| Message ID | 20241120000826.335387-3-surenb@google.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | move per-vma lock into vm_area_struct | expand |
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@google.com/ > [2] https://lore.kernel.org/all/ZsQyI%2F087V34JoIt@xsang-OptiPlex-9020/ > [3] https://lore.kernel.org/all/CAJuCfpEisU8Lfe96AYJDZ+OM4NoPmnw9bP53cT_kbfP_pR+-2g@mail.gmail.com/ > > Signed-off-by: Suren Baghdasaryan <surenb@google.com> > Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com> Reviewed-by: Shakeel Butt <shakeel.butt@linux.dev> 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?
On Wed, Nov 20, 2024 at 3:33 PM Shakeel Butt <shakeel.butt@linux.dev> 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@google.com/ > > [2] https://lore.kernel.org/all/ZsQyI%2F087V34JoIt@xsang-OptiPlex-9020/ > > [3] https://lore.kernel.org/all/CAJuCfpEisU8Lfe96AYJDZ+OM4NoPmnw9bP53cT_kbfP_pR+-2g@mail.gmail.com/ > > > > Signed-off-by: Suren Baghdasaryan <surenb@google.com> > > Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com> > > Reviewed-by: Shakeel Butt <shakeel.butt@linux.dev> 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. >
On Wed, Nov 20, 2024 at 03:44:29PM -0800, Suren Baghdasaryan wrote: > On Wed, Nov 20, 2024 at 3:33 PM Shakeel Butt <shakeel.butt@linux.dev> 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@google.com/ > > > [2] https://lore.kernel.org/all/ZsQyI%2F087V34JoIt@xsang-OptiPlex-9020/ > > > [3] https://lore.kernel.org/all/CAJuCfpEisU8Lfe96AYJDZ+OM4NoPmnw9bP53cT_kbfP_pR+-2g@mail.gmail.com/ > > > > > > Signed-off-by: Suren Baghdasaryan <surenb@google.com> > > > Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com> > > > > Reviewed-by: Shakeel Butt <shakeel.butt@linux.dev> > > 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. > For later, you will need to add a pad after vm_lock as well, so any future addition will not share the cacheline with vm_lock. I would do something like below. This is a nit and can be done later. diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h index 7654c766cbe2..5cc4fff163a0 100644 --- a/include/linux/mm_types.h +++ b/include/linux/mm_types.h @@ -751,10 +751,12 @@ struct vm_area_struct { #endif struct vm_userfaultfd_ctx vm_userfaultfd_ctx; #ifdef CONFIG_PER_VMA_LOCK + CACHELINE_PADDING(__pad1__); /* Unstable RCU readers are allowed to read this. */ - struct vma_lock vm_lock ____cacheline_aligned_in_smp; + struct vma_lock vm_lock; + CACHELINE_PADDING(__pad2__); #endif -} __randomize_layout; +} __randomize_layout ____cacheline_aligned_in_smp; #ifdef CONFIG_NUMA #define vma_policy(vma) ((vma)->vm_policy)
