linux-kernel - Re: [RFC PATCH] tasklet: ignore disabled tasklet in tasklet

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Message-ID: <CAJn8CcF=2=XBV0T661LnAxHi8+fT8KAVapCFaiDZgS6TyieNOA@mail.gmail.com>
Date:	Tue, 6 Nov 2012 09:22:16 +0800
From:	Xiaotian Feng <xtfeng@...il.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	linux-kernel@...r.kernel.org,
	Xiaotian Feng <dannyfeng@...cent.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [RFC PATCH] tasklet: ignore disabled tasklet in tasklet_action

On Tue, Nov 6, 2012 at 6:52 AM, Andrew Morton <akpm@...ux-foundation.org> wrote:
> On Fri,  2 Nov 2012 10:48:54 +0800
> Xiaotian Feng <xtfeng@...il.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)?

hmm, that will be a nightmare...
tasklet_schedule(t)/tasklet_hi_schedule(t) doesn't use list_head, they
simply
make t->next = NULL, then put t on the tail of
tasklet_vec/tasklet_hi_vec. If the code does a tasklet_hi_schedule()
and then a tasklet_schedule, the tasklet will stay on tasklet_vec and
tasklet_hi_vec .... tasklet_hi_action will handle it first and clear
the TASKLET_SCHED_SCHED bit, later, in tasklet_action, it will be
handled again and hit a BUG_ON ...

But if code does a tasklet_hi_schedule(), then tasklet_kil and later
does a tasklet_schedule(), we do need clear the TASKLET_STATE_HI. Also
we need to remove the tasklet_hi_enable() as it is the same as
tasklet_enable() and there's
only one user..

I'll send you V2 patch soon, thanks.

>
>
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