Re: [RFC][PATCH] perf: Rewrite core context handling
From: Peter Zijlstra
Date: Tue Oct 16 2018 - 05:51:16 EST
On Sat, Oct 13, 2018 at 08:31:37AM +0000, Song Liu wrote:
> The only suggestion I have right now is on which struct owns which
> 1. perf_cpu_context owns two perf_event_context: ctx and *task_ctx.
> This is the same as right now.
> 2. perf_event_context owns multiple perf_event_pmu_context:
> One perf_event_pmu_context for software groups;
> One perf_event_pmu_context for each hardware PMU.
It does now already, right? Through the pmu_ctx_list we can, given an
perf_event_context, find all associated perf_event_pmu_context's.
> 3. perf_event_pmu_context owns RB tree of events. Since we don't
> need rotation across multiple hardware PMUs, the rotation is
> within same perf_event_pmu_context.
By keeping the RB trees in perf_event_context, we get bigger trees,
which is more efficient (log(n+m) < log(n) + log(m))
Also, specifically, it means we only need a single merge sort /
iteration to schedule in a full context, instead of (again) doing 'n' of
Also, given a context and a pmu, it is cheaper for finding the relevant
events; this is needed for big.little for instance. Something the
proposed patch doesn't fully flesh out.
> 4. perf_cpu_context owns multiple perf_cpu_pmu_context:
> One perf_cpu_pmu_context for each hardware PMU.
What would we need that relation for?
> perf_cpu_pmu_context is tot needed for software only groups(?).
Yes, that is a very good question; it mostly centers around what we want
to do with perf_event_attr::exclusive for software events -- which is
currently dodgy at best.
Also, allocating the structure and keeping it around is probably less
code than explicitly not doing it.
> 5. perf_cpu_pmu_context has two pointers of perf_event_pmu_context.
Instead of embedding the thing? Yeah, not sure. Either way around we'd
not want to free the CPU perf_event_pmu_context that is associated with
the perf_cpu_pmu_context, and embedding it saves a pointer chase.
Not sure it actually makes a lot of difference either way around.