On Mon, Feb 26, 2024 at 01:55:10PM -0800, Paul E. McKenney wrote: > On Mon, Feb 26, 2024 at 04:19:14PM -0500, Kent Overstreet wrote: > > +cc Paul > > > > On Mon, Feb 26, 2024 at 04:17:19PM -0500, Kent Overstreet wrote: > > > On Mon, Feb 26, 2024 at 09:07:51PM +0000, Matthew Wilcox wrote: > > > > On Mon, Feb 26, 2024 at 09:17:33AM -0800, Linus Torvalds wrote: > > > > > Willy - tangential side note: I looked closer at the issue that you > > > > > reported (indirectly) with the small reads during heavy write > > > > > activity. > > > > > > > > > > Our _reading_ side is very optimized and has none of the write-side > > > > > oddities that I can see, and we just have > > > > > > > > > > filemap_read -> > > > > > filemap_get_pages -> > > > > > filemap_get_read_batch -> > > > > > folio_try_get_rcu() > > > > > > > > > > and there is no page locking or other locking involved (assuming the > > > > > page is cached and marked uptodate etc, of course). > > > > > > > > > > So afaik, it really is just that *one* atomic access (and the matching > > > > > page ref decrement afterwards). > > > > > > > > Yep, that was what the customer reported on their ancient kernel, and > > > > we at least didn't make that worse ... > > > > > > > > > We could easily do all of this without getting any ref to the page at > > > > > all if we did the page cache release with RCU (and the user copy with > > > > > "copy_to_user_atomic()"). Honestly, anything else looks like a > > > > > complete disaster. For tiny reads, a temporary buffer sounds ok, but > > > > > really *only* for tiny reads where we could have that buffer on the > > > > > stack. > > > > > > > > > > Are tiny reads (handwaving: 100 bytes or less) really worth optimizing > > > > > for to that degree? > > > > > > > > > > In contrast, the RCU-delaying of the page cache might be a good idea > > > > > in general. We've had other situations where that would have been > > > > > nice. The main worry would be low-memory situations, I suspect. > > > > > > > > > > The "tiny read" optimization smells like a benchmark thing to me. Even > > > > > with the cacheline possibly bouncing, the system call overhead for > > > > > tiny reads (particularly with all the mitigations) should be orders of > > > > > magnitude higher than two atomic accesses. > > > > > > > > Ah, good point about the $%^&^*^ mitigations. This was pre mitigations. > > > > I suspect that this customer would simply disable them; afaik the machine > > > > is an appliance and one interacts with it purely by sending transactions > > > > to it (it's not even an SQL system, much less a "run arbitrary javascript" > > > > kind of system). But that makes it even more special case, inapplicable > > > > to the majority of workloads and closer to smelling like a benchmark. > > > > > > > > I've thought about and rejected RCU delaying of the page cache in the > > > > past. With the majority of memory in anon memory & file memory, it just > > > > feels too risky to have so much memory waiting to be reused. We could > > > > also improve gup-fast if we could rely on RCU freeing of anon memory. > > > > Not sure what workloads might benefit from that, though. > > > > > > RCU allocating and freeing of memory can already be fairly significant > > > depending on workload, and I'd expect that to grow - we really just need > > > a way for reclaim to kick RCU when needed (and probably add a percpu > > > counter for "amount of memory stranded until the next RCU grace > > > period"). > > There are some APIs for that, though the are sharp-edged and mainly > intended for rcutorture, and there are some hooks for a CI Kconfig > option called RCU_STRICT_GRACE_PERIOD that could be organized into > something useful. > > Of course, if there is a long-running RCU reader, there is nothing > RCU can do. By definition, it must wait on all pre-existing readers, > no exceptions. > > But my guess is that you instead are thinking of memory-exhaustion > emergencies where you would like RCU to burn more CPU than usual to > reduce grace-period latency, there are definitely things that can be done. > > I am sure that there are more questions that I should ask, but the one > that comes immediately to mind is "Is this API call an occasional thing, > or does RCU need to tolerate many CPUs hammering it frequently?" > Either answer is fine, I just need to know. ;-) Well, we won't want it getting hammered on continuously - we should be able to tune reclaim so that doesn't happen. I think getting numbers on the amount of memory stranded waiting for RCU is probably first order of business - minor tweak to kfree_rcu() et all for that; there's APIs they can query to maintain that counter. then, we can add a heuristic threshhold somewhere, something like if (rcu_stranded * multiplier > reclaimable_memory) kick_rcu()
