On Fri, Jan 10, 2025 at 06:01:05PM +0100, Peter Zijlstra wrote: > On Thu, Jan 09, 2025 at 07:48:32AM -0800, Suren Baghdasaryan wrote: > > On Thu, Jan 9, 2025 at 3:51 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote: > > > > > > On Wed, Jan 08, 2025 at 06:30:09PM -0800, Suren Baghdasaryan wrote: > > > > Back when per-vma locks were introduces, vm_lock was moved out of > > > > vm_area_struct in [1] because of the performance regression caused by > > > > false cacheline sharing. Recent investigation [2] revealed that the > > > > regressions is limited to a rather old Broadwell microarchitecture and > > > > even there it can be mitigated by disabling adjacent cacheline > > > > prefetching, see [3]. > > > > Splitting single logical structure into multiple ones leads to more > > > > complicated management, extra pointer dereferences and overall less > > > > maintainable code. When that split-away part is a lock, it complicates > > > > things even further. With no performance benefits, there are no reasons > > > > for this split. Merging the vm_lock back into vm_area_struct also allows > > > > vm_area_struct to use SLAB_TYPESAFE_BY_RCU later in this patchset. > > > > This patchset: > > > > 1. moves vm_lock back into vm_area_struct, aligning it at the cacheline > > > > boundary and changing the cache to be cacheline-aligned to minimize > > > > cacheline sharing; > > > > 2. changes vm_area_struct initialization to mark new vma as detached until > > > > it is inserted into vma tree; > > > > 3. replaces vm_lock and vma->detached flag with a reference counter; > > > > 4. changes vm_area_struct cache to SLAB_TYPESAFE_BY_RCU to allow for their > > > > reuse and to minimize call_rcu() calls. > > > > > > Does not clean up that reattach nonsense :-( > > > > Oh, no. I think it does. That's why in [1] I introduce > > vma_iter_store_attached() to be used on already attached vmas and to > > avoid marking them attached again. Also I added assertions in > > vma_mark_attached()/vma_mark_detached() to avoid re-attaching or > > re-detaching. Unless I misunderstood your comment? > > Hmm, I'll go read the thing again, maybe I missed it. You're right. I was looking for the approach that changed the need to reattach, by moving the point of no return. This should do for now. Let me see if I can find time today to finally do a proper reading. Thanks!
