On Fri, Oct 17, 2025 at 9:04 PM Xu Kuohai xukuohai@huaweicloud.com wrote:
From: Xu Kuohai xukuohai@huawei.com
When the BPF ring buffer is full, a new event cannot be recorded until one or more old events are consumed to make enough space for it. In cases such as fault diagnostics, where recent events are more useful than older ones, this mechanism may lead to critical events being lost.
So add overwrite mode for BPF ring buffer to address it. In this mode, the new event overwrites the oldest event when the buffer is full.
The basic idea is as follows:
producer_pos tracks the next position to record new event. When there is enough free space, producer_pos is simply advanced by producer to make space for the new event.
To avoid waiting for consumer when the buffer is full, a new variable, overwrite_pos, is introduced for producer. It points to the oldest event committed in the buffer. It is advanced by producer to discard one or more oldest events to make space for the new event when the buffer is full.
pending_pos tracks the oldest event to be committed. pending_pos is never passed by producer_pos, so multiple producers never write to the same position at the same time.
The following example diagrams show how it works in a 4096-byte ring buffer.
At first, {producer,overwrite,pending,consumer}_pos are all set to 0.
0 512 1024 1536 2048 2560 3072 3584 4096 +-----------------------------------------------------------------------+ | | | | | | +-----------------------------------------------------------------------+ ^ | |
producer_pos = 0 overwrite_pos = 0 pending_pos = 0 consumer_pos = 0
Now reserve a 512-byte event A.
There is enough free space, so A is allocated at offset 0. And producer_pos is advanced to 512, the end of A. Since A is not submitted, the BUSY bit is set.
0 512 1024 1536 2048 2560 3072 3584 4096 +-----------------------------------------------------------------------+ | | | | A | | | [BUSY] | | +-----------------------------------------------------------------------+ ^ ^ | | | | | producer_pos = 512 |
overwrite_pos = 0 pending_pos = 0 consumer_pos = 0
Reserve event B, size 1024.
B is allocated at offset 512 with BUSY bit set, and producer_pos is advanced to the end of B.
0 512 1024 1536 2048 2560 3072 3584 4096 +-----------------------------------------------------------------------+ | | | | | A | B | | | [BUSY] | [BUSY] | | +-----------------------------------------------------------------------+ ^ ^ | | | | | producer_pos = 1536 |
overwrite_pos = 0 pending_pos = 0 consumer_pos = 0
Reserve event C, size 2048.
C is allocated at offset 1536, and producer_pos is advanced to 3584.
0 512 1024 1536 2048 2560 3072 3584 4096 +-----------------------------------------------------------------------+ | | | | | | A | B | C | | | [BUSY] | [BUSY] | [BUSY] | | +-----------------------------------------------------------------------+ ^ ^ | | | | | producer_pos = 3584 |
overwrite_pos = 0 pending_pos = 0 consumer_pos = 0
Submit event A.
The BUSY bit of A is cleared. B becomes the oldest event to be committed, so pending_pos is advanced to 512, the start of B.
0 512 1024 1536 2048 2560 3072 3584 4096 +-----------------------------------------------------------------------+ | | | | | | A | B | C | | | | [BUSY] | [BUSY] | | +-----------------------------------------------------------------------+ ^ ^ ^ | | | | | | | pending_pos = 512 producer_pos = 3584 |
overwrite_pos = 0 consumer_pos = 0
Submit event B.
The BUSY bit of B is cleared, and pending_pos is advanced to the start of C, which is now the oldest event to be committed.
0 512 1024 1536 2048 2560 3072 3584 4096 +-----------------------------------------------------------------------+ | | | | | | A | B | C | | | | | [BUSY] | | +-----------------------------------------------------------------------+ ^ ^ ^ | | | | | | | pending_pos = 1536 producer_pos = 3584 |
overwrite_pos = 0 consumer_pos = 0
Reserve event D, size 1536 (3 * 512).
There are 2048 bytes not being written between producer_pos (currently 3584) and pending_pos, so D is allocated at offset 3584, and producer_pos is advanced by 1536 (from 3584 to 5120).
Since event D will overwrite all bytes of event A and the first 512 bytes of event B, overwrite_pos is advanced to the start of event C, the oldest event that is not overwritten.
