Also adding Corey. ;-) On Thu, Dec 20, 2018 at 08:03:13AM -0800, Paul E. McKenney wrote: > On Thu, Dec 20, 2018 at 07:42:17AM -0800, Tejun Heo wrote: > > Hello, Angel. > > > > (cc'ing Paul for SRCU) > > > > On Thu, Dec 20, 2018 at 09:55:10AM +0200, Angel Shtilianov wrote: > > > Hi everybody. > > > A couple of days I've decided to migrate several servers on > > > linux-4.19. What I've observed is that I have no /dev/ipmi. After > > > taking a look into the boot log I've found that ipmi modules are > > > complaining about percpu memory allocation failures: > > > https://pastebin.com/MCDssZzV > > ... > > > -#define PERCPU_DYNAMIC_RESERVE (28 << 10) > > > +#define PERCPU_DYNAMIC_RESERVE (28 << 11) > > > > So, you prolly just needed to bump this number. The reserved percpu > > area is used to accommodate static percpu variables used by modules. > > They are special because code generation assumes static symbols aren't > > too far from the program counter. The usual dynamic percpu area is > > way high up in vmalloc area, so if we put static percpu allocations > > there, they go out of range for module symbol relocations. > > > > The reserved area has some issues. > > > > 1. The area is not dynamically mapped, meaning that however much we > > reserve is hard allocated on boot for future module uses, so we > > don't can't increase it willy-nilly. > > > > 2. There is no mechanism to adjust the size dynamically. 28k is just > > a number I pulled out of my ass after looking at some common > > configs like a decade ago, so it being low now isn't too > > surprising. Provided that we can't make it run-time dynamic (and I > > can't think of a way to do that), the right thing to do would be > > sizing it during build with some buffer and allow it to be > > overridden boot time. This is definitely doable. > > > > BTW, ipmi's extra usage, 8k, is coming from the use of static SRCU. > > Paul, that's quite a bit of percpu memory to reserve statically. > > Would it be possible to make srcu_struct init dynamic so that it can > > use the normal percpu_alloc? That way, this problem can be completely > > side-stepped and it only occupies percpu memory which tends to be > > pretty expensive unless ipmi is actually initialized. > > Yes, it is possible. Just do something like this: > > struct srcu_struct my_srcu_struct; > > And before the first use of my_srcu_struct, do this: > > init_srcu_struct(&my_srcu_struct); > > This will result in alloc_percpu() being invoked to allocate the > needed per-CPU space. > > If my_srcu_struct is used in a module or some such, then to avoid memory > leaks, after the last use of my_srcu_struct, do this: > > cleanup_srcu_struct(&my_srcu_struct); > > There are several places in the kernel that take this approach. > > Thanx, Paul
