linux-kernel - Re: [PATCH 1/3] perf: Optimize perf_install_in

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Date:   Wed, 23 Oct 2019 15:30:27 +0300
From:   Alexander Shishkin <alexander.shishkin@...ux.intel.com>
To:     Peter Zijlstra <peterz@...radead.org>, mingo@...nel.org,
        peterz@...radead.org, linux-kernel@...r.kernel.org
Cc:     acme@...nel.org, mark.rutland@....com, jolsa@...hat.com,
        namhyung@...nel.org, andi@...stfloor.org,
        kan.liang@...ux.intel.com, alexander.shishkin@...ux.intel.com
Subject: Re: [PATCH 1/3] perf: Optimize perf_install_in_event()

Peter Zijlstra <peterz@...radead.org> writes:

> +	/*
> +	 * perf_event_attr::disabled events will not run and can be initialized
> +	 * without IPI. Except when this is the first event for the context, in
> +	 * that case we need the magic of the IPI to set ctx->is_active.
> +	 *
> +	 * The IOC_ENABLE that is sure to follow the creation of a disabled
> +	 * event will issue the IPI and reprogram the hardware.
> +	 */
> +	if (__perf_effective_state(event) == PERF_EVENT_STATE_OFF && ctx->nr_events) {
> +		raw_spin_lock_irq(&ctx->lock);
> +		if (task && ctx->task == TASK_TOMBSTONE) {

Confused: isn't that redundant? If ctx->task reads TASK_TOMBSTONE, task
is always !NULL, afaict. And in any case, if a task context is going
away, we shouldn't probably be adding events there. Or am I missing
something?

Other than that, this makes sense to me, fwiw.

Regards,
--
Alex