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

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Date:	Wed, 03 Dec 2014 05:37:51 +0100
From:	Juergen Gross <jgross@...e.com>
To:	"Luis R. Rodriguez" <mcgrof@...e.com>,
	David Vrabel <david.vrabel@...rix.com>
CC:	Joerg Roedel <jroedel@...e.de>, kvm@...r.kernel.org,
	Davidlohr Bueso <dbueso@...e.de>,
	Peter Zijlstra <peterz@...radead.org>, x86@...nel.org,
	Oleg Nesterov <oleg@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Jan Beulich <JBeulich@...e.com>,
	xen-devel@...ts.xenproject.org, boris.ostrovsky@...cle.com,
	Borislav Petkov <bp@...e.de>, Olaf Hering <ohering@...e.de>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [Xen-devel] [PATCH] xen: privcmd: schedule() after private hypercall
 when non CONFIG_PREEMPT

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

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