On Wed, Nov 13, 2024 at 6:28 AM Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx> wrote: > > On Tue, Nov 12, 2024 at 11:46:32AM -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]. > > I don't see a motivating reason as to why we want to do this? We increase > memory usage here which is not good, but later lock optimisation mitigates > it, but why wouldn't we just do the lock optimisations and use less memory > overall? > > > This patchset moves vm_lock back into vm_area_struct, aligning it at the > > cacheline boundary and changing the cache to be cache-aligned as well. > > 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 : ... > > Pedantry, but it might be worth mentioning how much this can vary by config. > > For instance, on my machine: > > vm_area_struct 125238 138820 184 44 Ack. > > > > > 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. This memory consumption growth can be > > addressed later by optimizing the vm_lock. > > Yes grabbing this back is of critical importance I'd say! :) > > Functionally it looks ok to me but would like to see a stronger > justification in the commit msg! :) > > > > > [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> > > --- > > include/linux/mm.h | 28 +++++++++++++---------- > > include/linux/mm_types.h | 6 +++-- > > kernel/fork.c | 49 ++++------------------------------------ > > 3 files changed, 25 insertions(+), 58 deletions(-) > > > > diff --git a/include/linux/mm.h b/include/linux/mm.h > > index 01ce619f3d17..a5eb0be3e351 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; > > @@ -729,7 +735,7 @@ 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); > > } > > > > /* > > @@ -739,13 +745,13 @@ 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(); > > } > > > > @@ -774,7 +780,7 @@ 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(). > > @@ -782,7 +788,7 @@ 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) > > @@ -794,7 +800,7 @@ 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); > > } > > > > @@ -827,6 +833,7 @@ 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) {} > > @@ -861,10 +868,6 @@ 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)); > > @@ -873,6 +876,7 @@ 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); > > Why the SLAB_NO_MERGE? Ah, I had it there for convenience to be able to see them separately in /proc/slabinfo. With SLAB_HWCACHE_ALIGN I don't think it matters for cacheline sharing... Once we add SLAB_TYPESAFE_BY_RCU this flag won't matter anyway because it will prevent slab merging. > > > mmap_init(); > > nsproxy_cache_init(); > > } > > -- > > 2.47.0.277.g8800431eea-goog > >
