Re: [PATCH RFC] sched: introduce add_wait_queue_exclusive_head

From: Peng Tao
Date: Tue Mar 18 2014 - 09:51:38 EST

On Tue, Mar 18, 2014 at 9:33 PM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> On Tue, Mar 18, 2014 at 09:10:08PM +0800, Peng Tao wrote:
>> Normally wait_queue_t is a FIFO list for exclusive waiting tasks.
>> As a side effect, if there are many threads waiting on the same
>> condition (which is common for data servers like Lustre), all
>> threads will be waken up again and again, causing unnecessary cache
>> line polution. Instead of FIFO lists, we can use LIFO lists to always
>> wake up the most recent active threads.
>> Lustre implements this add_wait_queue_exclusive_head() privately but we
>> think it might be useful as a generic function. With it being moved to
>> generic layer, the rest of Lustre private wrappers for wait queue can be
>> all removed.
>> Of course there is an alternative approach to just open code it but we'd
>> like to ask first to see if there is objection to making it generic.
> OK, so I don't particularly mind LIFO, but there are a few problems with
> the patch.
> Firstly I think the _head postfix for LIFO is a bad name,
Do you have any preference on the name? add_wait_queue_exclusive_lifo()?

> and secondly,
> and most important, this breaks __wake_up_common().
> So waitqueue wakeups are specified as waking all !exclusive tasks and @n
> exclusive tasks. The way this works is that !exclusive tasks are added
> to the head (LIFO) and exclusive tasks are added to the tail
> (FIFO).
> We can then iterate the list until @n exclusive tasks have been
> observed.
> However if you start adding exclusive tasks to the head this all comes
> apart.
> If you don't mix exclusive and !exclusive tasks on the same waitqueue
> this isn't a problem, but I'm sure people will eventually do this and
> get a nasty surprise.
Yes, Lustre takes care not to mix exclusive and !exclusive tasks in this case.

> I'm not sure what the best way around this would be; but I can see two
> options:
> - add enough debugging bits to detect this fail case.
> - extend wait_queue_head_t to keep a pointer to the first !exclusive
> element and insert exclusive LIFO tasks there -- thereby keeping
> !exclusive tasks at the front.
Thank you for the suggestions. Personally I am in favor of the second
one but I'll wait others to comment first.

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