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

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Message-ID: <20191023140806.GE19358@hirez.programming.kicks-ass.net>
Date:   Wed, 23 Oct 2019 16:08:06 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Alexander Shishkin <alexander.shishkin@...ux.intel.com>
Cc:     mingo@...nel.org, linux-kernel@...r.kernel.org, acme@...nel.org,
        mark.rutland@....com, jolsa@...hat.com, namhyung@...nel.org,
        andi@...stfloor.org, kan.liang@...ux.intel.com
Subject: Re: [PATCH 1/3] perf: Optimize perf_install_in_event()

On Wed, Oct 23, 2019 at 03:44:44PM +0200, Peter Zijlstra wrote:
> On Wed, Oct 23, 2019 at 03:30:27PM +0300, Alexander Shishkin wrote:
> > 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,
> 
> The test is only relevant for task contexts, that's what the first
> 'task' clause tests for, then we need to check the ctx isn't dying,
> which is the second clause 'ctx->task == TASK_TOMBSTONE'.

Urgh, n/m that. I got confused and that can indeed be simplified.