Re: question on vhost, limiting kernel threads and NPROC

On Mon, Sep 13, 2021 at 12:04:04PM -0500, Mike Christie wrote: > I just realized I forgot to cc the virt list so adding now. > > Christian see the very bottom for a different fork patch. > > On 7/12/21 7:05 AM, Stefan Hajnoczi wrote: > > On Fri, Jul 09, 2021 at 11:25:37AM -0500, Mike Christie wrote: > >> Hi, > >> > >> The goal of this email is to try and figure how we want to track/limit the > >> number of kernel threads created by vhost devices. > >> > >> Background: > >> ----------- > >> For vhost-scsi, we've hit a issue where the single vhost worker thread can't > >> handle all IO the being sent from multiple queues. IOPs is stuck at around > >> 500K. To fix this, we did this patchset: > >> > >> https://lore.kernel.org/linux-scsi/20210525180600.6349-1-michael.christie... > >> > >> which allows userspace to create N threads and map them to a dev's virtqueues. > >> With this we can get around 1.4M IOPs. > >> > >> Problem: > >> -------- > >> While those patches were being reviewed, a concern about tracking all these > >> new possible threads was raised here: > >> > >> https://lore.kernel.org/linux-scsi/YL45CfpHyzSEcAJv@stefanha-x1.localdomain/ > >> > >> To save you some time, the question is what does other kernel code using the > >> kthread API do to track the number of kernel threads created on behalf of > >> a userspace thread. The answer is they don't do anything so we will have to > >> add that code. > >> > >> I started to do that here: > >> > >> https://lkml.org/lkml/2021/6/23/1233 > >> > >> where those patches would charge/check the vhost device owner's RLIMIT_NPROC > >> value. But, the question of if we really want to do this has come up which is > >> why I'm bugging lists like libvirt now. > >> > >> Question/Solution: > >> ------------------ > >> I'm bugging everyone so we can figure out: > >> > >> If we need to specifically track the number of kernel threads being made > >> for the vhost kernel use case by the RLIMIT_NPROC limit? > >> > >> Or, is it ok to limit the number of devices with the RLIMIT_NOFILE limit. > >> Then each device has a limit on the number of threads it can create. > > > > Do we want to add an interface where an unprivileged userspace process > > can create large numbers of kthreads? The number is indirectly bounded > > by RLIMIT_NOFILE * num_virtqueues, but there is no practical way to > > use that rlimit since num_virtqueues various across vhost devices and > > RLIMIT_NOFILE might need to have a specific value to control file > > descriptors. > > > > io_uring worker threads are limited by RLIMIT_NPROC. I think it makes > > sense in vhost too where the device instance is owned by a specific > > userspace process and can be accounted against that process' rlimit. > > > > I don't have a specific use case other than that I think vhost should be > > safe and well-behaved. > > > > Sorry for the late reply. I finally got to go on PTO and used like 2 > years worth in one super long vacation :) > > I still don't have a RLIMIT_NPROC use case and it wasn't not clear to > me if that has to be handled before merging. However, I might have got > lucky and found a bug where the fix will handle your request too. > > It looks like cgroup v2 is supposed to work, but for vhost threads > it doesn't because the kernel functions we use just support v1. If > we change the vhost layer to create threads like how io_uring does > then we get the RLIMIT_NPROC checks and also cgroup v2 support. > > Christian, If you didn't like this patch > > https://lkml.org/lkml/2021/6/23/1233 > > then I'm not sure how