On Wed, Jan 17, 2024 at 01:02:19PM -0800, Shakeel Butt wrote: > On Wed, Jan 17, 2024 at 12:21 PM Linus Torvalds > <torvalds@xxxxxxxxxxxxxxxxxxxx> 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? See commit 3754707bcc3e (Revert "memcg: enable accounting for > > file lock caches") for the history here. > > > > Roman last looked into optimizing this code path. I suspect > mod_objcg_state() to be more costly than obj_cgroup_charge(). I will > try to measure this path and see if I can improve it. It's roughly an equal split between mod_objcg_state() and obj_cgroup_charge(). And each is comparable (by order of magnitude) to the slab allocation cost itself. On the free() path a significant cost comes simple from reading the objcg pointer (it's usually a cache miss). So I don't see how we can make it really cheap (say, less than 5% overhead) without caching pre-accounted objects. I thought about merging of charge and stats handling paths, which _maybe_ can shave off another 20-30%, but there still will be a double-digit% accounting overhead. I'm curious to hear other ideas and suggestions. Thanks!
