On Wed, Nov 20, 2024 at 3:33 PM Shakeel Butt <shakeel.butt@xxxxxxxxx> wrote: > > On Tue, Nov 19, 2024 at 04:08:23PM -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. > > Move vm_lock back into vm_area_struct, aligning it at the cacheline > > boundary and changing the cache to be cacheline-aligned as well. > > With kernel compiled using defconfig, this causes VMA memory consumption > > to grow from 160 (vm_area_struct) + 40 (vm_lock) bytes to 256 bytes: > > > > slabinfo before: > > <name> ... <objsize> <objperslab> <pagesperslab> : ... > > vma_lock ... 40 102 1 : ... > > vm_area_struct ... 160 51 2 : ... > > > > slabinfo after moving vm_lock: > > <name> ... <objsize> <objperslab> <pagesperslab> : ... > > vm_area_struct ... 256 32 2 : ... > > > > Aggregate VMA memory consumption per 1000 VMAs grows from 50 to 64 pages, > > which is 5.5MB per 100000 VMAs. Note that the size of this structure is > > dependent on the kernel configuration and typically the original size is > > higher than 160 bytes. Therefore these calculations are close to the > > worst case scenario. A more realistic vm_area_struct usage before this > > change is: > > > > <name> ... <objsize> <objperslab> <pagesperslab> : ... > > vma_lock ... 40 102 1 : ... > > vm_area_struct ... 176 46 2 : ... > > > > Aggregate VMA memory consumption per 1000 VMAs grows from 54 to 64 pages, > > which is 3.9MB per 100000 VMAs. > > This memory consumption growth can be addressed later by optimizing the > > vm_lock. > > > > [1] https://lore.kernel.org/all/20230227173632.3292573-34-surenb@xxxxxxxxxx/ > > [2] https://lore.kernel.org/all/ZsQyI%2F087V34JoIt@xsang-OptiPlex-9020/ > > [3] https://lore.kernel.org/all/CAJuCfpEisU8Lfe96AYJDZ+OM4NoPmnw9bP53cT_kbfP_pR+-2g@xxxxxxxxxxxxxx/ > > > > Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx> > > Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx> > > Reviewed-by: Shakeel Butt <shakeel.butt@xxxxxxxxx> Thanks! > > > One question below. > > > --- a/include/linux/mm_types.h > > +++ b/include/linux/mm_types.h > > @@ -716,8 +716,6 @@ struct vm_area_struct { > > * slowpath. > > */ > > unsigned int vm_lock_seq; > > - /* Unstable RCU readers are allowed to read this. */ > > - struct vma_lock *vm_lock; > > #endif > > > > /* > > @@ -770,6 +768,10 @@ struct vm_area_struct { > > struct vma_numab_state *numab_state; /* NUMA Balancing state */ > > #endif > > struct vm_userfaultfd_ctx vm_userfaultfd_ctx; > > +#ifdef CONFIG_PER_VMA_LOCK > > + /* Unstable RCU readers are allowed to read this. */ > > + struct vma_lock vm_lock ____cacheline_aligned_in_smp; > > +#endif > > } __randomize_layout; > > Do we just want 'struct vm_area_struct' to be cacheline aligned or do we > want 'struct vma_lock vm_lock' to be on a separate cacheline as well? We want both to minimize cacheline sharing. >
