Si-Wei Liu <si-wei....@oracle.com> writes: > On 5/26/2025 2:16 AM, Markus Armbruster wrote: >> Si-Wei Liu <si-wei....@oracle.com> writes: >> >>> On 5/15/2025 11:40 PM, Markus Armbruster wrote: >>>> Jason Wang <jasow...@redhat.com> writes: >>>> >>>>> On Thu, May 8, 2025 at 2:47 AM Jonah Palmer <jonah.pal...@oracle.com> >>>>> wrote: >>>>>> Current memory operations like pinning may take a lot of time at the >>>>>> destination. Currently they are done after the source of the migration >>>>>> is >>>>>> stopped, and before the workload is resumed at the destination. This is >>>>>> a >>>>>> period where neigher traffic can flow, nor the VM workload can continue >>>>>> (downtime). >>>>>> >>>>>> We can do better as we know the memory layout of the guest RAM at the >>>>>> destination from the moment that all devices are initializaed. So >>>>>> moving that operation allows QEMU to communicate the kernel the maps >>>>>> while the workload is still running in the source, so Linux can start >>>>>> mapping them. >>>>>> >>>>>> As a small drawback, there is a time in the initialization where QEMU >>>>>> cannot respond to QMP etc. By some testing, this time is about >>>>>> 0.2seconds. >>>>> Adding Markus to see if this is a real problem or not. >>>> I guess the answer is "depends", and to get a more useful one, we need >>>> more information. >>>> >>>> When all you care is time from executing qemu-system-FOO to guest >>>> finish booting, and the guest takes 10s to boot, then an extra 0.2s >>>> won't matter much. >>> >>> There's no such delay of an extra 0.2s or higher per se, it's just shifting >>> around the page pinning hiccup, no matter it is 0.2s or something else, >>> from the time of guest booting up to before guest is booted. This saves >>> back guest boot time or start up delay, but in turn the same delay >>> effectively will be charged to VM launch time. We follow the same model >>> with VFIO, which would see the same hiccup during launch (at an early stage >>> where no real mgmt software would care about). >>> >>>> When a management application runs qemu-system-FOO several times to >>>> probe its capabilities via QMP, then even milliseconds can hurt. >>>> >>> Not something like that, this page pinning hiccup is one time only that >>> occurs in the very early stage when launching QEMU, i.e. there's no >>> consistent delay every time when QMP is called. The delay in QMP response >>> at that very point depends on how much memory the VM has, but this is just >>> specif to VM with VFIO or vDPA devices that have to pin memory for DMA. >>> Having said, there's no extra delay at all if QEMU args has no vDPA device >>> assignment, on the other hand, there's same delay or QMP hiccup when VFIO >>> is around in QEMU args. >>> >>>> In what scenarios exactly is QMP delayed? >>> >>> Having said, this is not a new problem to QEMU in particular, this QMP >>> delay is not peculiar, it's existent on VFIO as well. >> >> In what scenarios exactly is QMP delayed compared to before the patch? > > The page pinning process now runs in a pretty early phase at > qemu_init() e.g. machine_run_board_init(),
It runs within qemu_init() qmp_x_exit_preconfig() qemu_init_board() machine_run_board_init() Except when --preconfig is given, it instead runs within QMP command x-exit-preconfig. Correct? > before any QMP command can be serviced, the latter of which typically > would be able to get run from qemu_main_loop() until the AIO gets > chance to be started to get polled and dispatched to bh. We create the QMP monitor within qemu_create_late_backends(), which runs before qmp_x_exit_preconfig(), but commands get processed only in the main loop, which we enter later. Correct? > Technically it's not a real delay for specific QMP command, but rather > an extended span of initialization process may take place before the > very first QMP request, usually qmp_capabilities, will be > serviced. It's natural for mgmt software to expect initialization > delay for the first qmp_capabilities response if it has to immediately > issue one after launching qemu, especially when you have a large guest > with hundred GBs of memory and with passthrough device that has to pin > memory for DMA e.g. VFIO, the delayed effect from the QEMU > initialization process is very visible too. > On the other hand, before > the patch, if memory happens to be in the middle of being pinned, any > ongoing QMP can't be serviced by the QEMU main loop, either. > > I'd also like to highlight that without this patch, the pretty high > delay due to page pinning is even visible to the guest in addition to > just QMP delay, which largely affected guest boot time with vDPA > device already. It is long standing, and every VM user with vDPA > device would like to avoid such high delay for the first boot, which > is not seen with similar device e.g. VFIO passthrough. > >> >>> Thanks, >>> -Siwei >>> >>>> You told us an absolute delay you observed. What's the relative delay, >>>> i.e. what's the delay with and without these patches? >> >> Can you answer this question? > > I thought I already got that answered in earlier reply. The relative > delay is subject to the size of memory. Usually mgmt software won't be > able to notice, unless the guest has more than 100GB of THP memory to > pin, for DMA or whatever reason. > > >> >>>> We need QMP to become available earlier in the startup sequence for >>>> other reasons. Could we bypass the delay that way? Please understand >>>> that this would likely be quite difficult: we know from experience that >>>> messing with the startup sequence is prone to introduce subtle >>>> compatility breaks and even bugs. >>>> >>>>> (I remember VFIO has some optimization in the speed of the pinning, >>>>> could vDPA do the same?) >>>> >>>> That's well outside my bailiwick :) > > Please be understood that any possible optimization is out of scope of > this patch series, while there's certainly way around that already and > to be carry out in the future, as Peter alluded to in earlier > discussion thread: > > https://lore.kernel.org/qemu-devel/ZZT7wuq-_IhfN_wR@x1n/ > https://lore.kernel.org/qemu-devel/ZZZUNsOVxxqr-H5S@x1n/ > > Thanks, > -Siwei > >>>> >>>> [...] >>>>