On Thu, Jun 30, 2022 at 09:47:15AM +0800, Feng Tang wrote: > kmalloc's API family is critical for mm, with one shortcoming that > its object size is fixed to be power of 2. When user requests memory > for '2^n + 1' bytes, actually 2^(n+1) bytes will be allocated, so > in worst case, there is around 50% memory space waste. > > We've met a kernel boot OOM panic, and from the dumped slab info: > > [ 26.062145] kmalloc-2k 814056KB 814056KB > > From debug we found there are huge number of 'struct iova_magazine', > whose size is 1032 bytes (1024 + 8), so each allocation will waste > 1016 bytes. Though the issue is solved by giving the right(bigger) > size of RAM, it is still better to optimize the size (either use > a kmalloc friendly size or create a dedicated slab for it). > > And from lkml archive, there was another crash kernel OOM case [1] > back in 2019, which seems to be related with the similar slab waste > situation, as the log is similar: > > [ 4.332648] iommu: Adding device 0000:20:02.0 to group 16 > [ 4.338946] swapper/0 invoked oom-killer: gfp_mask=0x6040c0(GFP_KERNEL|__GFP_COMP), nodemask=(null), order=0, oom_score_adj=0 > ... > [ 4.857565] kmalloc-2048 59164KB 59164KB > > The crash kernel only has 256M memory, and 59M is pretty big here. > > So add an way to track each kmalloc's memory waste info, and leverage > the existing SLUB debug framework to show its call stack info, so > that user can evaluate the waste situation, identify some hot spots > and optimize accordingly, for a better utilization of memory. > > The waste info is integrated into existing interface: > /sys/kernel/debug/slab/kmalloc-xx/alloc_traces, one example of > 'kmalloc-4k' after boot is: > > 126 ixgbe_alloc_q_vector+0xa5/0x4a0 [ixgbe] waste: 233856/1856 age=1493302/1493830/1494358 pid=1284 cpus=32 nodes=1 > __slab_alloc.isra.86+0x52/0x80 > __kmalloc_node+0x143/0x350 > ixgbe_alloc_q_vector+0xa5/0x4a0 [ixgbe] > ixgbe_init_interrupt_scheme+0x1a6/0x730 [ixgbe] > ixgbe_probe+0xc8e/0x10d0 [ixgbe] > local_pci_probe+0x42/0x80 > work_for_cpu_fn+0x13/0x20 > process_one_work+0x1c5/0x390 > worker_thread+0x1b9/0x360 > kthread+0xe6/0x110 > ret_from_fork+0x1f/0x30 > > which means in 'kmalloc-4k' slab, there are 126 requests of > 2240 bytes which got a 4KB space (wasting 1856 bytes each > and 233856 bytes in total). And when system starts some real > workload like multiple docker instances, there are more > severe waste. > > [1]. https://lkml.org/lkml/2019/8/12/266 > > Signed-off-by: Feng Tang <feng.tang@xxxxxxxxx> > --- > Note: > * this is based on linux-next tree with tag next-20220628 So this makes use of the fact that orig_size differ from s->object_size when allocated from kmalloc, and for non-kmalloc caches it doesn't track waste because s->object_size == orig_size. Am I following? And then it has overhead of 'waste' field for every non-kmalloc objects because track is saved per object. Also the field is not used at free. (Maybe that would be okay as it's only for debugging, just noting.) > mm/slub.c | 45 ++++++++++++++++++++++++++++++--------------- > 1 file changed, 30 insertions(+), 15 deletions(-) > > diff --git a/mm/slub.c b/mm/slub.c > index 26b00951aad1..bc4f9d4fb1e2 100644 > --- a/mm/slub.c > +++ b/mm/slub.c > @@ -271,6 +271,7 @@ struct track { > #endif > int cpu; /* Was running on cpu */ > int pid; /* Pid context */ > + unsigned long waste; /* memory waste for a kmalloc-ed object */ > unsigned long when; /* When did the operation occur */ > }; > > @@ -747,6 +748,7 @@ static inline depot_stack_handle_t set_track_prepare(void) > > static void set_track_update(struct kmem_cache *s, void *object, > enum track_item alloc, unsigned long addr, > + unsigned long waste, > depot_stack_handle_t handle) > { > struct track *p = get_track(s, object, alloc); > @@ -758,14 +760,16 @@ static void set_track_update(struct kmem_cache *s, void *object, > p->cpu = smp_processor_id(); > p->pid = current->pid; > p->when = jiffies; > + p->waste = waste; > } > > static __always_inline void set_track(struct kmem_cache *s, void *object, > - enum track_item alloc, unsigned long addr) > + enum track_item alloc, unsigned long addr, > + unsigned long waste) > { > depot_stack_handle_t handle = set_track_prepare(); > > - set_track_update(s, object, alloc, addr, handle); > + set_track_update(s, object, alloc, addr, waste, handle); > } > > static void init_tracking(struct kmem_cache *s, void *object) > @@ -1325,7 +1329,9 @@ static inline int alloc_consistency_checks(struct kmem_cache *s, > > static noinline int alloc_debug_processing(struct kmem_cache *s, > struct slab *slab, > - void *object, unsigned long addr) > + void *object, unsigned long addr, > + unsigned long waste > + ) > { > if (s->flags & SLAB_CONSISTENCY_CHECKS) { > if (!alloc_consistency_checks(s, slab, object)) > @@ -1334,7 +1340,7 @@ static noinline int