On Thu, Apr 18, 2024 at 5:26 PM Andreas Karis <akaris@xxxxxxxxxx> wrote: > > Not a kernel developer, so sorry if this here is pointless spam. > > But qemu is not the only thing consuming /dev/vhost-net: > https://doc.dpdk.org/guides/howto/virtio_user_as_exception_path.html > > Before the series of patches around > https://github.com/torvalds/linux/commit/e297cd54b3f81d652456ae6cb93941fc6b5c6683, > you would run a DPDK application inside namespaces of container > "blue", but then the vhost-... threads were spawned as children of > kthreadd and run in the global namespaces. That seemed counter > intuitive, and what's worse, the DPDK application running inside the > "blue" container namespaces cannot see and thus cannot modify > scheduling attributes and affinity of its own vhost-... threads. In > scenarios with pretty strict isolation/pinning requirements, the > vhost-... threads could easily be scheduled on the same CPUs as the > DPDK poll mode drivers (because they inherit the calling process' CPU > affinity), and there's no way for the DPDK process itself to move the > vhost-... threads to other CPUs. Also, if the DPDK process ran as > SCHED_RR, the vhost-.... thread would still be SCHED_NORMAL. > > After the patch series, the fact that the vhost-... threads run as > tasks of the process that requests them seems more natural and gives > us as users the kind of control that we'd want from within the > container to modify the vhost-... threads' scheduling and affinity. > The vhost-... thread as a child of the DPDK application inherits the > same scheduling class, CPU set, etc and the DPDK process can easily > change those attributes. > > However, if another user was used to the old behavior, and their > entire tooling was created for the old behavior (imagine someone wrote > a bunch of scripts/services to force affinity from the global PID > namespace for those vhost-... threads), and now this change was > introduced, it would break their tooling. So because _I_ might fancy > the new behavior, but user _B_ might be all set up for the old > kthreadd, shouldn't there be a flag or configuration for the user to: > > if (new_flag) > vhost_task_create() > else > kthread_create() > > I don't know what kind of flag or knob I'd expect here, but it could > be granular for the calling process (a new syscall?), or a kernel > flag, etc. But something that let's the admin choose how the kernel > spawns the vhost-... threads? A flag via uAPI that could be controlled by the user of vhost-net via syscall. For example, it could be done via set/get_backend_feautre() syscall. Thanks > > On Thu, Apr 18, 2024 at 9:07 AM Michael S. Tsirkin <mst@xxxxxxxxxx> wrote: > > > > On Thu, Apr 18, 2024 at 12:08:52PM +0800, Jason Wang wrote: > > > On Thu, Apr 18, 2024 at 12:10 AM Mike Christie > > > <michael.christie@xxxxxxxxxx> wrote: > > > > > > > > On 4/16/24 10:50 PM, Jason Wang wrote: > > > > > On Mon, Apr 15, 2024 at 4:52 PM Jason Wang <jasowang@xxxxxxxxxx> wrote: > > > > >> > > > > >> On Sat, Apr 13, 2024 at 12:53 AM <michael.christie@xxxxxxxxxx> wrote: > > > > >>> > > > > >>> On 4/11/24 10:28 PM, Jason Wang wrote: > > > > >>>> On Fri, Apr 12, 2024 at 12:19 AM Mike Christie > > > > >>>> <michael.christie@xxxxxxxxxx> wrote: > > > > >>>>> > > > > >>>>> On 4/11/24 3:39 AM, Jason Wang wrote: > > > > >>>>>> On Sat, Mar 16, 2024 at 8:47 AM Mike Christie > > > > >>>>>> <michael.christie@xxxxxxxxxx> wrote: > > > > >>>>>>> > > > > >>>>>>> The following patches were made over Linus's tree and also apply over > > > > >>>>>>> mst's vhost branch. The patches add the ability for vhost_tasks to > > > > >>>>>>> handle SIGKILL by flushing queued works, stop new works from being > > > > >>>>>>> queued, and prepare the task for an early exit. > > > > >>>>>>> > > > > >>>>>>> This removes the need for the signal/coredump hacks added in: > > > > >>>>>>> > > > > >>>>>>> Commit f9010dbdce91 ("fork, vhost: Use CLONE_THREAD to fix freezer/ps regression") > > > > >>>>>>> > > > > >>>>>>> when the vhost_task patches were initially merged and fix the issue > > > > >>>>>>> in this thread: > > > > >>>>>>> > > > > >>>>>>> https://lore.kernel.org/all/000000000000a41b82060e875721@xxxxxxxxxx/ > > > > >>>>>>> > > > > >>>>>>> Long Background: > > > > >>>>>>> > > > > >>>>>>> The original vhost worker code didn't support any signals. If the > > > > >>>>>>> userspace