On Wed, Jan 17, 2024 at 12:20:59PM -0800, Linus Torvalds wrote: > On Wed, 17 Jan 2024 at 11:39, Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote: > > > > That's a good point. If the microbenchmark isn't likely to be even > > remotely realistic, maybe we should just revert the revert until if/when > > somebody shows a real world impact. > > > > Linus, any objections to that? > > We use SLAB_ACCOUNT for much more common allocations like queued > signals, so I would tend to agree with Jeff that it's probably just > some not very interesting microbenchmark that shows any file locking > effects from SLAB_ALLOC, not any real use. > > That said, those benchmarks do matter. It's very easy to say "not > relevant in the big picture" and then the end result is that > everything is a bit of a pig. > > And the regression was absolutely *ENORMOUS*. We're not talking "a few > percent". We're talking a 33% regression that caused the revert: > > https://lore.kernel.org/lkml/20210907150757.GE17617@xsang-OptiPlex-9020/ > > I wish our SLAB_ACCOUNT wasn't such a pig. Rather than account every > single allocation, it would be much nicer to account at a bigger > granularity, possibly by having per-thread counters first before > falling back to the obj_cgroup_charge. Whatever. > > It's kind of stupid to have a benchmark that just allocates and > deallocates a file lock in quick succession spend lots of time > incrementing and decrementing cgroup charges for that repeated > alloc/free. > > However, that problem with SLAB_ACCOUNT is not the fault of file > locking, but more of a slab issue. > > End result: I think we should bring in Vlastimil and whoever else is > doing SLAB_ACCOUNT things, and have them look at that side. > > And then just enable SLAB_ACCOUNT for file locks. But very much look > at silly costs in SLAB_ACCOUNT first, at least for trivial > "alloc/free" patterns.. > > Vlastimil? Who would be the best person to look at that SLAB_ACCOUNT > thing? Probably me. I recently did some work on improving the kmem accounting performance, which is mentioned in this thread and shaves off about 30%: https://lore.kernel.org/lkml/20231019225346.1822282-1-roman.gushchin@xxxxxxxxx/ Overall the SLAB_ACCOUNT overhead looks big on micro-benchmarks simple because SLAB allocation path is really fast, so even touching a per-cpu variable adds a noticeable overhead. There is nothing particularly slow on the kmem allocation and release paths, but saving a memcg/objcg pointer, bumping the charge and stats adds up, even though we have batching in place. I believe the only real way to make it significantly faster is to cache pre-charged slab objects, but it adds to the complexity and increases the memory footprint. So far it was all about micro-benchmarks, I haven't seen any complaints on the performance of real workloads. Thanks!
