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
