linux-kernel - Re: [PATCH] nohz/cpuset: Make a CPU stick with do_timer() duty in the presence of nohz cpusets

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Message-ID: <CAFTL4hwSva0eBK1f=K7Hus0ybiEACtebQbhEUDsJjLZcmmeCtA@mail.gmail.com>
Date:	Fri, 30 Nov 2012 18:38:16 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Hakan Akkan <hakanakkan@...il.com>
Cc:	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH] nohz/cpuset: Make a CPU stick with do_timer() duty in the
 presence of nohz cpusets

2012/11/28 Hakan Akkan <hakanakkan@...il.com>:
> +static int check_drop_timer_duty(int cpu)
> +{
> +       int curr_handler, prev_handler, new_handler;
> +       int nrepeat = -1;
> +       bool drop_recheck;
> +
> +repeat:
> +       WARN_ON_ONCE(++nrepeat > 1);
> +       drop_recheck = false;
> +       curr_handler = cpu;
> +       new_handler = TICK_DO_TIMER_NONE;
> +
> +#ifdef CONFIG_CPUSETS_NO_HZ
> +       if (atomic_read(&nr_cpus_user_nohz) > 0) {

Note atomic_read() is not SMP ordered. If another CPU does
atomic_add() or atomic_add_return(), you may not see the new value as
expected with atomic_read(). Doing atomic_add_return(0, &atomic_thing)
returns the fully ordered result.

You also need to do that to ensure full ordering against
tick_cpu_sched.user_nohz.

On the current layout we have:

(Write side)                                      (Read side)

ts->user_nohz = 1;
atomic_inc(&nr_cpus_user_nohz)

                                                      if
(atomic_read(&nr_cpus_user_nohz))
                                                            if
(per_cpu(tick_cpu_sched, curr_handler).user_nohz)
                                                                ....

If you want to make sure that you see the expected value on user_nohz
from the read side, you need to correctly order the write and read
against nr_cpus_user_nohz.

For this you can use atomic_add_return() which implies the full barrier:


(Write side)                                      (Read side)

ts->user_nohz = 1;
atomic_inc_return(&nr_cpus_user_nohz)

                                                      if
(atomic_add_return(0, &nr_cpus_user_nohz))
                                                            if
(per_cpu(tick_cpu_sched, curr_handler).user_nohz)
                                                                ....

I have much more comments on this patch, I will come back on this soon.

Thanks.
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