On 12/16/19 2:08 PM, Mike Kravetz wrote: > On 12/16/19 8:17 AM, Davidlohr Bueso wrote: >> On Mon, 16 Dec 2019, Michal Hocko wrote: >>> I am afraid that work_struct is too large to be stuffed into the struct >>> page array (because of the lockdep part). >> Yeah, this needs to be done without touching struct page. >> >> Which is why I had done the stack allocated way in this patch, but we >> cannot wait for it to complete in irq, so that's out the window. Andi >> had suggested percpu allocated work items, but having played with the >> idea over the weekend, I don't see how we can prevent another page being >> freed on the same cpu before previous work on the same cpu is complete >> (cpu0 wants to free pageA, schedules the work, in the mean time cpu0 >> wants to free pageB and workerfn for pageA still hasn't been called). >> >>> I think that it would be just safer to make hugetlb_lock irq safe. Are >>> there any other locks that would require the same? >> It would be simpler. Any performance issues that arise would probably >> be only seen in microbenchmarks, assuming we want to have full irq safety. >> If we don't need to worry about hardirq, then even better. >> >> The subpool lock would also need to be irq safe. > I do think we need to worry about hardirq. There are no restruictions that > put_page can not be called from hardirq context. > > I am concerned about the latency of making hugetlb_lock (and potentially > subpool lock) hardirq safe. When these locks were introduced (before my > time) the concept of making them irq safe was not considered. Recently, > I learned that the hugetlb_lock is held for a linear scan of ALL hugetlb > pages during a cgroup reparentling operation. That is just too long. > > If there is no viable work queue solution, then I think we would like to > restructure the hugetlb locking before a change to just make hugetlb_lock > irq safe. The idea would be to split the scope of what is done under > hugetlb_lock. Most of it would never be executed in irq context. Then > have a small/limited set of functionality that really needs to be irq > safe protected by an irq safe lock. > Please take a look at my recently posted patch to see if that is an acceptable workqueue based solution. Thanks, Longman
