On Tue, Mar 18, 2014 at 9:33 PM, Peter Zijlstra <pet...@infradead.org> 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. Thanks, Tao -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
