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

From: Peter Zijlstra <peterz@infradead.org>
To: Mark Rutland <mark.rutland@arm.com>
Cc: mingo@redhat.com, tglx@linutronix.de, juri.lelli@redhat.com,
	vincent.guittot@linaro.org, dietmar.eggemann@arm.com,
	rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de,
	bristot@redhat.com, linux-kernel@vger.kernel.org,
	linux-mm@kvack.org, linux-api@vger.kernel.org, x86@kernel.org,
	pjt@google.com, posk@google.com, avagin@google.com,
	jannh@google.com, tdelisle@uwaterloo.ca, posk@posk.io
Subject: Re: [RFC][PATCH v2 5/5] sched: User Mode Concurency Groups
Date: Mon, 24 Jan 2022 11:07:04 +0100	[thread overview]
Message-ID: <20220124100704.GC22849@worktop.programming.kicks-ass.net> (raw)
In-Reply-To: <20220124100306.GO20638@worktop.programming.kicks-ass.net>

On Mon, Jan 24, 2022 at 11:03:06AM +0100, Peter Zijlstra wrote:

> > 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);
> > | }
> 
> Would something like:
> 
> #ifndef arch_irqentry_irq_enter
> static __always_inline bool arch_irqentry_irq_enter(struct pt_regs *regs)
> {
> 	if (!regs_irqs_disabled(regs)) {
> 		local_irq_enable();
> 		return true;
> 	}
> 	return false;
> }
> #endif
> 
> static __always_inline void irqentry_irq_enter(struct pt_regs *regs)
> {
> 	if (arch_irqentry_irq_inherit(regs)) {
> 		if (user_mode(regs) && (current->flags & PF_UMCG_WORKER))
> 			umcg_sys_enter(regs, -1);
> 	}
> }
> 
> Work? Then arm64 can do:
> 
> static __always_inline bool arch_irqentry_irq_enter(struct pt_regs *regs)
> {
> 	local_daif_inherit();
> 	return interrupts_enabled(regs);
> }
> 
> or somesuch...

Ah,.. just read your other email, so your concern is about the
user_mode() thing due to ARM64 taking a different exception path for
from-user vs from-kernel ?

I don't mind too much if arm64 decides to open-code the umcg hooks, but
please do it such that's hard to forget a spot.