much you will like what is needed to support the > above. Here is a patch which includes what we would need from the fork > related code. On one hand, it's nicer because it fits into the PF FLAG > code like you requested. But, I have to add a no_files arg. See below: > > > ---------------------------------------------- > > > >From 351d476e8db0a78b9bdf22d77dd1abe66c0eac40 Mon Sep 17 00:00:00 2001 > From: Mike Christie <michael.christie(a)oracle.com&gt; > Date: Mon, 13 Sep 2021 11:20:20 -0500 > Subject: [PATCH] fork: allow cloning of userspace procs from kernel > > Userspace apps/processes like Qemu call into the vhost layer to create > worker threads which execute IO on behalf of VMs. If users set RIMIT > or cgroup limits or setup v2 cgroups or namespaces, the worker thread > is not accounted for or even setup correctly. The reason is that vhost > uses the kthread api which inherits those attributes/values from the > kthreadd thread. This patch allows kernel modules to work like the > io_uring code which can call kernel_clone from the userspace thread's > context and directly inherit its attributes like cgroups from and will > check limits like RLIMIT_NPROC against that userspace thread. > > Note: this patch combines 2 changes that should be separate patches. I'm > including both in one patch to just make it easier to get an idea of what > needs to be done. If we are ok with this then I'll break it up into a > proper patchset. > > This patch does the following: > > 1. Separates the PF_IO_WORKER flag behavior that controls signals and exit > cleanup into a new flag PF_USER_WORKER, so the vhost layer can use it > without the PF_IO_WORKER scheduling/IO behavior. > > 2. It adds a new no_files kernel_clone_args field. This is needed by vhost > because tools like qemu/libvirt do not always do a close() on the vhost > device. For some devices they just rely on the process exit reaping/cleanup > code to do a close() on all open FDs. However, if the vhost worker threads > have the device open (CLONE_FILES not set) or have a refcount on the > files_struct (CLONE_FILES set) then we can leak or possibly crash. > > leak - qemu just exits and expects the put done by the process exit > code will be the last put on the fd. But becuase the worker thread has a > ref to the fd or to the process's files_struct then it will never get the > last put and so the vhost device's release function will never be called. > > crash - if we add signal handling to the worker threads then it can > happen where the worker thread might get the signal and exit before > qemu has called the vhost cleanup releated ioctls and we can end up > crashing referencing what should be a valid device still. > --- > arch/x86/kernel/process.c | 4 ++-- > include/linux/sched.h | 1 + > include/linux/sched/task.h | 5 ++++- > init/main.c | 4 ++-- > kernel/fork.c | 24 +++++++++++++++++++----- > kernel/kthread.c | 3 ++- > kernel/signal.c | 4 ++-- > kernel/umh.c | 5 +++-- > 8 files changed, 35 insertions(+), 15 deletions(-) > > diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c > index 1d9463e3096b..1c5d516fb508 100644 > --- a/arch/x86/kernel/process.c > +++ b/arch/x86/kernel/process.c > @@ -178,9 +178,9 @@ int copy_thread(unsigned long clone_flags, unsigned long sp, unsigned long arg, > task_user_gs(p) = get_user_gs(current_pt_regs()); > #endif > > - if (unlikely(p->flags & PF_IO_WORKER)) { > + if (unlikely(p->flags & PF_USER_WORKER)) { > /* > - * An IO thread is a user space thread, but it doesn't > + * A user worker thread is a user space thread, but it doesn't > * return to ret_after_fork(). > * > * In order to indicate