0 512 1024 1536 2048 2560 3072 3584 4096 +-----------------------------------------------------------------------+ | | | | | | D End | | C | D Begin| | [BUSY] | | [BUSY] | [BUSY] | +-----------------------------------------------------------------------+ ^ ^ ^ | | | | | pending_pos = 1536 | | overwrite_pos = 1536 | | | producer_pos=5120 |
consumer_pos = 0
Reserve event E, size 1024.
Although there are 512 bytes not being written between producer_pos and pending_pos, E cannot be reserved, as it would overwrite the first 512 bytes of event C, which is still being written.
Submit event C and D.
pending_pos is advanced to the end of D.
0 512 1024 1536 2048 2560 3072 3584 4096 +-----------------------------------------------------------------------+ | | | | | | D End | | C | D Begin| | | | | | +-----------------------------------------------------------------------+ ^ ^ ^ | | | | | overwrite_pos = 1536 | | | producer_pos=5120 | pending_pos=5120 |
consumer_pos = 0
The performance data for overwrite mode will be provided in a follow-up patch that adds overwrite-mode benchmarks.
A sample of performance data for non-overwrite mode, collected on an x86_64 CPU and an arm64 CPU, before and after this patch, is shown below. As we can see, no obvious performance regression occurs.
- x86_64 (AMD EPYC 9654)
Before:
Ringbuf, multi-producer contention
rb-libbpf nr_prod 1 11.623 ± 0.027M/s (drops 0.000 ± 0.000M/s) rb-libbpf nr_prod 2 15.812 ± 0.014M/s (drops 0.000 ± 0.000M/s) rb-libbpf nr_prod 3 7.871 ± 0.003M/s (drops 0.000 ± 0.000M/s) rb-libbpf nr_prod 4 6.703 ± 0.001M/s (drops 0.000 ± 0.000M/s) rb-libbpf nr_prod 8 2.896 ± 0.002M/s (drops 0.000 ± 0.000M/s) rb-libbpf nr_prod 12 2.054 ± 0.002M/s (drops 0.000 ± 0.000M/s) rb-libbpf nr_prod 16 1.864 ± 0.002M/s (drops 0.000 ± 0.000M/s) rb-libbpf nr_prod 20 1.580 ± 0.002M/s (drops 0.000 ± 0.000M/s) rb-libbpf nr_prod 24 1.484 ± 0.002M/s (drops 0.000 ± 0.000M/s) rb-libbpf nr_prod 28 1.369 ± 0.002M/s (drops 0.000 ± 0.000M/s) rb-libbpf nr_prod 32 1.316 ± 0.001M/s (drops 0.000 ± 0.000M/s) rb-libbpf nr_prod 36 1.272 ± 0.002M/s (drops 0.000 ± 0.000M/s) rb-libbpf nr_prod 40 1.239 ± 0.001M/s (drops 0.000 ± 0.000M/s) rb-libbpf nr_prod 44 1.226 ± 0.002M/s (drops 0.000 ± 0.000M/s) rb-libbpf nr_prod 48 1.213 ± 0.001M/s (drops 0.000 ± 0.000M/s) rb-libbpf nr_prod 52 1.193 ± 0.001M/s (drops 0.000 ± 0.000M/s)
After:
Ringbuf, multi-producer contention
rb-libbpf nr_prod 1 11.845 ± 0.036M/s (drops 0.000 ± 0.000M/s) rb-libbpf nr_prod 2 15.889 ± 0.006M/s (drops 0.000 ± 0.000M/s) rb-libbpf nr_prod 3 8.155 ± 0.002M/s (drops 0.000 ± 0.000M/s) rb-libbpf nr_prod 4 6.708 ± 0.001M/s (drops 0.000 ± 0.000M/s) rb-libbpf nr_prod 8 2.918 ± 0.001M/s (drops 0.000 ± 0.000M/s) rb-libbpf nr_prod 12 2.065 ± 0.002M/s (drops 0.000 ± 0.000M/s) rb-libbpf nr_prod 16 1.870 ± 0.002M/s (drops 0.000 ± 0.000M/s) rb-libbpf nr_prod 20 1.582 ± 0.002M/s (drops 