application that owned the worker got a SIGKILL, the app/ > > > > >>>>>>> process would exit dropping all references to the device and then the > > > > >>>>>>> file operation's release function would be called. From there we would > > > > >>>>>>> wait on running IO then cleanup the device's memory. > > > > >>>>>> > > > > >>>>>> A dumb question. > > > > >>>>>> > > > > >>>>>> Is this a user space noticeable change? For example, with this series > > > > >>>>>> a SIGKILL may shutdown the datapath ... > > > > >>>>> > > > > >>>>> It already changed in 6.4. We basically added a new interface to shutdown > > > > >>>>> everything (userspace and vhost kernel parts). So we won't just shutdown > > > > >>>>> the data path while userspace is still running. We will shutdown everything > > > > >>>>> now if you send a SIGKILL to a vhost worker's thread. > > > > >>>> > > > > >>>> If I understand correctly, for example Qemu can still live is SIGKILL > > > > >>>> is just send to vhost thread. > > > > >>> > > > > >>> Pre-6.4 qemu could still survive if only the vhost thread got a SIGKILL. > > > > >>> We used kthreads which are special and can ignore it like how userspace > > > > >>> can ignore SIGHUP. > > > > >>> > > > > >>> 6.4 and newer kernels cannot survive. Even if the vhost thread sort of > > > > >>> ignores it like I described below where, the signal is still delivered > > > > >>> to the other qemu threads due to the shared signal handler. Userspace > > > > >>> can't ignore SIGKILL. It doesn't have any say in the matter, and the > > > > >>> kernel forces them to exit. > > > > >> > > > > >> Ok, I see, so the reason is that vhost belongs to the same thread > > > > >> group as the owner now. > > > > >> > > > > >>> > > > > >>>> > > > > >>>> If this is correct, guests may detect this (for example virtio-net has > > > > >>>> a watchdog). > > > > >>>> > > > > >>> > > > > >>> What did you mean by that part? Do you mean if the vhost thread were to > > > > >>> exit, so drivers/vhost/net.c couldn't process IO, then the watchdog in > > > > >>> the guest (virtio-net driver in the guest kernel) would detect that? > > > > >> > > > > >> I meant this one. But since we are using CLONE_THREAD, we won't see these. > > > > >> > > > > >>> Or > > > > >>> are you saying the watchdog in the guest can detect signals that the > > > > >>> host gets? > > > > >>> > > > > >>> > > > > >>>>> > > > > >>>>> Here are a lots of details: > > > > >>>>> > > > > >>>>> - Pre-6.4 kernel, when vhost workers used kthreads, if you sent any signal > > > > >>>>> to a vhost worker, we ignore it. Nothing happens. kthreads are special and > > > > >>>>> can ignore all signals. > > > > >>>>> > > > > >>>>> You could think of it as the worker is a completely different process than > > > > >>>>> qemu/userspace so they have completely different signal handlers. The > > > > >>>>> vhost worker signal handler ignores all signals even SIGKILL. > > > > >>>> > > > > >>>> Yes. > > > > >>>> > > > > >>>>> > > > > >>>>> If you send a SIGKILL to a qemu thread, then it just exits right away. We > > > > >>>>> don't get to do an explicit close() on the vhost device and we don't get > > > > >>>>> to do ioctls like VHOST_NET_SET_BACKEND to clear backends. The kernel exit > > > > >>>>> code runs and releases refcounts on the device/file, then the vhost device's > > > > >>>>> file_operations->release function is called. vhost_dev_cleanup then stops > > > > >>>>> the vhost worker. > > > > >>>> > > > > >>>> Right. > > > > >>>> > > > > >>>>> > > > > >>>>> - In 6.4 and newer kernels, vhost workers use vhost_tasks, so the worker > > > > >>>>> can be thought of as a thread within the userspace process. With that > > > > >>>>> change we have the same signal handler as the userspace process. > > > > >>>>> > > > > >>>>> If you send a SIGKILL to a qemu thread then it works like above. > > > > >>>>> > > > > >>>>> If you send a SIGKILL to a vhost worker, the vhost worker still sort of > > > > >>>>> ignores it (that is the hack that I mentioned at the beginning of this > > > > >>>>> thread). kernel/vhost_task.c:vhost_task_fn will see the signal and > > > > >>>>> then just continue to process works until file_operations->release > > > > >>>>> calls > > > > >>>> > > > > >>>> Yes, so this sticks to the behaviour before vhost_tasks. > > > > >>> > > > > >>> Not exactly. The vhost_task stays alive