Re: [RFC PATCH v3 11/15] context-tracking: Introduce work deferral infrastructure

[Frederic Weisbecker]

Le Tue, Nov 19, 2024 at 04:34:58PM +0100, Valentin Schneider a écrit :
> +bool ct_set_cpu_work(unsigned int cpu, unsigned int work)
> +{
> +	struct context_tracking *ct = per_cpu_ptr(&context_tracking, cpu);
> +	unsigned int old;
> +	bool ret = false;
> +
> +	preempt_disable();
> +
> +	old = atomic_read(&ct->state);
> +	/*
> +	 * Try setting the work until either
> +	 * - the target CPU has entered kernelspace
> +	 * - the work has been set
> +	 */
> +	do {
> +		ret = atomic_try_cmpxchg(&ct->state, &old, old | (work << CT_WORK_START));
> +	} while (!ret && ((old & CT_STATE_MASK) != CT_STATE_KERNEL));
> +
> +	preempt_enable();
> +	return ret;

Does it ignore the IPI even if:

     (ret && (old & CT_STATE_MASK) == CT_STATE_KERNEL))

?

And what about CT_STATE_IDLE?

Is the work ignored in those two cases?

But would it be cleaner to never set the work if the target is elsewhere
than CT_STATE_USER. So you don't need to clear the work on kernel exit
but rather on kernel entry.

That is:

bool ct_set_cpu_work(unsigned int cpu, unsigned int work)
{
	struct context_tracking *ct = per_cpu_ptr(&context_tracking, cpu);
	unsigned int old;
	bool ret = false;

	preempt_disable();

	old = atomic_read(&ct->state);

	/* Start with our best wishes */
	old &= ~CT_STATE_MASK;
	old |= CT_STATE_USER

	/*
	 * Try setting the work until either
	 * - the target CPU has exited userspace
	 * - the work has been set
	 */
	do {
		ret = atomic_try_cmpxchg(&ct->state, &old, old | (work << CT_WORK_START));
	} while (!ret && ((old & CT_STATE_MASK) == CT_STATE_USER));

	preempt_enable();

	return ret;
}

Thanks.