From mboxrd@z Thu Jan 1 00:00:00 1970 From: Ingo Molnar Subject: Re: [PATCH 2/2] perf: Userspace software event and ioctl Date: Thu, 25 Sep 2014 20:33:43 +0200 Message-ID: <20140925183342.GB6854@gmail.com> References: <1411050873-9310-1-git-send-email-pawel.moll@arm.com> <1411050873-9310-3-git-send-email-pawel.moll@arm.com> <1411491764.3922.46.camel@hornet> <20140924074942.GB3797@gmail.com> <1411665649.4768.84.camel@hornet> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <1411665649.4768.84.camel@hornet> Sender: linux-kernel-owner@vger.kernel.org To: Pawel Moll Cc: Ingo Molnar , Arnaldo Carvalho de Melo , Richard Cochran , Steven Rostedt , Peter Zijlstra , Paul Mackerras , John Stultz , "linux-kernel@vger.kernel.org" , "linux-api@vger.kernel.org" List-Id: linux-api@vger.kernel.org * Pawel Moll wrote: > On Wed, 2014-09-24 at 08:49 +0100, Ingo Molnar wrote: > > * Pawel Moll wrote: > > > > > On Thu, 2014-09-18 at 15:34 +0100, Pawel Moll wrote: > > > > This patch adds a PERF_COUNT_SW_USERSPACE_EVENT type, > > > > which can be generated by user with PERF_EVENT_IOC_ENTRY > > > > ioctl command, which injects an event of said type into > > > > the perf buffer. > > > > > > It occurred to me last night that currently perf doesn't handle "write" > > > syscall at all, while this seems like the most natural way of > > > "injecting" userspace events into perf buffer. > > > > > > An ioctl would still be needed to set a type of the following events, > > > something like: > > > > > > ioctl(SET_TYPE, 0x42); > > > write(perf_fd, binaryblob, size); > > > ioctl(SET_TYPE, 0); > > > dprintf(perf_fd, "String"); > > > > > > which is fine for use cases when the type doesn't change often, > > > but would double the amount of syscalls when every single event > > > is of a different type. Perhaps there still should be a > > > "generating ioctl" taking both type and data/size in one go? > > > > Absolutely, there should be a single syscall. > > Yeah, it's my gut feeling as well. I just wonder if we still want to > keep write() handler for operations on perf fds? This seems natural - > takes data buffer and its size. The only issue is the type. > > > I'd even argue it should be a new prctl(): that way we could both > > generate user events for specific perf fds, but also into any > > currently active context (that allows just generation/injection > > of user events). In the latter case we might have no fd to work > > off from. > > When Arnaldo suggested that the "user events" could be used by perf > trace, it was exactly my first thought. I just didn't have answer how to > present it to the user (an extra syscall didn't seem like a good idea), > but prctl seems interesting, something like this? > > prctl(PR_TRACE_UEVENT, type, size, data, 0); Exactly! > How would we select tasks that can write to a given buffer? Maybe an > ioctl() on a perf fd? Something like this? > > ioctl(perf_fd, PERF_EVENT_IOC_ENABLE_UEVENT, pid); > ioctl(perf_fd, PERF_EVENT_IOC_DISABLE_UEVENT, pid); No, I think there's a simpler way: this should be a regular perf_attr flag, which defaults to '0' (tasks cannot do this), but which can be set to 1 if the profiler explicitly allows such event injection. perf-trace might want to set this flag by default. I.e. whether user-events are allowed is controlled by the profiling/tracing context, via the regular perf syscall. It would propagate into the perf context, so it would be easy to check at event generation time. Thanks, Ingo