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Diffstat (limited to 'tools/include/linux/ring_buffer.h')
-rw-r--r-- | tools/include/linux/ring_buffer.h | 73 |
1 files changed, 73 insertions, 0 deletions
diff --git a/tools/include/linux/ring_buffer.h b/tools/include/linux/ring_buffer.h new file mode 100644 index 000000000000..9a083ae60473 --- /dev/null +++ b/tools/include/linux/ring_buffer.h @@ -0,0 +1,73 @@ +#ifndef _TOOLS_LINUX_RING_BUFFER_H_ +#define _TOOLS_LINUX_RING_BUFFER_H_ + +#include <asm/barrier.h> + +/* + * Contract with kernel for walking the perf ring buffer from + * user space requires the following barrier pairing (quote + * from kernel/events/ring_buffer.c): + * + * Since the mmap() consumer (userspace) can run on a + * different CPU: + * + * kernel user + * + * if (LOAD ->data_tail) { LOAD ->data_head + * (A) smp_rmb() (C) + * STORE $data LOAD $data + * smp_wmb() (B) smp_mb() (D) + * STORE ->data_head STORE ->data_tail + * } + * + * Where A pairs with D, and B pairs with C. + * + * In our case A is a control dependency that separates the + * load of the ->data_tail and the stores of $data. In case + * ->data_tail indicates there is no room in the buffer to + * store $data we do not. + * + * D needs to be a full barrier since it separates the data + * READ from the tail WRITE. + * + * For B a WMB is sufficient since it separates two WRITEs, + * and for C an RMB is sufficient since it separates two READs. + * + * Note, instead of B, C, D we could also use smp_store_release() + * in B and D as well as smp_load_acquire() in C. + * + * However, this optimization does not make sense for all kernel + * supported architectures since for a fair number it would + * resolve into READ_ONCE() + smp_mb() pair for smp_load_acquire(), + * and smp_mb() + WRITE_ONCE() pair for smp_store_release(). + * + * Thus for those smp_wmb() in B and smp_rmb() in C would still + * be less expensive. For the case of D this has either the same + * cost or is less expensive, for example, due to TSO x86 can + * avoid the CPU barrier entirely. + */ + +static inline u64 ring_buffer_read_head(struct perf_event_mmap_page *base) +{ +/* + * Architectures where smp_load_acquire() does not fallback to + * READ_ONCE() + smp_mb() pair. + */ +#if defined(__x86_64__) || defined(__aarch64__) || defined(__powerpc64__) || \ + defined(__ia64__) || defined(__sparc__) && defined(__arch64__) + return smp_load_acquire(&base->data_head); +#else + u64 head = READ_ONCE(base->data_head); + + smp_rmb(); + return head; +#endif +} + +static inline void ring_buffer_write_tail(struct perf_event_mmap_page *base, + u64 tail) +{ + smp_store_release(&base->data_tail, tail); +} + +#endif /* _TOOLS_LINUX_RING_BUFFER_H_ */ |