Re: Why do_swap() sets cpu.current before context switch?


Yasushi SHOJI
 

Hi,

On Thu, Sep 9, 2021 at 12:01 AM Katsuhiro Suzuki
<katsuhiro@katsuster.net> wrote:
On 2021/09/08 16:22, Yasushi SHOJI wrote:
On Wed, Sep 8, 2021 at 10:23 AM Katsuhiro Suzuki <katsuhiro@katsuster.net> wrote:
Why do you `use _current_cpu` at all? `_arch_switch()` or
`arch_switch()` on the main branch takes
both new and old thread handles.
Because to keep consistency for another context switching (by preemption) and
other interrupts.
Only _current_cpu.current is available when an interrupt occurred.
The reason why we set _current to the new thread is that we can't set it after
we switch to the new thread. The newly switched thread will just start
running from
the point it left off. Otherwise, we have to make sure that each and
every arch must
set _current to the new thread in `arch_switch()`.

At an IRQ, you know the `_current` is the currently executing thread and you can
get a new thread from ready_q, if needed.
At explicit switch, you are given both old and new threads.
So in both cases, we _can_ implement the switch.

I agree that we need to do similar things for arch_switch and irq, and
love to use
exact same code for both, but it might be better to have separate code for
each situation. Or, at least use .macro to construct parts of it.

ps. Unfortunately, unlike the Linux kernel, we don't have any way to
get the thread struct
from a stack pointer, IIRC.

just my 2 cents,
--
yashi

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