On Thu, 4 Apr 2019, Vlastimil Babka wrote: > I looked a bit at SLUB debugging capabilities and first thing I noticed is > there's no static key guarding the runtime enablement as is common for similar > debugging functionalities, so here's a RFC to add it. Can be further improved > if there's interest. Well the runtime enablement is per slab cache and static keys are global. Adding static key adds code to the critical paths. Since the flags for a kmem_cache have to be inspected anyways there may not be that much of a benefit. > It's true that in the alloc fast path the debugging check overhead is AFAICS > amortized by the per-cpu cache, i.e. when the allocation is from there, no > debugging functionality is performed. IMHO that's kinda a weakness, especially > for SLAB_STORE_USER, so I might also look at doing something about it, and then > the static key might be more critical for overhead reduction. Moving debugging out of the per cpu fastpath allows that fastpath to be much simpler and faster. SLAB_STORE_USER is mostly used only for debugging in which case we are less concerned with performance. If you want to use SLAB_STORE_USER in the fastpath then we have to do some major redesign there. > In the freeing fast path I quickly checked the stats and it seems that in > do_slab_free(), the "if (likely(page == c->page))" is not as likely as it > declares, as in the majority of cases, freeing doesn't happen on the object > that belongs to the page currently cached. So the advantage of a static key in > slow path __slab_free() should be more useful immediately. Right. The freeing logic is actuall a weakness in terms of performance for SLUB due to the need to operate on a per page queue immediately.
