linux-kernel - [RFC] perf_events: ctx_flexible_sched_in() not maximizing PMU utilization

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Date:	Thu, 6 May 2010 16:03:26 +0200
From:	Stephane Eranian <eranian@...gle.com>
To:	LKML <linux-kernel@...r.kernel.org>
Cc:	Peter Zijlstra <peterz@...radead.org>, mingo@...e.hu,
	Paul Mackerras <paulus@...ba.org>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	"David S. Miller" <davem@...emloft.net>,
	perfmon2-devel@...ts.sf.net
Subject: [RFC] perf_events: ctx_flexible_sched_in() not maximizing PMU 
	utilization

Hi,

Looking at ctx_flexible_sched_in(), the logic is that if  group_sched_in()
fails for a HW group, then no other HW group in the list is even tried.
I don't understand this restriction. Groups are independent of each other.
The failure of one group should not block others from being scheduled,
otherwise you under-utilize the PMU.

What is the reason for this restriction? Can we lift it somehow?

static void
ctx_flexible_sched_in(struct perf_event_context *ctx,
                      struct perf_cpu_context *cpuctx)
{
        struct perf_event *event;
        int can_add_hw = 1;

        list_for_each_entry(event, &ctx->flexible_groups, group_entry) {
                /* Ignore events in OFF or ERROR state */
                if (event->state <= PERF_EVENT_STATE_OFF)
                        continue;
                /*
                 * Listen to the 'cpu' scheduling filter constraint
                 * of events:
                 */
                if (event->cpu != -1 && event->cpu != smp_processor_id())
                        continue;

                if (group_can_go_on(event, cpuctx, can_add_hw))
                        if (group_sched_in(event, cpuctx, ctx))
                                can_add_hw = 0;
        }
}
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