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

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Date:   Mon, 5 Sep 2022 08:34:47 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Guo Ren <guoren@...nel.org>
Cc:     Jisheng Zhang <jszhang@...nel.org>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Anup Patel <anup@...infault.org>,
        Atish Patra <atishp@...shpatra.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Steven Rostedt <rostedt@...dmis.org>,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, kvm-riscv@...ts.infradead.org
Subject: Re: [PATCH v2 4/5] riscv: add lazy preempt support

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.
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