From mboxrd@z Thu Jan 1 00:00:00 1970 From: Alexey Budankov Subject: Re: [PATCH v2 2/2]: perf record: enable asynchronous trace writing Date: Mon, 27 Aug 2018 13:48:19 +0300 Message-ID: <2b595b40-fe31-2ced-6077-e972e4aaf561@linux.intel.com> References: <54cc11d7-3ef2-c856-052e-6e2c309ff743@linux.intel.com> <20180827083852.GF24695@krava> <20180827100536.GA8065@sejong> <3c0a8ecc-a68e-7e19-066c-096f7e318e1f@linux.intel.com> <20180827103819.GB3725@krava> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20180827103819.GB3725@krava> Content-Language: en-US Sender: linux-kernel-owner@vger.kernel.org To: Jiri Olsa Cc: Namhyung Kim , Peter Zijlstra , Ingo Molnar , Arnaldo Carvalho de Melo , Alexander Shishkin , Andi Kleen , linux-kernel , linux-perf-users@vger.kernel.org, kernel-team@lge.com List-Id: linux-perf-users.vger.kernel.org On 27.08.2018 13:38, Jiri Olsa wrote: > On Mon, Aug 27, 2018 at 01:25:35PM +0300, Alexey Budankov wrote: >> Hi Namhyung, >> >> On 27.08.2018 13:05, Namhyung Kim wrote: >>> Hello, >>> >>> On Mon, Aug 27, 2018 at 12:33:07PM +0300, Alexey Budankov wrote: >>>> Hi, >>>> >>>> On 27.08.2018 11:38, Jiri Olsa wrote: >>>>> On Thu, Aug 23, 2018 at 07:47:01PM +0300, Alexey Budankov wrote: >>>>> >>>>> SNIP >>>>> >>>>>> static int record__mmap_read_evlist(struct record *rec, struct perf_evlist *evlist, >>>>>> bool overwrite) >>>>>> { >>>>>> u64 bytes_written = rec->bytes_written; >>>>>> - int i; >>>>>> - int rc = 0; >>>>>> + int i, rc = 0; >>>>>> struct perf_mmap *maps; >>>>>> + int trace_fd = rec->session->data->file.fd; >>>>>> + struct aiocb **mmap_aio = rec->evlist->mmap_aio; >>>>>> + int mmap_aio_size = 0; >>>>>> + off_t off; >>>>>> >>>>>> if (!evlist) >>>>>> return 0; >>>>>> @@ -546,14 +620,17 @@ static int record__mmap_read_evlist(struct record *rec, struct perf_evlist *evli >>>>>> if (overwrite && evlist->bkw_mmap_state != BKW_MMAP_DATA_PENDING) >>>>>> return 0; >>>>>> Hi >>>>>> + off = lseek(trace_fd, 0, SEEK_CUR); >>>>>> + >>>>> >>>>> with async write, do we need to query/set the offset like this >>>>> all the time? >>>> >>>> It looks like we need it this way. Internally glibc AIO implements writes >>>> using pwrite64 syscall in our case. The sycall requires offset as a parameter >>>> and doesn't update file position on the completion. >>>> >>>>> >>>>> could we just keep/update the offset value in the 'struct perf_data_file' >>>>> and skip both lseek calls? >>>> >>>> Don't see how it is possible. offset is different for every enqeued write >>>> operation and write areas don't intersect for the whole writing loop. >>>> To know the final file position it is required to iterate thru >>>> the loop. >>> >>> But as far as I can see the offset is linearly updated in >>> perf_mmap__push() and I guess those two lseek() calls will return >>> a same value as the last updated offset, no? >> >> Yes, offset is linearly calculated by perf_mmap__push() code for >> the next possible write operation, but file position is update by >> the kernel only in the second lseek() syscall after the loop. >> The first lseek() syscall reads that file position for >> the next loop iterations. > > does the file's offset need to get updated with lseek at all? > the async write gets offset.. so I'd think as long as we keep > offset value, we don't need to call lseek at all Yep, I expected the same from pwrite64() syscall which eventually gets this offset but that is not the case. > > jirka >