Messages in this thread |  | | Date | Thu, 4 Apr 2019 15:57:24 +0000 | From | Christopher Lameter <> | Subject | Re: [RFC 0/2] add static key for slub_debug |
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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.
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