On Wed, Nov 20, 2024 at 4:05 PM Shakeel Butt <shakeel.butt@linux.dev> wrote: > > On Wed, Nov 20, 2024 at 03:44:29PM -0800, Suren Baghdasaryan wrote: > > On Wed, Nov 20, 2024 at 3:33 PM Shakeel Butt <shakeel.butt@linux.dev> 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@google.com/ > > > > [2] https://lore.kernel.org/all/ZsQyI%2F087V34JoIt@xsang-OptiPlex-9020/ > > > > [3] https://lore.kernel.org/all/CAJuCfpEisU8Lfe96AYJDZ+OM4NoPmnw9bP53cT_kbfP_pR+-2g@mail.gmail.com/ > > > > > > > > Signed-off-by: Suren Baghdasaryan <surenb@google.com> > > > > Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com> > > > > > > Reviewed-by: Shakeel Butt <shakeel.butt@linux.dev> > > > > 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. > > > > For later, you will need to add a pad after vm_lock as well, so any > future addition will not share the cacheline with vm_lock. I would do > something like below. This is a nit and can be done later. > > diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h > index 7654c766cbe2..5cc4fff163a0 100644 > --- a/include/linux/mm_types.h > +++ b/include/linux/mm_types.h > @@ -751,10 +751,12 @@ struct vm_area_struct { > #endif > struct vm_userfaultfd_ctx vm_userfaultfd_ctx; > #ifdef CONFIG_PER_VMA_LOCK > + CACHELINE_PADDING(__pad1__); > /* Unstable RCU readers are allowed to read this. */ > - struct vma_lock vm_lock ____cacheline_aligned_in_smp; > + struct vma_lock vm_lock; > + CACHELINE_PADDING(__pad2__); > #endif > -} __randomize_layout; > +} __randomize_layout ____cacheline_aligned_in_smp; I thought SLAB_HWCACHE_ALIGN for vm_area_cachep added in this patch would have the same result, no? > > #ifdef CONFIG_NUMA > #define vma_policy(vma) ((vma)->vm_policy)
On Wed, Nov 20, 2024 at 04:33:37PM -0800, Suren Baghdasaryan wrote: [...] > > > > > > > > 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. > > > > > > > For later, you will need to add a pad after vm_lock as well, so any > > future addition will not share the cacheline with vm_lock. I would do > > something like below. This is a nit and can be done later. > > > > diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h > > index 7654c766cbe2..5cc4fff163a0 100644 > > --- a/include/linux/mm_types.h > > +++ b/include/linux/mm_types.h > > @@ -751,10 +751,12 @@ struct vm_area_struct { > > #endif > > struct vm_userfaultfd_ctx vm_userfaultfd_ctx; > > #ifdef CONFIG_PER_VMA_LOCK > > + CACHELINE_PADDING(__pad1__); > > /* Unstable RCU readers are allowed to read this. */ > > - struct vma_lock vm_lock ____cacheline_aligned_in_smp; > > + struct vma_lock vm_lock; > > + CACHELINE_PADDING(__pad2__); > > #endif > > -} __randomize_layout; > > +} __randomize_layout ____cacheline_aligned_in_smp; > > I thought SLAB_HWCACHE_ALIGN for vm_area_cachep added in this patch > would have the same result, no? > SLAB_HWCACHE_ALIGN is more about the slub allocator allocating cache aligned memory. It does not say anything about the internals of the struct for which the kmem_cache is being created. The ____cacheline_aligned_in_smp tag in your patch made sure that the field vm_lock will be put in a new cacheline and there can be a hole between vm_lock and the previous field if the previous field is not ending at the cacheline boundary. Please note that if you add a new field after vm_lock (without cacheline alignment tag), it will be on the same cacheline as vm_lock. So, your code is achieving the vm_lock on its own cacheline goal but vm_lock being the only field on that cacheline is not being achieved.
