From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758303AbcEFNlU (ORCPT ); Fri, 6 May 2016 09:41:20 -0400 Received: from szxga03-in.huawei.com ([119.145.14.66]:53824 "EHLO szxga03-in.huawei.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757774AbcEFNlS (ORCPT ); Fri, 6 May 2016 09:41:18 -0400 Subject: Re: [PATCH v2 3/4] perf tools: Support reading from backward ring buffer To: Arnaldo Carvalho de Melo References: <1461723563-67451-1-git-send-email-wangnan0@huawei.com> <1461723563-67451-4-git-send-email-wangnan0@huawei.com> <20160505200723.GI11069@kernel.org> CC: , Arnaldo Carvalho de Melo , Peter Zijlstra , Zefan Li , From: "Wangnan (F)" Message-ID: <572C9EE4.5020903@huawei.com> Date: Fri, 6 May 2016 21:40:52 +0800 User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:38.0) Gecko/20100101 Thunderbird/38.5.0 MIME-Version: 1.0 In-Reply-To: <20160505200723.GI11069@kernel.org> Content-Type: text/plain; charset="utf-8"; format=flowed Content-Transfer-Encoding: 7bit X-Originating-IP: [10.111.66.109] X-CFilter-Loop: Reflected X-Mirapoint-Virus-RAPID-Raw: score=unknown(0), refid=str=0001.0A090204.572C9EF5.0029,ss=1,re=0.000,recu=0.000,reip=0.000,cl=1,cld=1,fgs=0, ip=0.0.0.0, so=2013-05-26 15:14:31, dmn=2013-03-21 17:37:32 X-Mirapoint-Loop-Id: ffc4f4429b8b6be72fa3da8c6793e1de Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 >> Cc: Arnaldo Carvalho de Melo >> Cc: Peter Zijlstra >> Cc: Zefan Li >> 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. >> + 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