linux-kernel - Re: [tip:perfcounters/core] perf_counter: Optimize context switch between identical inherited contexts

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Date:	Sun, 24 May 2009 13:33:15 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	mingo@...hat.com, hpa@...or.com, paulus@...ba.org, acme@...hat.com,
	linux-kernel@...r.kernel.org, a.p.zijlstra@...llo.nl,
	mtosatti@...hat.com, tglx@...utronix.de,
	cjashfor@...ux.vnet.ibm.com
Cc:	linux-tip-commits@...r.kernel.org
Subject: Re: [tip:perfcounters/core] perf_counter: Optimize context switch
	between identical inherited contexts


* tip-bot for Paul Mackerras <paulus@...ba.org> wrote:

> @@ -885,6 +934,16 @@ void perf_counter_task_sched_out(struct task_struct *task, int cpu)
>  
>  	regs = task_pt_regs(task);
>  	perf_swcounter_event(PERF_COUNT_CONTEXT_SWITCHES, 1, 1, regs, 0);
> +
> +	next_ctx = next->perf_counter_ctxp;
> +	if (next_ctx && context_equiv(ctx, next_ctx)) {
> +		task->perf_counter_ctxp = next_ctx;
> +		next->perf_counter_ctxp = ctx;
> +		ctx->task = next;
> +		next_ctx->task = task;
> +		return;
> +	}

there's one complication that this trick is causing - the migration 
counter relies on ctx->task to get per task migration stats:

 static inline u64 get_cpu_migrations(struct perf_counter *counter)
 {
        struct task_struct *curr = counter->ctx->task;

        if (curr)
                return curr->se.nr_migrations;
        return cpu_nr_migrations(smp_processor_id());
 }

as ctx->task is now jumping (while we keep the context), the 
migration stats are out of whack.

	Ingo
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