On Fri, Oct 23, 2020 at 03:44:16PM +0200, Peter Zijlstra wrote: > On Fri, Oct 23, 2020 at 02:29:54PM +0100, Suzuki Poulose wrote: > > On 10/23/20 2:16 PM, Peter Zijlstra wrote: > > > On Fri, Oct 23, 2020 at 01:56:47PM +0100, Suzuki Poulose wrote: > > > > > That way another session could use the same sink if it is free. i.e > > > > > > > > perf record -e cs_etm/@sink0/u --per-thread app1 > > > > > > > > and > > > > > > > > perf record -e cs_etm/@sink0/u --per-thread app2 > > > > > > > > both can work as long as the sink is not used by the other session. > > > > > > Like said above, if sink is shared between CPUs, that's going to be a > > > trainwreck :/ Why do you want that? > > > > That ship has sailed. That is how the current generation of systems are, > > unfortunately. But as I said, this is changing and there are guidelines > > in place to avoid these kind of topologies. With the future > > technologies, this will be completely gone. > > I understand that the hardware is like that, but why do you want to > support this insanity in software? > > If you only allow a single sink user (group) at the same time, your > problem goes away. Simply disallow the above scenario, do not allow > concurrent sink users if sinks are shared like this. > > Have the perf-record of app2 above fail because the sink is in-user > already. I agree with you that --per-thread scenarios are easy to deal with, but to support cpu-wide scenarios events must share a sink (because there is one event per CPU). CPU-wide support can't be removed because it has been around for close to a couple of years and heavily used. I also think using the pid of the process that created the events, i.e perf, is a good idea. We just need to agree on how to gain access to it. In Sai's patch you objected to the following: > + struct task_struct *task = READ_ONCE(event->owner); > + > + if (!task || is_kernel_event(event)) Would it be better to use task_nr_pid(current) instead of event->owner? The end result will be exactly the same. There is also no need to check the validity of @current since it is a user process. Thanks, Mathieu [1]. https://elixir.bootlin.com/linux/latest/source/kernel/events/core.c#L6170 > > Only if the hardware has per-CPU sinks can you allow this.
