linux-kernel - Re: [PATCH v6 16/18] perf: enable reading of perf.data files from different ABI rev

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Message-ID: <CABPqkBTz10h0rWbhnLjPiOSfG1QG_MjmRs1+t79C5FA2XwHRBw@mail.gmail.com>
Date:	Thu, 9 Feb 2012 23:49:58 +0100
From:	Stephane Eranian <eranian@...gle.com>
To:	David Ahern <dsahern@...il.com>
Cc:	linux-kernel@...r.kernel.org, peterz@...radead.org, mingo@...e.hu,
	acme@...hat.com, robert.richter@....com, ming.m.lin@...el.com,
	andi@...stfloor.org, asharma@...com, ravitillo@....gov,
	vweaver1@...s.utk.edu, khandual@...ux.vnet.ibm.com
Subject: Re: [PATCH v6 16/18] perf: enable reading of perf.data files from
 different ABI rev

On Thu, Feb 9, 2012 at 11:46 PM, David Ahern <dsahern@...il.com> wrote:
>
>
> On 02/09/2012 03:41 PM, Stephane Eranian wrote:
>>>> +     /* read perf_file_section, ids are read in caller */
>>>> +     ret = readn(fd, &f_attr->ids, sizeof(f_attr->ids));
>>>
>>> I thought the above 2 lines were to be removed.
>>>
>>
>> No, They are needed to get the offsets of the ids[] table.
>> The field name are confusing. This is not reading the ids[],
>> but just the offset of where the table is. Without that you
>> get bogus offsets. I ran into that bug during testing...
>
> Hmm.... ok, so the perf_file_attr is now read in back to back reads --
> attr then ids -- versus today where it gets both in one readn.
>
>
Yes, you have to do that in case on file sizeof(perf_event_attr) <
tool sizeof(perf_event_attr). perf_event_attr is encapsulated into
perf_file_attr.

>>
>>
>>> David
>>>
>>>
>>>> +
>>>> +     return ret <= 0 ? -1 : 0;
>>>> +}
>>>> +
>>>>  int perf_session__read_header(struct perf_session *session, int fd)
>>>>  {
>>>>       struct perf_header *header = &session->header;
>>>> @@ -1974,19 +2019,17 @@ int perf_session__read_header(struct perf_session *session, int fd)
>>>>       if (session->fd_pipe)
>>>>               return perf_header__read_pipe(session, fd);
>>>>
>>>> -     if (perf_file_header__read(&f_header, header, fd) < 0) {
>>>> -             pr_debug("incompatible file format\n");
>>>> +     if (perf_file_header__read(&f_header, header, fd) < 0)
>>>>               return -EINVAL;
>>>> -     }
>>>>
>>>> -     nr_attrs = f_header.attrs.size / sizeof(f_attr);
>>>> +     nr_attrs = f_header.attrs.size / f_header.attr_size;
>>>>       lseek(fd, f_header.attrs.offset, SEEK_SET);
>>>>
>>>>       for (i = 0; i < nr_attrs; i++) {
>>>>               struct perf_evsel *evsel;
>>>>               off_t tmp;
>>>>
>>>> -             if (readn(fd, &f_attr, sizeof(f_attr)) <= 0)
>>>> +             if (read_attr(fd, header, &f_attr) < 0)
>>>>                       goto out_errno;
>>>>
>>>>               if (header->needs_swap)
>>>> diff --git a/tools/perf/util/session.c b/tools/perf/util/session.c
>>>> index 3f3afed..5476320 100644
>>>> --- a/tools/perf/util/session.c
>>>> +++ b/tools/perf/util/session.c
>>>> @@ -24,7 +24,7 @@ static int perf_session__open(struct perf_session *self, bool force)
>>>>               self->fd = STDIN_FILENO;
>>>>
>>>>               if (perf_session__read_header(self, self->fd) < 0)
>>>> -                     pr_err("incompatible file format");
>>>> +                     pr_err("incompatible file format (rerun with -v to learn more)");
>>>>
>>>>               return 0;
>>>>       }
>>>> @@ -56,7 +56,7 @@ static int perf_session__open(struct perf_session *self, bool force)
>>>>       }
>>>>
>>>>       if (perf_session__read_header(self, self->fd) < 0) {
>>>> -             pr_err("incompatible file format");
>>>> +             pr_err("incompatible file format (rerun with -v to learn more)");
>>>>               goto out_close;
>>>>       }
>>>>
--
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