0.000 ± 0.000M/s) rb-libbpf nr_prod 24 1.482 ± 0.001M/s (drops 0.000 ± 0.000M/s) rb-libbpf nr_prod 28 1.372 ± 0.002M/s (drops 0.000 ± 0.000M/s) rb-libbpf nr_prod 32 1.323 ± 0.002M/s (drops 0.000 ± 0.000M/s) rb-libbpf nr_prod 36 1.264 ± 0.001M/s (drops 0.000 ± 0.000M/s) rb-libbpf nr_prod 40 1.236 ± 0.002M/s (drops 0.000 ± 0.000M/s) rb-libbpf nr_prod 44 1.209 ± 0.002M/s (drops 0.000 ± 0.000M/s) rb-libbpf nr_prod 48 1.189 ± 0.001M/s (drops 0.000 ± 0.000M/s) rb-libbpf nr_prod 52 1.165 ± 0.002M/s (drops 0.000 ± 0.000M/s)
- arm64 (HiSilicon Kunpeng 920)
Before:
Ringbuf, multi-producer contention
rb-libbpf nr_prod 1 11.310 ± 0.623M/s (drops 0.000 ± 0.000M/s) rb-libbpf nr_prod 2 9.947 ± 0.004M/s (drops 0.000 ± 0.000M/s) rb-libbpf nr_prod 3 6.634 ± 0.011M/s (drops 0.000 ± 0.000M/s) rb-libbpf nr_prod 4 4.502 ± 0.003M/s (drops 0.000 ± 0.000M/s) rb-libbpf nr_prod 8 3.888 ± 0.003M/s (drops 0.000 ± 0.000M/s) rb-libbpf nr_prod 12 3.372 ± 0.005M/s (drops 0.000 ± 0.000M/s) rb-libbpf nr_prod 16 3.189 ± 0.010M/s (drops 0.000 ± 0.000M/s) rb-libbpf nr_prod 20 2.998 ± 0.006M/s (drops 0.000 ± 0.000M/s) rb-libbpf nr_prod 24 3.086 ± 0.018M/s (drops 0.000 ± 0.000M/s) rb-libbpf nr_prod 28 2.845 ± 0.004M/s (drops 0.000 ± 0.000M/s) rb-libbpf nr_prod 32 2.815 ± 0.008M/s (drops 0.000 ± 0.000M/s) rb-libbpf nr_prod 36 2.771 ± 0.009M/s (drops 0.000 ± 0.000M/s) rb-libbpf nr_prod 40 2.814 ± 0.011M/s (drops 0.000 ± 0.000M/s) rb-libbpf nr_prod 44 2.752 ± 0.006M/s (drops 0.000 ± 0.000M/s) rb-libbpf nr_prod 48 2.695 ± 0.006M/s (drops 0.000 ± 0.000M/s) rb-libbpf nr_prod 52 2.710 ± 0.006M/s (drops 0.000 ± 0.000M/s)
After:
Ringbuf, multi-producer contention
rb-libbpf nr_prod 1 11.283 ± 0.550M/s (drops 0.000 ± 0.000M/s) rb-libbpf nr_prod 2 9.993 ± 0.003M/s (drops 0.000 ± 0.000M/s) rb-libbpf nr_prod 3 6.898 ± 0.006M/s (drops 0.000 ± 0.000M/s) rb-libbpf nr_prod 4 5.257 ± 0.001M/s (drops 0.000 ± 0.000M/s) rb-libbpf nr_prod 8 3.830 ± 0.005M/s (drops 0.000 ± 0.000M/s) rb-libbpf nr_prod 12 3.528 ± 0.013M/s (drops 0.000 ± 0.000M/s) rb-libbpf nr_prod 16 3.265 ± 0.018M/s (drops 0.000 ± 0.000M/s) rb-libbpf nr_prod 20 2.990 ± 0.007M/s (drops 0.000 ± 0.000M/s) rb-libbpf nr_prod 24 2.929 ± 0.014M/s (drops 0.000 ± 0.000M/s) rb-libbpf nr_prod 28 2.898 ± 0.010M/s (drops 0.000 ± 0.000M/s) rb-libbpf nr_prod 32 2.818 ± 0.006M/s (drops 0.000 ± 0.000M/s) rb-libbpf nr_prod 36 2.789 ± 0.012M/s (drops 0.000 ± 0.000M/s) rb-libbpf nr_prod 40 2.770 ± 0.006M/s (drops 0.000 ± 0.000M/s) rb-libbpf nr_prod 44 2.651 ± 0.007M/s (drops 0.000 ± 0.000M/s) rb-libbpf nr_prod 48 2.669 ± 0.005M/s (drops 0.000 ± 0.000M/s) rb-libbpf nr_prod 52 2.695 ± 0.009M/s (drops 0.000 ± 0.000M/s)
Signed-off-by: Xu Kuohai xukuohai@huawei.com
include/uapi/linux/bpf.h | 4 ++ kernel/bpf/ringbuf.c | 109 +++++++++++++++++++++++++++------ tools/include/uapi/linux/bpf.h | 4 ++ 3 files changed, 98 insertions(+), 19 deletions(-)
[...]
