On Mon, 2013-01-07 at 15:28 +0530, Viresh Kumar wrote: > Hi Tejun, > > On 4 January 2013 20:39, Tejun Heo <tj@xxxxxxxxxx> wrote: > > I don't know either. Changing behavior subtly like this is hard. I > > usually try to spot some problem cases and try to identify patterns > > there. Once you identify a few of them, understanding and detecting > > other problem cases get a lot easier. In this case, maybe there are > > too many places to audit and the problems are too subtle, and, if we > > *have* to do it, the only thing we can do is implementing a debug > > option to make such problems more visible - say, always schedule to a > > different cpu on queue_work(). > > > > That said, at this point, the patchset doesn't seem all that > > convincing to me and if I'm comprehending responses from others > > correctly that seems to be the consensus. It might be a better > > approach to identify the specific offending workqueue users and make > > them handle the situation more intelligently than trying to impose the > > behavior on all workqueue users. At any rate, we need way more data > > showing this actually helps and if so why. > > I understand your concerns and believe me, even i feel the same :) > I had another idea, that i wanted to share. > > Firstly the root cause of this patchset. > > Myself and some others in Linaro are working on ARM future cores: > big.LITTLE systems. > Here we have few very powerful, high power consuming cores (big, > currently A15's) and > few very efficient ones (LITTLE, currently A7's). > > The ultimate goal is to save as much power as possible without compromising > much with performance. For, that we wanted most of the stuff to run on LITTLE > cores and some performance-demanding stuff on big Cores. There are > multiple things > going around in this direction. Now, we thought A15's or big cores > shouldn't be used > for running small tasks like timers/workqueues and hence this patch is > an attempt > towards reaching that goal. > > Over that we can do some load balancing of works within multiple alive > cpus, so that > it can get done quickly. Also, we shouldn't start using an idle cpu > just for processing > work :) > > I have another idea that we can try: > > queue_work_on_any_cpu(). I think this is a good idea. > > With this we would not break any existing code and can try to migrate > old users to > this new infrastructure (atleast the ones which are rearming works from their > work_handlers). What do you say? > > To take care of the cache locality issue, we can pass an argument to > this routine, > that can provide > - the mask of cpus to schedule this work on > OR > - Sched Level (SD_LEVEL) of cpus to run it. I wouldn't give a mask. If one is needed, we could have a queue_work_on_mask_cpus(), or something. I think the "any" in the name should be good enough to let developers know that this will not be on the CPU that is called. By default, I would suggest for cache locality, that we try to keep it on the same CPU. But if there's a better CPU to run on, it runs there. Also, we could still add a debug option that makes it always run on other CPUs to slap developers that don't read. -- Steve > > Waiting for your view on it :) > > -- > viresh -- To unsubscribe from this list: send the line "unsubscribe linux-rt-users" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
