On Fri, Oct 12, 2018 at 11:50:01AM +0200, Peter Zijlstra wrote: > > Can we please not top-post? > > On Thu, Oct 11, 2018 at 10:37:14PM +0000, Song Liu wrote: > > Thanks Peter! These are really really helpful. > > > > I am trying to think through the case of a group of two events on two > > separate hardware PMUs. In current implementation, this will not trigger > > move_group, > > Right, currently this is disallowed (or should be, I'll need to double > check the code). > > > so they will not be able to rotate together? And actually, > > they may not be able to run at all? Maybe this case is never supported? > > Indeed, we do not allow mixing events of different PMUs, with the > explicit exception of software events. Since software events must always > schedule, they're allowed to be fitted into any group. > > > On the other hand, would something like this work: > > > > perf_cpu_context <-[1:2]-> perf_event_context <-[1:n]-> perf_event > > | | > > `----[1:n]----> pmu <----- [1:n]----------' > > > > 1. Every cpu has only one perf_cpu_context. No perf_cpu_pmu_context. > > The perf_event_pmu_context is currently needed to efficiently track > which events are active. And to determine if rotation is needed at all. > > And the perf_cpu_pmu_context is needed because the rotation is per PMU > in ABI. > > > 2. perf_cpu_context has two perf_event_context, one for the cpu, the > > other for the task. > > That doesn't work (or I'm not understanding), tasks come and go on CPUs, > at best it has a reference to the current active task's context. But it > already had that, and it still does, see perf_cpu_context::task_ctx. > > > 3. Each perf_event_context has 3 perf_event_groups, pinned_groups, > > flexible_groups, and software_groups (for sw event only groups). > > So I'm thinking you want to split off the software groups because they > don't need rotation? > > While doing this patch I noticed that we need to ignore attr.exclusive > for software events. Not sure that was intentional or not, but certainly > inconsistent.
That sentence is confused; what I meant to say was that I noticed that attr.exclusive for software events is currently confused. > > 4. All flexible_groups of the same cpu rotate a the same time. If > > there are two hardware PMUs on the cpu, the rotation will look > > like: 1) stop both PMUs; 2) rotate events; 3) start both PMUs. > > ABI precludes that currently, we have per PMU rotation intervals exposed > in sysfs. > > > I feel this will make the implementation simpler. Is it too broken in > > some cases? Or did I miss anything obvious? One thing I noticed is > > that we need to drop per PMU config perf_event_mux_interval_ms. > > Right that. People added that for a reason (although it eludes me atm). > I don't think we can drop that easily.
