Re: [RFC PATCH] tasklet: ignore disabled tasklet in tasklet_action

[Andrew Morton <akpm@linux-foundation.org>]

On Fri,  2 Nov 2012 10:48:54 +0800
Xiaotian Feng <xtfeng@gmail.com> wrote:

> We met a ksoftirqd 100% issue, the perf top shows kernel is busy
> with tasklet_action(), but no actual action is shown. From dumped
> kernel, there's only one disabled tasklet on the tasklet_vec.
> 
> tasklet_action might be handled after tasklet is disabled, this will
> make disabled tasklet stayed on tasklet_vec. tasklet_action will not
> handle disabled tasklet, but place it on the tail of tasklet_vec,
> still raise softirq for this tasklet. Things will become worse if
> device driver uses tasklet_disable on its device remove/close code.
> The disabled tasklet will stay on the vec, frequently __raise_softirq_off()
> and make ksoftirqd wakeup even if no tasklets need to be handled.
> 
> This patch introduced a new TASKLET_STATE_HI bit to indicate HI_SOFTIRQ,
> in tasklet_action(), simply ignore the disabled tasklet and don't raise
> the softirq nr. In my previous patch, I remove tasklet_hi_enable() since
> it is the same as tasklet_enable(). So only tasklet_enable() needs to be
> modified, if tasklet state is changed from disable to enable, use
> __tasklet_schedule() to put it on the right vec.

gee, I haven't looked at the tasklet code in 100 years.  I think I'll
send this in Thomas's direction ;)

The race description seems real and the patch looks sane to me.  Are
you sure we can get away with never clearing TASKLET_STATE_HI?  For
example, what would happen if code does a tasklet_hi_schedule(t) and
later does a tasklet_schedule(t)?