75 lines
2.4 KiB
C
75 lines
2.4 KiB
C
|
#ifndef _TOOLS_LINUX_RING_BUFFER_H_
|
||
|
|
#define _TOOLS_LINUX_RING_BUFFER_H_
|
||
|
|
|
||
|
|
#include <asm/barrier.h>
|
||
|
|
#include <linux/perf_event.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_ */
|