On 1/17/24 21:20, 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. Counters are one thing (afaik some batching happens on the memcg side via "stocks"), but another is associating the memcg with the allocated objects in slab pages, so kmem_cache_free() knows which counter to decrement. We'll have to see where the overhead is today. If there's overhead due to calls between mm/slub.c and mm/memcontrol.c we can now reduce that with SLAB gone. > 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. Roman and Shakeel are already Cc'd. Roman recently did https://lore.kernel.org/lkml/20231019225346.1822282-1-roman.gushchin@xxxxxxxxx/ which is mentioned in the cover letter and was merged in 6.7, but cover says it didn't help much, too bad. So is it still 33% or how much? > 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. > > Linus
