Re: [PATCH 6/7] perf: Centralise context pmu disabling

[Peter Zijlstra <peterz@infradead.org>]

On Mon, Feb 10, 2014 at 05:44:23PM +0000, Mark Rutland wrote:
> Commit 443772776c69 (perf: Disable all pmus on unthrottling and
> rescheduling) identified an issue with having multiple PMUs sharing a
> perf_event_context, but only partially solved the issue.
> 
> While ctx::pmu will be disabled across all of its events being
> scheduled, pmus which are not ctx::pmu will be repeatedly enabled and
> disabled between events being added, possibly counting imbetween
> pmu::add calls. This could be expensive and could lead to events
> counting for differing periods.
> 
> Instead, this patch adds new helpers to disable/enable all pmus which
> have events in a context. While perf_pmu_{dis,en}able may be called
> repeatedly for a particular pmu, disabling is reference counted such
> that the real pmu::{dis,en}able callbacks are only called once (were
> this not the case, the current code would be broken for ctx::pmu).
> 
> Uses of perf_pmu{disable,enable}(ctx->pmu) are replaced with
> perf_ctx_pmus_{disable,enable}(ctx). The now unnecessary calls to
> perf_pmu_enable and perf_pmu_disable added by 443772776c69 are removed.

Hurmn; instead of adding more for_each_event iterations we should be
reducing them.

Given that we currently schedule first to last and stop on the first
event that fails to schedule, we can terminate the ctx_sched_out() loop
when it finds the first event that wasn't actually scheduled.