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

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Message-ID: <20141203022833.GS25677@wotan.suse.de>
Date:	Wed, 3 Dec 2014 03:28:33 +0100
From:	"Luis R. Rodriguez" <mcgrof@...e.com>
To:	David Vrabel <david.vrabel@...rix.com>
Cc:	Juergen Gross <jgross@...e.com>, 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 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?

  Luis
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