linux-kernel - Re: [PATCH v2 3/4] perf tools: Support reading from backward ring buffer

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Date:	Mon, 9 May 2016 09:13:00 +0800
From:	"Wangnan (F)" <wangnan0@...wei.com>
To:	Arnaldo Carvalho de Melo <acme@...nel.org>
CC:	<linux-kernel@...r.kernel.org>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Zefan Li <lizefan@...wei.com>, <pi3orama@....com>
Subject: Re: [PATCH v2 3/4] perf tools: Support reading from backward ring
 buffer



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@...wei.com>
>>> Cc: Arnaldo Carvalho de Melo <acme@...hat.com>
>>> Cc: Peter Zijlstra <peterz@...radead.org>
>>> Cc: Zefan Li <lizefan@...wei.com>
>>> Cc: pi3orama@....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
>