Message-ID: <5579F3A5.7010103@redhat.com>
Date: Thu, 11 Jun 2015 16:46:29 -0400
From: Rik van Riel <riel@redhat.com>
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To: Frederic Weisbecker <fweisbec@gmail.com>,
        LKML <linux-kernel@vger.kernel.org>
CC: Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Preeti U Murthy <preeti@linux.vnet.ibm.com>,
        Christoph Lameter <cl@linux.com>, Ingo Molnar <mingo@kernel.org>,
        Viresh Kumar <viresh.kumar@linaro.org>
Subject: Re: [PATCH 7/8] nohz: Evaluate tick dependency once on context switch
References: <1434044168-23173-1-git-send-email-fweisbec@gmail.com> <1434044168-23173-8-git-send-email-fweisbec@gmail.com>
In-Reply-To: <1434044168-23173-8-git-send-email-fweisbec@gmail.com>
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On 06/11/2015 01:36 PM, Frederic Weisbecker wrote:
> The tick dependency is evaluated on every irq. This is a batch of checks
> which determine whether it is safe to stop the tick or not. These checks
> are often split in many details: posix cpu timers, scheduler, sched clock,
> perf events. Each of which are made of smaller details: posix cpu
> timer involves checking process wide timers then thread wide timers. Perf
> involves checking freq events then more per cpu details.
> 
> Checking these details every time we update the full dynticks state
> bring avoidable overhead.
> 
> So lets evaluate these dependencies once on context switch. Then the
> further dependency checks will be performed through a single state check.
> 
> This is a first step that can be later optimized by dividing task level
> dependency, CPU level dependency and global dependency and update
> each at the right time.

> +static void tick_nohz_full_update_dependencies(void)
> +{
> +	struct tick_sched *ts = this_cpu_ptr(&tick_cpu_sched);
> +
> +	if (!posix_cpu_timers_can_stop_tick(current))
> +		ts->tick_needed |= TICK_NEEDED_POSIX_CPU_TIMER;
> +
> +	if (!perf_event_can_stop_tick())
> +		ts->tick_needed |= TICK_NEEDED_PERF_EVENT;
> +
> +	if (!sched_can_stop_tick())
> +		ts->tick_needed |= TICK_NEEDED_SCHED;
>  

I see this getting kicked from task work and from ipi
context, but does it get kicked on task wakeup, when
we have a second runnable task on a CPU, but we decide
not to preempt the currently running task to switch to
it yet, but we will want to preempt the currently running
task at a later point in time?
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