hwpoison) usage. Interestingly, unpoison_memory() documents: "This is only done on the software-level, so it only works for linux injected failures, not real hardware failures" -- ehm? -- Thanks, David / dhildenb _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
