From mboxrd@z Thu Jan 1 00:00:00 1970 From: Namhyung Kim Subject: Re: [PATCH v2 2/2]: perf record: enable asynchronous trace writing Date: Mon, 27 Aug 2018 19:05:36 +0900 Message-ID: <20180827100536.GA8065@sejong> References: <54cc11d7-3ef2-c856-052e-6e2c309ff743@linux.intel.com> <20180827083852.GF24695@krava> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Return-path: Content-Disposition: inline In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org To: Alexey Budankov Cc: Jiri Olsa , 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 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; > >> > >> + 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? Thanks, Namhyung