diff --git a/include/linux/mm.h b/include/linux/mm.h index 1ba2e480ae63..737c003b0a1e 100644 --- a/include/linux/mm.h +++ b/include/linux/mm.h @@ -684,6 +684,12 @@ static inline void vma_numab_state_free(struct vm_area_struct *vma) {} #endif /* CONFIG_NUMA_BALANCING */ #ifdef CONFIG_PER_VMA_LOCK +static inline void vma_lock_init(struct vm_area_struct *vma) +{ + init_rwsem(&vma->vm_lock.lock); + vma->vm_lock_seq = UINT_MAX; +} + /* * Try to read-lock a vma. The function is allowed to occasionally yield false * locked result to avoid performance overhead, in which case we fall back to @@ -701,7 +707,7 @@ static inline bool vma_start_read(struct vm_area_struct *vma) if (READ_ONCE(vma->vm_lock_seq) == READ_ONCE(vma->vm_mm->mm_lock_seq.sequence)) return false; - if (unlikely(down_read_trylock(&vma->vm_lock->lock) == 0)) + if (unlikely(down_read_trylock(&vma->vm_lock.lock) == 0)) return false; /* @@ -716,7 +722,7 @@ static inline bool vma_start_read(struct vm_area_struct *vma) * This pairs with RELEASE semantics in vma_end_write_all(). */ if (unlikely(vma->vm_lock_seq == raw_read_seqcount(&vma->vm_mm->mm_lock_seq))) { - up_read(&vma->vm_lock->lock); + up_read(&vma->vm_lock.lock); return false; } return true; @@ -731,7 +737,7 @@ static inline bool vma_start_read(struct vm_area_struct *vma) static inline void vma_start_read_locked_nested(struct vm_area_struct *vma, int subclass) { mmap_assert_locked(vma->vm_mm); - down_read_nested(&vma->vm_lock->lock, subclass); + down_read_nested(&vma->vm_lock.lock, subclass); } /* @@ -743,13 +749,13 @@ static inline void vma_start_read_locked_nested(struct vm_area_struct *vma, int static inline void vma_start_read_locked(struct vm_area_struct *vma) { mmap_assert_locked(vma->vm_mm); - down_read(&vma->vm_lock->lock); + down_read(&vma->vm_lock.lock); } static inline void vma_end_read(struct vm_area_struct *vma) { rcu_read_lock(); /* keeps vma alive till the end of up_read */ - up_read(&vma->vm_lock->lock); + up_read(&vma->vm_lock.lock); rcu_read_unlock(); } @@ -778,7 +784,7 @@ static inline void vma_start_write(struct vm_area_struct *vma) if (__is_vma_write_locked(vma, &mm_lock_seq)) return; - down_write(&vma->vm_lock->lock); + down_write(&vma->vm_lock.lock); /* * We should use WRITE_ONCE() here because we can have concurrent reads * from the early lockless pessimistic check in vma_start_read(). @@ -786,7 +792,7 @@ static inline void vma_start_write(struct vm_area_struct *vma) * we should use WRITE_ONCE() for cleanliness and to keep KCSAN happy. */ WRITE_ONCE(vma->vm_lock_seq, mm_lock_seq); - up_write(&vma->vm_lock->lock); + up_write(&vma->vm_lock.lock); } static inline void vma_assert_write_locked(struct vm_area_struct *vma) @@ -798,7 +804,7 @@ static inline void vma_assert_write_locked(struct vm_area_struct *vma) static inline void vma_assert_locked(struct vm_area_struct *vma) { - if (!rwsem_is_locked(&vma->vm_lock->lock)) + if (!rwsem_is_locked(&vma->vm_lock.lock)) vma_assert_write_locked(vma); } @@ -831,6 +837,7 @@ struct vm_area_struct *lock_vma_under_rcu(struct mm_struct *mm, #else /* CONFIG_PER_VMA_LOCK */ +static inline void vma_lock_init(struct vm_area_struct *vma) {} static inline bool vma_start_read(struct vm_area_struct *vma) { return false; } static inline void