On Wed, Nov 01 2017 at 12:23pm -0400, Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> wrote: > On Wed, Nov 01, 2017 at 11:48:44AM -0400, Mike Snitzer wrote: > > [cc'ing Paul, and LKML, to get his/others' take on SRCU cpu scaling] > > > > On Tue, Oct 31 2017 at 7:33pm -0400, > > Mikulas Patocka <mpatocka@xxxxxxxxxx> wrote: > > > > > The structure srcu_struct can be very big, its size is proportional to the > > > value CONFIG_NR_CPUS. The Fedora kernel has CONFIG_NR_CPUS 8192, the field > > > io_barrier in the struct mapped_device has 84kB in the debugging kernel > > > and 50kB in the non-debugging kernel. The large size may result in failure > > > of the function kzalloc_node. > > > > > > In order to avoid the allocation failure, we use the function > > > kvzalloc_node, this function falls back to vmalloc if a large contiguous > > > chunk of memory is not available. This patch also moves the field > > > io_barrier to the last position of struct mapped_device - the reason is > > > that on many processor architectures, short memory offsets result in > > > smaller code than long memory offsets - on x86-64 it reduces code size by > > > 320 bytes. > > > > > > Note to stable kernel maintainers - the kernels 4.11 and older don't have > > > the function kvzalloc_node, you can use the function vzalloc_node instead. > > > > > > Signed-off-by: Mikulas Patocka <mpatocka@xxxxxxxxxx> > > > Cc: stable@xxxxxxxxxxxxxxx > > > > This looks reasonable as a near-term workaround.. BUT: > > Paul has there been any discussion about how to make SRCU support > > dynamically scaling up to NR_CPUS maximum as 'nr_cpus' changes (rather > > than accounting for worst case of NR_CPUS up-front)? > > This is the first I have heard of this being a problem. > > For static instances of srcu_struct, life is hard. > > But it should not be all that difficult for SRCU to provide an allocator > for the dynamic cases, which given your kzalloc_node() above is the case > you are worried about, at least assuming that these allocations happen > after rcu_init() is invoked (which is pretty early). > > My approach would be to move the srcu_struct ->node[] array to its > own structure, with a pointer from srcu_struct, allowing short-sized > allocations to be used. (But I do need to check to make sure that there > are no gotchas, and with RCU there usually are a few.) Obviously some > -serious- testing would be required -- do you have a range of systems > to test on? If you'd like to give it a try I'd be happy to work on getting you test coverage. I do have access to a pretty wide range of systems. What type of testing would you like to see? (From where I sit as DM maintainer my testing would be DM-specific, just loading a DM device would make use of the SRCU code in question, so please let me know if there is anything more general you'd like done) > However, you would still have your potential failure case for systems > that really did have large numbers of CPUs, some of which really do > exist in the wild. > > > (But I had a quick look at scrutree.h and I'm not seeing explicit use of > > NR_CPUS, so it is likely occuring via implicit percpu through some > > member of 'struct srcu_struct', e.g. 'sda'?) > > The srcu_struct structure sees NR_CPUS via include/linux/rcu_node_tree.h, > which sizes the srcu_node array at build time. > > The sda pointer references a per-CPU allocation, which I believe already > is sized to the actual system rather than to NR_CPUS. OK, thanks for clarifying. Mike -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
