Re: [RFC][PATCH v2 5/5] sched: User Mode Concurency Groups

Mark Rutland
2022-01-21T16:58:46Z

On Thu, Jan 20, 2022 at 04:55:22PM +0100, Peter Zijlstra wrote:
> User Managed Concurrency Groups is an M:N threading toolkit that allows
> constructing user space schedulers designed to efficiently manage
> heterogeneous in-process workloads while maintaining high CPU
> utilization (95%+).
>
> XXX moar changelog explaining how this is moar awesome than
> traditional user-space threading.

Awaiting a commit message that I can parse, I'm just looking at the entry bits
for now. TBH I have no idea what this is actually trying to do...

[...]

> --- a/include/linux/entry-common.h
> +++ b/include/linux/entry-common.h
> @@ -23,6 +23,10 @@
> # define _TIF_UPROBE (0)
> #endif
>
> +#ifndef _TIF_UMCG
> +# define _TIF_UMCG (0)
> +#endif
> +
> /*
> * SYSCALL_WORK flags handled in syscall_enter_from_user_mode()
> */
> @@ -43,11 +47,13 @@
> SYSCALL_WORK_SYSCALL_EMU | \
> SYSCALL_WORK_SYSCALL_AUDIT | \
> SYSCALL_WORK_SYSCALL_USER_DISPATCH | \
> + SYSCALL_WORK_SYSCALL_UMCG | \
> ARCH_SYSCALL_WORK_ENTER)
> #define SYSCALL_WORK_EXIT (SYSCALL_WORK_SYSCALL_TRACEPOINT | \
> SYSCALL_WORK_SYSCALL_TRACE | \
> SYSCALL_WORK_SYSCALL_AUDIT | \
> SYSCALL_WORK_SYSCALL_USER_DISPATCH | \
> + SYSCALL_WORK_SYSCALL_UMCG | \
> SYSCALL_WORK_SYSCALL_EXIT_TRAP | \
> ARCH_SYSCALL_WORK_EXIT)
>
> @@ -221,8 +227,11 @@ static inline void local_irq_disable_exi
> */
> static inline void irqentry_irq_enable(struct pt_regs *regs)
> {
> - if (!regs_irqs_disabled(regs))
> + if (!regs_irqs_disabled(regs)) {
> local_irq_enable();
> + if (user_mode(regs) && (current->flags & PF_UMCG_WORKER))
> + umcg_sys_enter(regs, -1);
> + }
> }

Perhaps it would make sense to have separate umcg_sys_enter(regs) and
umcg_sys_enter_syscall(regs, syscallno)? Even if the former is just a wrapper,
to make the entry/exit bits clearly correspond for all the !syscall cases?

Also, is the syscall case meant to nest within this, or syscall entry paths not
supposed to call irqentry_irq_enable() ?

>
> /**
> @@ -232,8 +241,11 @@ static inline void irqentry_irq_enable(s
> */
> static inline void irqentry_irq_disable(struct pt_regs *regs)
> {
> - if (!regs_irqs_disabled(regs))
> + if (!regs_irqs_disabled(regs)) {
> + if (user_mode(regs) && (current->flags & PF_UMCG_WORKER))
> + umcg_sys_exit(regs);
> local_irq_disable();
> + }
> }

Do the umcg_sys_{enter,exit}() calls need to happen with IRQs unmasked?

* If not (and this nests): for arm64 these can live in our
enter_from_user_mode() and exit_to_user_mode() helpers.

* If so (or this doesn't nest): for arm64 we'd need to rework our
local_daif_{inherit,restore,mask}() calls to handle this, though I've been
meaning to do that anyway to handle pseudo-NMI better.

Either way, it looks like we'd need helpers along the lines of:

| static __always_inline void umcg_enter_from_user(struct pt_regs *regs)
| {
| if (current->flags & PF_UMCG_WORKER)
| umcg_sys_enter(regs, -1);
| }
|
| static __always_inline void umcg_exit_to_user(struct pt_regs *regs)
| {
| if (current->flags & PF_UMCG_WORKER)
| umcg_sys_exit(regs);
| }

Thanks,
Mark.