vma_end_read(struct vm_area_struct *vma) {} @@ -865,10 +872,6 @@ static inline void assert_fault_locked(struct vm_fault *vmf) extern const struct vm_operations_struct vma_dummy_vm_ops; -/* - * WARNING: vma_init does not initialize vma->vm_lock. - * Use vm_area_alloc()/vm_area_free() if vma needs locking. - */ static inline void vma_init(struct vm_area_struct *vma, struct mm_struct *mm) { memset(vma, 0, sizeof(*vma)); @@ -877,6 +880,7 @@ static inline void vma_init(struct vm_area_struct *vma, struct mm_struct *mm) INIT_LIST_HEAD(&vma->anon_vma_chain); vma_mark_detached(vma, false); vma_numab_state_init(vma); + vma_lock_init(vma); } /* Use when VMA is not part of the VMA tree and needs no locking */ diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h index 80fef38d9d64..5c4bfdcfac72 100644 --- 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; #ifdef CONFIG_NUMA diff --git a/kernel/fork.c b/kernel/fork.c index 0061cf2450ef..7823797e31d2 100644 --- a/kernel/fork.c +++ b/kernel/fork.c @@ -436,35 +436,6 @@ static struct kmem_cache *vm_area_cachep; /* SLAB cache for mm_struct structures (tsk->mm) */ static struct kmem_cache *mm_cachep; -#ifdef CONFIG_PER_VMA_LOCK - -/* SLAB cache for vm_area_struct.lock */ -static struct kmem_cache *vma_lock_cachep; - -static bool vma_lock_alloc(struct vm_area_struct *vma) -{ - vma->vm_lock = kmem_cache_alloc(vma_lock_cachep, GFP_KERNEL); - if (!vma->vm_lock) - return false; - - init_rwsem(&vma->vm_lock->lock); - vma->vm_lock_seq = UINT_MAX; - - return true; -} - -static inline void vma_lock_free(struct vm_area_struct *vma) -{ - kmem_cache_free(vma_lock_cachep, vma->vm_lock); -} - -#else /* CONFIG_PER_VMA_LOCK */ - -static inline bool vma_lock_alloc(struct vm_area_struct *vma) { return true; } -static inline void vma_lock_free(struct vm_area_struct *vma) {} - -#endif /* CONFIG_PER_VMA_LOCK */ - struct vm_area_struct *vm_area_alloc(struct mm_struct *mm) { struct vm_area_struct *vma; @@ -474,10 +445,6 @@ struct vm_area_struct *vm_area_alloc(struct mm_struct *mm) return NULL; vma_init(vma, mm); - if (!vma_lock_alloc(vma)) { - kmem_cache_free(vm_area_cachep, vma); - return NULL; - } return vma; } @@ -496,10 +463,7 @@ struct vm_area_struct *vm_area_dup(struct vm_area_struct *orig) * will be reinitialized. */ data_race(memcpy(new, orig, sizeof(*new))); - if (!vma_lock_alloc(new)) { - kmem_cache_free(vm_area_cachep, new); - return NULL; - } + vma_lock_init(new); INIT_LIST_HEAD(&new->anon_vma_chain); vma_numab_state_init(new); dup_anon_vma_name(orig, new); @@ -511,7 +475,6 @@ void __vm_area_free(struct vm_area_struct *vma) { vma_numab_state_free(vma); free_anon_vma_name(vma); - vma_lock_free(vma); kmem_cache_free(vm_area_cachep, vma); } @@ -522,7 +485,7 @@ static void vm_area_free_rcu_cb(struct rcu_head *head) vm_rcu); /* The vma should not be locked while being destroyed. */ - VM_BUG_ON_VMA(rwsem_is_locked(&vma->vm_lock->lock), vma); + VM_BUG_ON_VMA(rwsem_is_locked(&vma->vm_lock.lock), vma); __vm_area_free(vma); } #endif @@ -3168,11 +3131,9 @@ void __init proc_caches_init(void) sizeof(struct fs_struct), 0, SLAB_HWCACHE_ALIGN|SLAB_PANIC|SLAB_ACCOUNT, NULL); - - vm_area_cachep = KMEM_CACHE(vm_area_struct, SLAB_PANIC|SLAB_ACCOUNT); -#ifdef CONFIG_PER_VMA_LOCK - vma_lock_cachep = KMEM_CACHE(vma_lock, SLAB_PANIC|SLAB_ACCOUNT); -#endif + vm_area_cachep = KMEM_CACHE(vm_area_struct, + SLAB_HWCACHE_ALIGN|SLAB_NO_MERGE|SLAB_PANIC| + SLAB_ACCOUNT); mmap_init(); nsproxy_cache_init(); } diff --git a/tools/testing/vma/vma_internal.h b/tools/testing/vma/vma_internal.h index 1d9fc97b8e80..11c2c38ca4e8 100644 --- a/tools/testing/vma/vma_internal.h +++ b/tools/testing/vma/vma_internal.h @@ -230,10 +230,10 @@ struct vm_area_struct { /* * Can only be written (using WRITE_ONCE()) while holding both: * - mmap_lock (in write mode) - * - vm_lock->lock (in write mode) + * - vm_lock.lock (in write mode) * Can be read reliably while holding one of: * - mmap_lock (in read or write mode) - * - vm_lock->lock (in read or write mode) + * - vm_lock.lock (in read or write mode) * Can be read unreliably (using READ_ONCE()) for pessimistic bailout * while holding nothing (except RCU to keep the VMA struct allocated). * @@ -242,7 +242,7 @@ struct vm_area_struct { * slowpath. */ unsigned int vm_lock_seq; - struct vma_lock *vm_lock; + struct vma_lock vm_lock; #endif /* @@ -408,17 +408,10 @@ static inline struct vm_area_struct *vma_next(struct vma_iterator *vmi) return mas_find(&vmi->mas, ULONG_MAX); } -static inline bool vma_lock_alloc(struct vm_area_struct *vma) +static inline void vma_lock_init(struct vm_area_struct *vma) { - vma->vm_lock = calloc(1, sizeof(struct vma_lock)); - - if (!vma->vm_lock) - return false; - - init_rwsem(&vma->vm_lock->lock); + init_rwsem(&vma->vm_lock.lock); vma->vm_lock_seq = UINT_MAX; - - return true; } static inline void vma_assert_write_locked(struct vm_area_struct *); @@ -439,6 +432,7 @@ static inline void vma_init(struct vm_area_struct *vma, struct mm_struct *mm) vma->vm_ops = &vma_dummy_vm_ops; INIT_LIST_HEAD(&vma->anon_vma_chain); vma_mark_detached(vma, false); + vma_lock_init(vma); } static inline struct vm_area_struct *vm_area_alloc(struct mm_struct *mm) @@ -449,10 +443,6 @@ static inline struct vm_area_struct *vm_area_alloc(struct mm_struct *mm) return NULL; vma_init(vma, mm); - if (!vma_lock_alloc(vma)) { - free(vma); - return NULL; - } return vma; } @@ -465,10 +455,7 @@ static inline struct vm_area_struct *vm_area_dup(struct vm_area_struct *orig) return NULL; memcpy(new, orig, sizeof(*new)); - if (!vma_lock_alloc(new)) { - free(new); - return NULL; - } + vma_lock_init(new); INIT_LIST_HEAD(&new->anon_vma_chain); return new; @@ -638,14 +625,8 @@ static inline void mpol_put(struct mempolicy *) { } -static inline void vma_lock_free(struct vm_area_struct *vma) -{ - free(vma->vm_lock); -} - static inline void __vm_area_free(struct vm_area_struct *vma) { - vma_lock_free(vma); free(vma); }
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@google.com/ [2] https://lore.kernel.org/all/ZsQyI%2F087V34JoIt@xsang-OptiPlex-9020/ [3] https://lore.kernel.org/all/CAJuCfpEisU8Lfe96AYJDZ+OM4NoPmnw9bP53cT_kbfP_pR+-2g@mail.gmail.com/ Signed-off-by: Suren Baghdasaryan <surenb@google.com> Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com> --- include/linux/mm.h | 28 ++++++++++-------- include/linux/mm_types.h | 6 ++-- kernel/fork.c | 49 ++++---------------------------- tools/testing/vma/vma_internal.h | 33 +++++---------------- 4 files changed, 32 insertions(+), 84 deletions(-)