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Date:   Tue, 16 Oct 2018 11:50:56 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Song Liu <songliubraving@fb.com>
Cc:     Ingo Molnar <mingo@kernel.org>,
        lkml <linux-kernel@vger.kernel.org>,
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        "alexey.budankov@linux.intel.com" <alexey.budankov@linux.intel.com>,
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        "megha.dey@intel.com" <megha.dey@intel.com>,
        "frederic@kernel.org" <frederic@kernel.org>
Subject: Re: [RFC][PATCH] perf: Rewrite core context handling
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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
> data:
> 
> 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
them.

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.