Re: [Xen-devel] [PATCH] xen: privcmd: schedule() after private hypercall when non CONFIG_PREEMPT

[Juergen Gross <jgross@suse.com>]

On 12/03/2014 03:28 AM, Luis R. Rodriguez wrote:
> On Tue, Dec 02, 2014 at 11:11:18AM +0000, David Vrabel wrote:
>> On 01/12/14 22:36, Luis R. Rodriguez wrote:
>>>
>>> Then I do agree its a fair analogy (and find this obviously odd that how
>>> widespread cond_resched() is), we just don't have an equivalent for IRQ
>>> context, why not avoid the special check then and use this all the time in the
>>> middle of a hypercall on the return from an interrupt (e.g., the timer
>>> interrupt)?
>>
>> http://lists.xen.org/archives/html/xen-devel/2014-02/msg01101.html
>
> OK thanks! That explains why we need some asm code but in that submission you
> still also had used is_preemptible_hypercall(regs) and in the new
> implementation you use a CPU variable xen_in_preemptible_hcall prior to calling
> preempt_schedule_irq(). I believe you added the CPU variable because
> preempt_schedule_irq() will preempt first without any checks if it should, I'm
> asking why not do something like cond_resched_irq() where we check with
> should_resched() prior to preempting and that way we can avoid having to use
> the CPU variable?

Because that could preempt at any asynchronous interrupt making the
no-preempt kernel fully preemptive. How would you know you are just
doing a critical hypercall which should be preempted?

Juergen