Messages in this thread | | | Subject | Re: [PATCH v2 3/4] perf tools: Support reading from backward ring buffer | From | "Wangnan (F)" <> | Date | Mon, 9 May 2016 09:13:00 +0800 |
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On 2016/5/6 21:40, Wangnan (F) wrote: > > > On 2016/5/6 4:07, Arnaldo Carvalho de Melo wrote: >> Em Wed, Apr 27, 2016 at 02:19:22AM +0000, Wang Nan escreveu: >>> perf_evlist__mmap_read_backward() is introduced for reading backward >>> ring buffer. Different from reading forward, before reading, caller >>> needs to call perf_evlist__mmap_read_catchup() first. >>> >>> Backward ring buffer should be read from 'head' pointer, not '0'. >>> perf_evlist__mmap_read_catchup() saves 'head' to 'md->prev', then >>> make it remember the start position after each reading. >>> >>> Signed-off-by: Wang Nan <wangnan0@huawei.com> >>> Cc: Arnaldo Carvalho de Melo <acme@redhat.com> >>> Cc: Peter Zijlstra <peterz@infradead.org> >>> Cc: Zefan Li <lizefan@huawei.com> >>> Cc: pi3orama@163.com >>> --- >>> tools/perf/util/evlist.c | 39 +++++++++++++++++++++++++++++++++++++++ >>> tools/perf/util/evlist.h | 4 ++++ >>> 2 files changed, 43 insertions(+) >>> >>> diff --git a/tools/perf/util/evlist.c b/tools/perf/util/evlist.c >>> index 17cd014..2e0b7b0 100644 >>> --- a/tools/perf/util/evlist.c >>> +++ b/tools/perf/util/evlist.c >>> @@ -766,6 +766,45 @@ union perf_event *perf_evlist__mmap_read(struct >>> perf_evlist *evlist, int idx) >>> return perf_mmap__read(md, evlist->overwrite, old, head, >>> &md->prev); >>> } >>> +union perf_event * >>> +perf_evlist__mmap_read_backward(struct perf_evlist *evlist, int idx) >>> +{ >>> + struct perf_mmap *md = &evlist->mmap[idx]; >>> + u64 head, end; >>> + u64 start = md->prev; >>> + >>> + /* >>> + * Check if event was unmapped due to a POLLHUP/POLLERR. >>> + */ >>> + if (!atomic_read(&md->refcnt)) >>> + return NULL; >>> + >>> + /* NOTE: head is negative in this case */ >> What is this comment for, can you ellaborate? Are you double sure this >> arithmetic with u64, negative values, that -head below are all ok? > > Yes. In backward ring buffer, kernel write data from '0'. Each > time it write a record, it minus sizeof(record) from 'head' > pointer. So '-head' means number of bytes already written. > >> I've applied the first two patches in this series. >> >> I also need to check why we now need that catchup thing :-\ > > I think catchup is necessary. I tried to get rid of it by setting > md->prev to -1 and initialize it to 0 when reading forwardly and > to head when reading backwardly, but it require too much code > adjustments. >
In addtion, reader of backward ring buffer tends to fetch the most recent record. Since each reading gets earlier record, a catchup function is always required.
Thank you. >>> + head = perf_mmap__read_head(md); >>> + >>> + if (!head) >>> + return NULL; >>> + >>> + end = head + md->mask + 1; >>> + >>> + if ((end - head) > -head) >>> + end = 0; >>> + > > This '-head' is used to detect wrapping. Never use positive pointer. > >>> + return perf_mmap__read(md, false, start, end, &md->prev); >>> +} >>> + >>> +void perf_evlist__mmap_read_catchup(struct perf_evlist *evlist, int >>> idx) >>> +{ >>> + struct perf_mmap *md = &evlist->mmap[idx]; >>> + u64 head; >>> + >>> + if (!atomic_read(&md->refcnt)) >>> + return; >>> + >>> + head = perf_mmap__read_head(md); >>> + md->prev = head; >>> +} >>> + >>> static bool perf_mmap__empty(struct perf_mmap *md) >>> { >>> return perf_mmap__read_head(md) == md->prev && >>> !md->auxtrace_mmap.base; >>> diff --git a/tools/perf/util/evlist.h b/tools/perf/util/evlist.h >>> index 208897a..85d1b59 100644 >>> --- a/tools/perf/util/evlist.h >>> +++ b/tools/perf/util/evlist.h >>> @@ -129,6 +129,10 @@ struct perf_sample_id >>> *perf_evlist__id2sid(struct perf_evlist *evlist, u64 id); >>> union perf_event *perf_evlist__mmap_read(struct perf_evlist >>> *evlist, int idx); >>> +union perf_event *perf_evlist__mmap_read_backward(struct >>> perf_evlist *evlist, >>> + int idx); >>> +void perf_evlist__mmap_read_catchup(struct perf_evlist *evlist, int >>> idx); >>> + >>> void perf_evlist__mmap_consume(struct perf_evlist *evlist, int idx); >>> int perf_evlist__open(struct perf_evlist *evlist); >>> -- >>> 1.8.3.4 >
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