RE: [PATCH 2/8] Staging: hv: vmbus: Invoke vmbus_on_msg_dpc() directly

KY Srinivasan
2011-08-17T18:43:26Z

> -----Original Message-----
> From: Sasha Levin [mailto:levinsasha928@gmail.com]
> Sent: Wednesday, August 17, 2011 8:48 AM
> To: KY Srinivasan
> Cc: gregkh@suse.de; linux-kernel@vger.kernel.org;
> devel@linuxdriverproject.org; virtualization@lists.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@microsoft.com>
> > Signed-off-by: Haiyang Zhang <haiyangz@microsoft.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