Re: [PATCH v2 7/9] sched: define TIF_ALLOW_RESCHED

Sergey Senozhatsky
2023-10-26T07:50:28Z

On (23/10/24 10:34), Steven Rostedt wrote:
> On Tue, 19 Sep 2023 01:42:03 +0200
> Thomas Gleixner <tglx@linutronix.de> wrote:
>
> > 2) When the scheduler wants to set NEED_RESCHED due it sets
> > NEED_RESCHED_LAZY instead which is only evaluated in the return to
> > user space preemption points.
> >
> > As NEED_RESCHED_LAZY is not folded into the preemption count the
> > preemption count won't become zero, so the task can continue until
> > it hits return to user space.
> >
> > That preserves the existing behaviour.
>
> I'm looking into extending this concept to user space and to VMs.
>
> I'm calling this the "extended scheduler time slice" (ESTS pronounced "estis")
>
> The ideas is this. Have VMs/user space share a memory region with the
> kernel that is per thread/vCPU. This would be registered via a syscall or
> ioctl on some defined file or whatever. Then, when entering user space /
> VM, if NEED_RESCHED_LAZY (or whatever it's eventually called) is set, it
> checks if the thread has this memory region and a special bit in it is
> set, and if it does, it does not schedule. It will treat it like a long
> kernel system call.
>
> The kernel will then set another bit in the shared memory region that will
> tell user space / VM that the kernel wanted to schedule, but is allowing it
> to finish its critical section. When user space / VM is done with the
> critical section, it will check the bit that may be set by the kernel and
> if it is set, it should do a sched_yield() or VMEXIT so that the kernel can
> now schedule it.
>
> What about DOS you say? It's no different than running a long system call.
> No task can run forever. It's not a "preempt disable", it's just "give me
> some more time". A "NEED_RESCHED" will always schedule, just like a kernel
> system call that takes a long time. The goal is to allow user space to get
> out of critical sections that we know can cause problems if they get
> preempted. Usually it's a user space / VM lock is held or maybe a VM
> interrupt handler that needs to wake up a task on another vCPU.
>
> If we are worried about abuse, we could even punish tasks that don't call
> sched_yield() by the time its extended time slice is taken. Even without
> that punishment, if we have EEVDF, this extension will make it less
> eligible the next time around.
>
> The goal is to prevent a thread / vCPU being preempted while holding a lock
> or resource that other threads / vCPUs will want. That is, prevent
> contention, as that's usually the biggest issue with performance in user
> space and VMs.

I think some time ago we tried to check guest's preempt count on each vm-exit
and we'd vm-enter if guest exited from a critical section (those that bump
preempt count) so that it can hopefully finish whatever is was going to
do and vmexit again. We didn't look into covering guest's RCU read-side
critical sections.

Can you educate me, is your PoC significantly different from guest preempt
count check?