From 09d62154f61316f7e97eae3f31ef8770c7e4b386 Mon Sep 17 00:00:00 2001 From: Daniel Borkmann Date: Fri, 19 Oct 2018 15:51:02 +0200 Subject: tools, perf: add and use optimized ring_buffer_{read_head, write_tail} helpers Currently, on x86-64, perf uses LFENCE and MFENCE (rmb() and mb(), respectively) when processing events from the perf ring buffer which is unnecessarily expensive as we can do more lightweight in particular given this is critical fast-path in perf. According to Peter rmb()/mb() were added back then via a94d342b9cb0 ("tools/perf: Add required memory barriers") at a time where kernel still supported chips that needed it, but nowadays support for these has been ditched completely, therefore we can fix them up as well. While for x86-64, replacing rmb() and mb() with smp_*() variants would result in just a compiler barrier for the former and LOCK + ADD for the latter (__sync_synchronize() uses slower MFENCE by the way), Peter suggested we can use smp_{load_acquire,store_release}() instead for architectures where its implementation doesn't resolve in slower smp_mb(). Thus, e.g. in x86-64 we would be able to avoid CPU barrier entirely due to TSO. For architectures where the latter needs to use smp_mb() e.g. on arm, we stick to cheaper smp_rmb() variant for fetching the head. This work adds helpers ring_buffer_read_head() and ring_buffer_write_tail() for tools infrastructure that either switches to smp_load_acquire() for architectures where it is cheaper or uses READ_ONCE() + smp_rmb() barrier for those where it's not in order to fetch the data_head from the perf control page, and it uses smp_store_release() to write the data_tail. Latter is smp_mb() + WRITE_ONCE() combination or a cheaper variant if architecture allows for it. Those that rely on smp_rmb() and smp_mb() can further improve performance in a follow up step by implementing the two under tools/arch/*/include/asm/barrier.h such that they don't have to fallback to rmb() and mb() in tools/include/asm/barrier.h. Switch perf to use ring_buffer_read_head() and ring_buffer_write_tail() so it can make use of the optimizations. Later, we convert libbpf as well to use the same helpers. Side note [0]: the topic has been raised of whether one could simply use the C11 gcc builtins [1] for the smp_load_acquire() and smp_store_release() instead: __atomic_load_n(ptr, __ATOMIC_ACQUIRE); __atomic_store_n(ptr, val, __ATOMIC_RELEASE); Kernel and (presumably) tooling shipped along with the kernel has a minimum requirement of being able to build with gcc-4.6 and the latter does not have C11 builtins. While generally the C11 memory models don't align with the kernel's, the C11 load-acquire and store-release alone /could/ suffice, however. Issue is that this is implementation dependent on how the load-acquire and store-release is done by the compiler and the mapping of supported compilers must align to be compatible with the kernel's implementation, and thus needs to be verified/tracked on a case by case basis whether they match (unless an architecture uses them also from kernel side). The implementations for smp_load_acquire() and smp_store_release() in this patch have been adapted from the kernel side ones to have a concrete and compatible mapping in place. [0] http://patchwork.ozlabs.org/patch/985422/ [1] https://gcc.gnu.org/onlinedocs/gcc/_005f_005fatomic-Builtins.html Signed-off-by: Daniel Borkmann Acked-by: Peter Zijlstra (Intel) Cc: "Paul E. McKenney" Cc: Will Deacon Cc: Arnaldo Carvalho de Melo Signed-off-by: Alexei Starovoitov --- tools/perf/util/mmap.h | 15 +++------------ 1 file changed, 3 insertions(+), 12 deletions(-) (limited to 'tools/perf/util') diff --git a/tools/perf/util/mmap.h b/tools/perf/util/mmap.h index 05a6d47c7956..8f6531fd4dad 100644 --- a/tools/perf/util/mmap.h +++ b/tools/perf/util/mmap.h @@ -4,7 +4,7 @@ #include #include #include -#include +#include #include #include "auxtrace.h" #include "event.h" @@ -71,21 +71,12 @@ void perf_mmap__consume(struct perf_mmap *map); static inline u64 perf_mmap__read_head(struct perf_mmap *mm) { - struct perf_event_mmap_page *pc = mm->base; - u64 head = READ_ONCE(pc->data_head); - rmb(); - return head; + return ring_buffer_read_head(mm->base); } static inline void perf_mmap__write_tail(struct perf_mmap *md, u64 tail) { - struct perf_event_mmap_page *pc = md->base; - - /* - * ensure all reads are done before we write the tail out. - */ - mb(); - pc->data_tail = tail; + ring_buffer_write_tail(md->base, tail); } union perf_event *perf_mmap__read_forward(struct perf_mmap *map); -- cgit v1.2.3