temporarily. > > > > >>> > > > > >>> The signal is still delivered to the userspace threads and they will > > > > >>> exit due to getting the SIGKILL also. SIGKILL goes to all the threads in > > > > >>> the process and all userspace threads exit like normal because the vhost > > > > >>> task and normal old userspace threads share a signal handler. When > > > > >>> userspace exits, the kernel force drops the refcounts on the vhost > > > > >>> devices and that runs the release function so the vhost_task will then exit. > > > > >>> > > > > >>> So what I'm trying to say is that in 6.4 we already changed the behavior. > > > > >> > > > > >> Yes. To say the truth, it looks even worse but it might be too late to fix. > > > > > > > > > > Andres (cced) has identified two other possible changes: > > > > > > > > > > 1) doesn't run in the global PID namespace but run in the namespace of owner > > > > > > > > Yeah, I mentioned that one in vhost.h like it's a feature and when posting > > > > the patches I mentioned it as a possible fix. I mean I thought we wanted it > > > > to work like qemu and iothreads where the iothread would inherit all those > > > > values automatically. > > > > > > Right, but it could be noticed by the userspace, especially for the > > > one that tries to do tweak on the performance. > > > > > > The root cause is the that now we do copy_processs() in the process of > > > Qemu instead of the kthreadd. Which result of the the differences of > > > namespace (I think PID namespace is not the only one we see > > > difference) and others for the vhost task. > > > > Leaking things out of a namespace looks more like a bug. > > If you really have to be pedantic, the thing to add would > > be a namespace flag not a qemu flag. Userspace running inside > > a namespace really must have no say about whether to leak > > info out of it. > > > > > > > > > > At the time, I thought we didn't inherit the namespace, like we did the cgroup, > > > > because there was no kernel function for it (like how we didn't inherit v2 > > > > cgroups until recently when someone added some code for that). > > > > > > > > I don't know if it's allowed to have something like qemu in namespace N but then > > > > have it's children (vhost thread in this case) in the global namespace. > > > > I'll > > > > look into it. > > > > > > Instead of moving vhost thread between difference namespaces, I wonder > > > if the following is simpler: > > > > > > if (new_flag) > > > vhost_task_create() > > > else > > > kthread_create() > > > > > > New flag inherits the attributes of Qemu (namespaces, rlimit, cgroup, > > > scheduling attributes ...) which is what we want. Without the new > > > flag, we stick exactly to the behaviour as in the past to unbreak > > > existing userspace. > > > > > > > > > > > > 2) doesn't inherit kthreadd's scheduling attributes but the owner > > > > > > > > Same as above for this one. I thought I was fixing a bug where before > > > > we had to manually tune the vhost thread's values but for iothreads they > > > > automatically got setup. > > > > > > > > Just to clarify this one. When we used kthreads, kthread() will reset the > > > > scheduler priority for the kthread that's created, so we got the default > > > > values instead of inheriting kthreadd's values. So we would want: > > > > > > > > + sched_setscheduler_nocheck(current, SCHED_NORMAL, ¶m); > > > > > > > > in vhost_task_fn() instead of inheriting kthreadd's values. > > > > > > > > > > > > > > Though such a change makes more sense for some use cases, it may break others. > > > > > > > > > > I wonder if we need to introduce a new flag and bring the old kthread > > > > > > > > Do you mean something like a module param? > > > > > > This requires the management layer to know if it has a new user space > > > or not which is hard. A better place is to introduce backend features. > > > > > > > > > > > > codes if the flag is not set? Then we would not end up trying to align > > > > > the behaviour? > > > > > > > > > > > > > Let me know what you guys prefer. The sched part is easy. The namespace > > > > part might be more difficult, but I will look into it if you want it. > > > > > > Thanks a lot. I think it would be better to have the namespace part > > > (as well as other namespaces) then we don't need to answer hard > > > questions like if it can break user space or not. > > > > > > > > > >