that to tools like gdb, > diff --git a/include/linux/sched.h b/include/linux/sched.h > index ec8d07d88641..0c9b3f62d85f 100644 > --- a/include/linux/sched.h > +++ b/include/linux/sched.h > @@ -1577,6 +1577,7 @@ extern struct pid *cad_pid; > #define PF_VCPU 0x00000001 /* I'm a virtual CPU */ > #define PF_IDLE 0x00000002 /* I am an IDLE thread */ > #define PF_EXITING 0x00000004 /* Getting shut down */ > +#define PF_USER_WORKER 0x00000008 /* Userspace kernel thread */ > #define PF_IO_WORKER 0x00000010 /* Task is an IO worker */ > #define PF_WQ_WORKER 0x00000020 /* I'm a workqueue worker */ > #define PF_FORKNOEXEC 0x00000040 /* Forked but didn't exec */ > diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h > index ef02be869cf2..2a8f9b8c3868 100644 > --- a/include/linux/sched/task.h > +++ b/include/linux/sched/task.h > @@ -32,6 +32,8 @@ struct kernel_clone_args { > size_t set_tid_size; > int cgroup; > int io_thread; > + int no_files; > + int user_worker; > struct cgroup *cgrp; > struct css_set *cset; > }; > @@ -86,7 +88,8 @@ extern pid_t kernel_clone(struct kernel_clone_args *kargs); > struct task_struct *create_io_thread(int (*fn)(void *), void *arg, int node); > struct task_struct *fork_idle(int); > struct mm_struct *copy_init_mm(void); > -extern pid_t kernel_thread(int (*fn)(void *), void *arg, unsigned long flags); > +extern pid_t kernel_thread(int (*fn)(void *), void *arg, unsigned long flags, > + int no_files, int user_worker); > extern long kernel_wait4(pid_t, int __user *, int, struct rusage *); > int kernel_wait(pid_t pid, int *stat); > > diff --git a/init/main.c b/init/main.c > index f5b8246e8aa1..18f3b126df93 100644 > --- a/init/main.c > +++ b/init/main.c > @@ -676,7 +676,7 @@ noinline void __ref rest_init(void) > * the init task will end up wanting to create kthreads, which, if > * we schedule it before we create kthreadd, will OOPS. > */ > - pid = kernel_thread(kernel_init, NULL, CLONE_FS); > + pid = kernel_thread(kernel_init, NULL, CLONE_FS, 0, 0); > /* > * Pin init on the boot CPU. Task migration is not properly working > * until sched_init_smp() has been run. It will set the allowed > @@ -689,7 +689,7 @@ noinline void __ref rest_init(void) > rcu_read_unlock(); > > numa_default_policy(); > - pid = kernel_thread(kthreadd, NULL, CLONE_FS | CLONE_FILES); > + pid = kernel_thread(kthreadd, NULL, CLONE_FS | CLONE_FILES, 0, 0); > rcu_read_lock(); > kthreadd_task = find_task_by_pid_ns(pid, &init_pid_ns); > rcu_read_unlock(); > diff --git a/kernel/fork.c b/kernel/fork.c > index bc94b2cc5995..9528940d83d7 100644 > --- a/kernel/fork.c > +++ b/kernel/fork.c > @@ -1458,7 +1458,8 @@ static int copy_fs(unsigned long clone_flags, struct task_struct *tsk) > return 0; > } > > -static int copy_files(unsigned long clone_flags, struct task_struct *tsk) > +static int copy_files(unsigned long clone_flags, struct task_struct *tsk, > + int no_files) > { > struct files_struct *oldf, *newf; > int error = 0; > @@ -1470,6 +1471,11 @@ static int copy_files(unsigned long clone_flags, struct task_struct *tsk) > if (!oldf) > goto out; > > + if (no_files) { > + tsk->files = NULL; > + goto out; > + } > + > if (clone_flags & CLONE_FILES) { > atomic_inc(&oldf->count); > goto out; > @@ -1954,11 +1960,14 @@ static __latent_entropy struct task_struct *copy_process( > goto fork_out; > if (args->io_thread) { > /* > - * Mark us an IO worker, and block any signal that isn't > - * fatal or STOP > + * Mark us an IO worker. > */ > p->flags |= PF_IO_WORKER; > + } > + > + if (args->user_worker) { > siginitsetinv(&p->blocked, sigmask(SIGKILL)|sigmask(SIGSTOP)); > + p->flags |= PF_USER_WORKER; > } > > /* > @@ -2104,7 +2113,7 @@ static __latent_entropy struct task_struct *copy_process( > retval = copy_semundo(clone_flags, p); > if (retval) > goto bad_fork_cleanup_security; > - retval = copy_files(clone_flags, p); > + retval = copy_files(clone_flags, p, args->no_files); > if (retval) > goto bad_fork_cleanup_semundo; > retval = copy_fs(clone_flags, p); > @@ -2452,6 +2461,7 @@ struct task_struct *create_io_thread(int (*fn)(void *), void *arg, int node) > .stack = (unsigned long)fn, > .stack_size = (unsigned long)arg, > .io_thread = 1, > + .user_worker = 1, > }; > > return copy_process(NULL, 0, node, &args); > @@ -2548,7 +2558,8 @@ pid_t kernel_clone(struct kernel_clone_args *args) > /* > * Create a kernel thread. > */ > -pid_t kernel_thread(int (*fn)(void *), void *arg, unsigned long flags) > +pid_t kernel_thread(int (*fn)(void *), void *arg, unsigned long flags, > + int no_files, int user_worker) > { > struct kernel_clone_args args = { > .flags = ((lower_32_bits(flags) | CLONE_VM | > @@ -2556,10 +2567,13 @@ pid_t kernel_thread(int (*fn)(void *), void *arg, unsigned long flags) > .exit_signal = (lower_32_bits(flags) & CSIGNAL), > .stack = (unsigned long)fn, > .stack_size = (unsigned long)arg, > + .no_files = no_files, > + .user_worker = user_worker, > }; > > return kernel_clone(&args); > } > +EXPORT_SYMBOL_GPL(kernel_thread); > > #ifdef __ARCH_WANT_SYS_FORK > SYSCALL_DEFINE0(fork) > diff --git a/kernel/kthread.c b/kernel/kthread.c > index 5b37a8567168..724c7ec63307 100644 > --- a/kernel/kthread.c > +++ b/kernel/kthread.c > @@ -339,7 +339,8 @@ static void create_kthread(struct kthread_create_info *create) > current->pref_node_fork = create->node; > #endif > /* We want our own signal handler (we take no signals by default). */ > - pid = kernel_thread(kthread, create, CLONE_FS | CLONE_FILES | SIGCHLD); > + pid = kernel_thread(kthread, create, CLONE_FS | CLONE_FILES | SIGCHLD, > + 0, 0); > if (pid < 0) { > /* If user was SIGKILLed, I release the structure. */ > struct completion *done = xchg(&create->done, NULL); > diff --git a/kernel/signal.c b/kernel/signal.c > index a3229add4455..3f901067b266 100644 > --- a/kernel/signal.c > +++ b/kernel/signal.c > @@ -2795,11 +2795,11 @@ bool get_signal(struct ksignal *ksig) > } > > /* > - * PF_IO_WORKER threads will catch and exit on fatal signals > + * PF_USER_WORKER threads will catch and exit on fatal signals > * themselves. They have cleanup that must be performed, so > * we cannot call do_exit() on their behalf. > */ > - if (current->flags & PF_IO_WORKER) > + if (current->flags & PF_USER_WORKER) > goto out; > > /* > diff --git a/kernel/umh.c b/kernel/umh.c > index 36c123360ab8..a6b7b733bd99 100644 > --- a/kernel/umh.c > +++ b/kernel/umh.c > @@ -132,7 +132,8 @@ static void call_usermodehelper_exec_sync(struct subprocess_info *sub_info) > > /* If SIGCLD is ignored do_wait won't populate the status. */ > kernel_sigaction(SIGCHLD, SIG_DFL); > - pid = kernel_thread(call_usermodehelper_exec_async, sub_info, SIGCHLD); > + pid = kernel_thread(call_usermodehelper_exec_async, sub_info, SIGCHLD, > + 0, 0); > if (pid < 0) > sub_info->retval = pid; > else > @@ -172,7 +173,7 @@ static void call_usermodehelper_exec_work(struct work_struct *work) > * that always ignores SIGCHLD to ensure auto-reaping. > */ > pid = kernel_thread(call_usermodehelper_exec_async, sub_info, > - CLONE_PARENT | SIGCHLD); > + CLONE_PARENT | SIGCHLD, 0, 0); > if (pid < 0) { > sub_info->retval = pid; > umh_complete(sub_info); Looks quite reasonable to me. You do of course want to post it and CC the proper people so it gets review. And add the vhost changes of course.