Messages in this thread | | | From | Song Liu <> | Subject | Re: [RFC][PATCH] perf: Rewrite core context handling | Date | Wed, 17 Oct 2018 18:57:25 +0000 |
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> On Oct 17, 2018, at 11:33 AM, Peter Zijlstra <peterz@infradead.org> wrote: > > On Wed, Oct 17, 2018 at 07:19:55PM +0200, Peter Zijlstra wrote: >> On Wed, Oct 17, 2018 at 04:43:27PM +0000, Song Liu wrote: >> >>>> That makes task and cpu contexts wildly different, which will complicate >>>> matters I feel. >>>> >>> >>> I think we only need different logic when adding events to the task/cpu >>> contexts. The ctx_sched_in() and ctx_sched_out() will need some extra >>> logic to filter out events that are not being scheduled (don't schedule >>> events on PMU-a when rotating PMU-b). This logic will be the same for >>> task and cpu context. The difference is, the CPU context will not have >>> such events, because we never added such event to CPU context. >>> >>> Does this make sense? I could try draft a RFC to see how difficult it is. >> >> I'm not sure it saves much, if we have multiple per-cpu contexts we get >> to re-introduce the active_ctx_list and loose the simplification for the >> online status. >> >> Plus that fundamental assymetry -- which would bother my OCD forever >> more :-) > > Worse, the whole syscall that installs the events will come apart. The > locking for the two cases is different :/
I agree... I didn't get into details of locking. I just consider these all as part of "adding event to context".
I believe this patch should give close to the optimal performance. However, I do feel it makes the logic more complicate. Before this patch, perf_cpu_context and perf_event_context don't need to know much about multiple PMUs. With this patch, the two extra *_pmu_context are necessary for performance (and maybe also for correctness).
If we take first a baby step, how about adding more perf_event_ctx to task_struct->perf_event_ctxp? We need one sw perf_event_ctx and a few hw perf_event_ctx (one for each hw PMU). (I haven't checked whether it is OK to allocate these when attaching events). (And I guess you don't really like this..)
On the other hand, this patch makes it possible to create groups of events from different hw PMUs. I guess that will be useful.
Thanks, Song
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