alloc_debug_processing(struct kmem_cache *s, > > /* Success perform special debug activities for allocs */ > if (s->flags & SLAB_STORE_USER) > - set_track(s, object, TRACK_ALLOC, addr); > + set_track(s, object, TRACK_ALLOC, addr, waste); > trace(s, slab, object, 1); > init_object(s, object, SLUB_RED_ACTIVE); > return 1; > @@ -1398,6 +1404,7 @@ static noinline int free_debug_processing( > int ret = 0; > depot_stack_handle_t handle = 0; > > + /* TODO: feng: we can slab->waste -= track?) or in set_track */ > if (s->flags & SLAB_STORE_USER) > handle = set_track_prepare(); > > @@ -1418,7 +1425,7 @@ static noinline int free_debug_processing( > } > > if (s->flags & SLAB_STORE_USER) > - set_track_update(s, object, TRACK_FREE, addr, handle); > + set_track_update(s, object, TRACK_FREE, addr, 0, handle); > trace(s, slab, object, 0); > /* Freepointer not overwritten by init_object(), SLAB_POISON moved it */ > init_object(s, object, SLUB_RED_INACTIVE); > @@ -2905,7 +2912,7 @@ static inline void *get_freelist(struct kmem_cache *s, struct slab *slab) > * already disabled (which is the case for bulk allocation). > */ > static void *___slab_alloc(struct kmem_cache *s, gfp_t gfpflags, int node, > - unsigned long addr, struct kmem_cache_cpu *c) > + unsigned long addr, struct kmem_cache_cpu *c, unsigned int orig_size) > { > void *freelist; > struct slab *slab; > @@ -3048,7 +3055,7 @@ static void *___slab_alloc(struct kmem_cache *s, gfp_t gfpflags, int node, > check_new_slab: > > if (kmem_cache_debug(s)) { > - if (!alloc_debug_processing(s, slab, freelist, addr)) { > + if (!alloc_debug_processing(s, slab, freelist, addr, s->object_size - orig_size)) { > /* Slab failed checks. Next slab needed */ > goto new_slab; > } else { > @@ -3102,7 +3109,7 @@ static void *___slab_alloc(struct kmem_cache *s, gfp_t gfpflags, int node, > * pointer. > */ > static void *__slab_alloc(struct kmem_cache *s, gfp_t gfpflags, int node, > - unsigned long addr, struct kmem_cache_cpu *c) > + unsigned long addr, struct kmem_cache_cpu *c, unsigned int orig_size) > { > void *p; > > @@ -3115,7 +3122,7 @@ static void *__slab_alloc(struct kmem_cache *s, gfp_t gfpflags, int node, > c = slub_get_cpu_ptr(s->cpu_slab); > #endif > > - p = ___slab_alloc(s, gfpflags, node, addr, c); > + p = ___slab_alloc(s, gfpflags, node, addr, c, orig_size); > #ifdef CONFIG_PREEMPT_COUNT > slub_put_cpu_ptr(s->cpu_slab); > #endif > @@ -3206,7 +3213,7 @@ static __always_inline void *slab_alloc_node(struct kmem_cache *s, struct list_l > */ > if (IS_ENABLED(CONFIG_PREEMPT_RT) || > unlikely(!object || !slab || !node_match(slab, node))) { > - object = __slab_alloc(s, gfpflags, node, addr, c); > + object = __slab_alloc(s, gfpflags, node, addr, c, orig_size); > } else { > void *next_object = get_freepointer_safe(s, object); > > @@ -3709,7 +3716,7 @@ int kmem_cache_alloc_bulk(struct kmem_cache *s, gfp_t flags, size_t size, > * of re-populating per CPU c->freelist > */ > p[i] = ___slab_alloc(s, flags, NUMA_NO_NODE, > - _RET_IP_, c); > + _RET_IP_, c, size); This looks wrong. size here is size of array. Maybe just s->object_size instead of size? > if (unlikely(!p[i])) > goto error; > > @@ -5068,6 +5075,7 @@ struct location { > depot_stack_handle_t handle; > unsigned long count; > unsigned long addr; > + unsigned long waste; > long long sum_time; > long min_time; > long max_time; > @@ -5138,11 +5146,12 @@ static int add_location(struct loc_track *t, struct kmem_cache *s, > if (pos == end) > break; > > - caddr = t->loc[pos].addr; > - chandle = t->loc[pos].handle; > - if ((track->addr == caddr) && (handle == chandle)) { > + l = &t->loc[pos]; > + caddr = l->addr; > + chandle = l->handle; > + if ((track->addr == caddr) && (handle == chandle) && > + (track->waste == l->waste)) { > > - l = &t->loc[pos]; > l->count++; > if (track->when) { > l->sum_time += age; > @@ -5190,6 +5199,7 @@ static int add_location(struct loc_track *t, struct kmem_cache *s, > l->min_pid = track->pid; > l->max_pid = track->pid; > l->handle = handle; > + l->waste = track->waste; I think this may be fooled when there are different wastes values from same caller (i.e. when a kmalloc_track_caller() is used.) because the array is sorted by caller address, but not sorted by waste. And writing this I noticed that it already can be fooled now :) It's also not sorted by handle. > cpumask_clear(to_cpumask(l->cpus)); > cpumask_set_cpu(track->cpu, to_cpumask(l->cpus)); > nodes_clear(l->nodes); > @@ -6078,6 +6088,11 @@ static int slab_debugfs_show(struct seq_file *seq, void *v) > else > seq_puts(seq, "<not-available>"); > > + > + if (l->waste) > + seq_printf(seq, " waste: %lu/%lu", Maybe waste=%lu/%lu like others? > + l->count * l->waste, l->waste); > + > if (l->sum_time != l->min_time) { > seq_printf(seq, " age=%ld/%llu/%ld", > l->min_time, div_u64(l->sum_time, l->count), > -- > 2.27.0 > -- Thanks, Hyeonggon
