Messages in this thread | | | Date | Tue, 6 Nov 2012 09:22:16 +0800 | Subject | Re: [RFC PATCH] tasklet: ignore disabled tasklet in tasklet_action | From | Xiaotian Feng <> |
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On Tue, Nov 6, 2012 at 6:52 AM, Andrew Morton <akpm@linux-foundation.org> wrote: > 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)?
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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