@@ -72,6 +73,8 @@ struct bpf_ringbuf { */ unsigned long consumer_pos __aligned(PAGE_SIZE); unsigned long producer_pos __aligned(PAGE_SIZE);
/* points to the record right after the last overwritten one */unsigned long overwrite_pos;
I moved this after pending_pos, as all these fields are actually exposed to the user space, so didn't want to unnecessarily shift pending_pos.
unsigned long pending_pos; char data[] __aligned(PAGE_SIZE);}; @@ -166,7 +169,7 @@ static void bpf_ringbuf_notify(struct irq_work *work)
- considering that the maximum value of data_sz is (4GB - 1), there
- will be no overflow, so just note the size limit in the comments.
*/ -static struct bpf_ringbuf *bpf_ringbuf_alloc(size_t data_sz, int numa_node) +static struct bpf_ringbuf *bpf_ringbuf_alloc(size_t data_sz, int numa_node, bool overwrite_mode) { struct bpf_ringbuf *rb;
@@ -183,17 +186,25 @@ static struct bpf_ringbuf *bpf_ringbuf_alloc(size_t data_sz, int numa_node) rb->consumer_pos = 0; rb->producer_pos = 0; rb->pending_pos = 0;
rb->overwrite_mode = overwrite_mode; return rb;}
static struct bpf_map *ringbuf_map_alloc(union bpf_attr *attr) {
bool overwrite_mode = false; struct bpf_ringbuf_map *rb_map; if (attr->map_flags & ~RINGBUF_CREATE_FLAG_MASK) return ERR_PTR(-EINVAL);if (attr->map_flags & BPF_F_RB_OVERWRITE) {if (attr->map_type == BPF_MAP_TYPE_USER_RINGBUF)
this seemed error prone if we ever add another ringbuf type (unlikely, but still), so I inverted this all to allow BPF_F_RB_OVERWRITE only for BPF_MAP_TYPE_RINGBUF. We should try to be as strict as possible by default.
return ERR_PTR(-EINVAL);overwrite_mode = true;}if (attr->key_size || attr->value_size || !is_power_of_2(attr->max_entries) || !PAGE_ALIGNED(attr->max_entries))@@ -205,7 +216,7 @@ static struct bpf_map *ringbuf_map_alloc(union bpf_attr *attr)
bpf_map_init_from_attr(&rb_map->map, attr);
rb_map->rb = bpf_ringbuf_alloc(attr->max_entries, rb_map->map.numa_node);
rb_map->rb = bpf_ringbuf_alloc(attr->max_entries, rb_map->map.numa_node, overwrite_mode); if (!rb_map->rb) { bpf_map_area_free(rb_map); return ERR_PTR(-ENOMEM);@@ -293,13 +304,25 @@ static int ringbuf_map_mmap_user(struct bpf_map *map, struct vm_area_struct *vma return remap_vmalloc_range(vma, rb_map->rb, vma->vm_pgoff + RINGBUF_PGOFF); }
+/* Return an estimate of the available data in the ring buffer.
Fixed up comment style
[...]
static u32 ringbuf_total_data_sz(const struct bpf_ringbuf *rb) @@ -402,11 +425,41 @@ bpf_ringbuf_restore_from_rec(struct bpf_ringbuf_hdr *hdr) return (void*)((addr & PAGE_MASK) - off); }
+static bool bpf_ringbuf_has_space(const struct bpf_ringbuf *rb,
unsigned long new_prod_pos,unsigned long cons_pos,unsigned long pend_pos)+{
/* no space if oldest not yet committed record until the newest* record span more than (ringbuf_size - 1).*/
same, keep in mind that we now use kernel-wide comment style with /* on separate line. Fixed up all other places as well.
if (new_prod_pos - pend_pos > rb->mask)return false;/* ok, we have space in overwrite mode */if (unlikely(rb->overwrite_mode))return true;/* no space if producer position advances more than (ringbuf_size - 1)* ahead of consumer position when not in overwrite mode.*/if (new_prod_pos - cons_pos > rb->mask)return false;return true;+}
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