Re: [PATCH 7/7] perf: kill perf_event_context::pmu

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Feb 10, 2014 at 05:44:24PM +0000, Mark Rutland wrote:
> Currently portions of the perf subsystem assume that a
> perf_event_context is associated with a single pmu while in reality a
> single perf_event_context may be shared by a number of pmus, as commit
> 443772776c69 (perf: Disable all pmus on unthrottling and rescheduling)
> describes.
> 
> This patch removes perf_event_context::pmu, replacing it with a direct
> pointer to the associated perf_cpu_context and a task_ctx_nr (as all
> pmus sharing a context have the same task_ctx_nr). This makes the
> relationship between pmus and perf_event_contexts clearer and allows us
> to save on some pointer chasing.
> 
> This also fixes a potential misuse of ctx->pmu introduced in commit
> bad7192b842c (perf: Fix PERF_EVENT_IOC_PERIOD to force-reset the
> period), where ctx->pmu is disabled before modifying state on
> event->pmu. In this case the two pmus are not guaranteed to be the same.
> 
> As perf_pmu_rotate_{start,stop} only really care about the context they
> are rotating, they are renamed to perf_event_ctx_{start,stop}.

This very much relies on the previous patch where you make pmu_disable
iterate all the events.

We could also change this to keep a pmu list for each context and
iterate that instead. Given there is indeed a fair limit on different
PMUs in the system that iteration should be much shorter.