linux-kernel - RE: [PATCH 2/8] Staging: hv: vmbus: Invoke vmbus_on_msg

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Message-ID: <6E21E5352C11B742B20C142EB499E04808194C66@TK5EX14MBXC126.redmond.corp.microsoft.com>
Date:	Wed, 17 Aug 2011 18:43:24 +0000
From:	KY Srinivasan <kys@...rosoft.com>
To:	Sasha Levin <levinsasha928@...il.com>
CC:	"gregkh@...e.de" <gregkh@...e.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"devel@...uxdriverproject.org" <devel@...uxdriverproject.org>,
	"virtualization@...ts.osdl.org" <virtualization@...ts.osdl.org>,
	Haiyang Zhang <haiyangz@...rosoft.com>
Subject: RE: [PATCH 2/8] Staging: hv: vmbus: Invoke vmbus_on_msg_dpc()
 directly



> -----Original Message-----
> From: Sasha Levin [mailto:levinsasha928@...il.com]
> Sent: Wednesday, August 17, 2011 8:48 AM
> To: KY Srinivasan
> Cc: gregkh@...e.de; linux-kernel@...r.kernel.org;
> devel@...uxdriverproject.org; virtualization@...ts.osdl.org; Haiyang Zhang
> Subject: Re: [PATCH 2/8] Staging: hv: vmbus: Invoke vmbus_on_msg_dpc()
> directly
> 
> On Mon, 2011-08-15 at 15:12 -0700, K. Y. Srinivasan wrote:
> > The message processing function needs to execute on the same CPU where
> > the interrupt was taken. tasklets cannot gurantee this; so, invoke the
> > function directly.
> >
> > Signed-off-by: K. Y. Srinivasan <kys@...rosoft.com>
> > Signed-off-by: Haiyang Zhang <haiyangz@...rosoft.com>
> > ---
> 
> tasklets are guaranteed to run on the same CPU as the function that
> scheduled them.
> 
> Unless I'm missing something?

I too was under this impression until I stumbled upon this comment in
include/Linux/interrupt.h where I see that there is no guarantee that
tasklet would run on the same CPU that it was scheduled on
(look at the first listed property):

/* Tasklets --- multithreaded analogue of BHs.

   Main feature differing them of generic softirqs: tasklet
   is running only on one CPU simultaneously.

   Main feature differing them of BHs: different tasklets
   may be run simultaneously on different CPUs.

   Properties:
   * If tasklet_schedule() is called, then tasklet is guaranteed
     to be executed on some cpu at least once after this.
.
.
*/

Given this comment here, I felt that safest thing to do would be to just
not use the tasklet in this scenario.

Regards,

K. Y

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