Re: [PATCH v2 4/5] riscv: add lazy preempt support

From: Guo Ren <guoren@kernel.org>
To: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: Jisheng Zhang <jszhang@kernel.org>,
	Paul Walmsley <paul.walmsley@sifive.com>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	Albert Ou <aou@eecs.berkeley.edu>,
	Anup Patel <anup@brainfault.org>,
	Atish Patra <atishp@atishpatra.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Steven Rostedt <rostedt@goodmis.org>,
	linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org,
	kvm@vger.kernel.org, kvm-riscv@lists.infradead.org
Subject: Re: [PATCH v2 4/5] riscv: add lazy preempt support
Date: Mon, 5 Sep 2022 16:33:54 +0800	[thread overview]
Message-ID: <CAJF2gTR=Cmcox5JrX2bB12MdmurY3vexSA6vw1cdXXbCNB8tXw@mail.gmail.com> (raw)
In-Reply-To: <YxWYh5C5swlyobi2@linutronix.de>

On Mon, Sep 5, 2022 at 2:34 PM Sebastian Andrzej Siewior
<bigeasy@linutronix.de> wrote:
>
> On 2022-09-04 23:16:12 [+0800], Guo Ren wrote:
> > > diff --git a/arch/riscv/kernel/entry.S b/arch/riscv/kernel/entry.S
> > > index b9eda3fcbd6d..595100a4c2c7 100644
> > > --- a/arch/riscv/kernel/entry.S
> > > +++ b/arch/riscv/kernel/entry.S
> > > @@ -361,9 +361,14 @@ restore_all:
> > >  resume_kernel:
> > >         REG_L s0, TASK_TI_PREEMPT_COUNT(tp)
> > >         bnez s0, restore_all
> > > -       REG_L s0, TASK_TI_FLAGS(tp)
> > > -       andi s0, s0, _TIF_NEED_RESCHED
> > > +       REG_L s1, TASK_TI_FLAGS(tp)
> > > +       andi s0, s1, _TIF_NEED_RESCHED
> > > +       bnez s0, 1f
> > > +       REG_L s0, TASK_TI_PREEMPT_LAZY_COUNT(tp)
> > > +       bnez s0, restore_all
> > > +       andi s0, s1, _TIF_NEED_RESCHED_LAZY
> > Can you tell me, who increased/decreased the PREEMPT_LAZY_COUNT? And
> > who set NEED_RESCHED_LAZY?
>
> There is "generic" code in the PREEMPT_RT patch doing that. The counter
> is incremented/ decremented via preempt_lazy_enable()/disable() and one
> of the user is migrate_disable()/enable().
> Basically if a task is task_is_realtime() then NEED_RESCHED is set for
> the wakeup. For the remaining states (SCHED_OTHER, …) NEED_RESCHED_LAZY
> is set for the wakeup. This can be delayed if the task is in a "preempt
> disable lazy" section (similar to a preempt_disable() section) but a
> task_is_realtime() can still be scheduled if needed.
Okay, It should be [PATCH RT]. RISC-V would also move to GENERIC_ENTRY
[1], so above assembly code would be replaced by generic one, right?

[1]: https://lore.kernel.org/linux-riscv/20220904072637.8619-3-guoren@kernel.org/T/#u


> See details at
>         https://git.kernel.org/pub/scm/linux/kernel/git/rt/linux-rt-devel.git/plain/patches/sched__Add_support_for_lazy_preemption.patch?h=linux-6.0.y-rt-patches


>
> Sebastian


-- 
Best Regards
 Guo Ren