linux-kernel - Re: tick/sched: Make jiffies update quick check more robust

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Date:   Mon, 07 Dec 2020 15:41:47 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Will Deacon <will@...nel.org>
Subject: Re: tick/sched: Make jiffies update quick check more robust

On Mon, Dec 07 2020 at 10:59, Peter Zijlstra wrote:
>> +	if (IS_ENABLED(CONFIG_64BIT)) {
>> +		if (ktime_before(now, smp_load_acquire(&tick_next_period)))
>> +			return;
>
> Explicit ACQUIRE
>
>> +	} else {
>> +		unsigned int seq;
>> +
>> +		/*
>> +		 * Avoid contention on jiffies_lock and protect the quick
>> +		 * check with the sequence count.
>> +		 */
>> +		do {
>> +			seq = read_seqcount_begin(&jiffies_seq);
>> +			nextp = tick_next_period;
>> +		} while (read_seqcount_retry(&jiffies_seq, seq));
>> +
>> +		if (ktime_before(now, nextp))
>> +			return;
>
> Actually has an implicit ACQUIRE:
>
> 	read_seqcount_retry() implies smp_rmb(), which ensures
> 	LOAD->LOAD order, IOW any later load must happen after our
> 	@tick_next_period load.
>
> 	Then it has a control dependency on ktime_before(,nextp), which
> 	ensures LOAD->STORE order.
>
> 	Combined we have a LOAD->{LOAD,STORE} order on the
> 	@tick_next_period load, IOW ACQUIRE.
>
>> +	}
>>  
>> +	/* Quick check failed, i.e. update is required. */
>>  	raw_spin_lock(&jiffies_lock);
>
> Another ACQUIRE, which means the above ACQUIRE only ensures we load the
> lock value after?
>
> Or are we trying to guarantee the caller is sure to observe the new
> jiffies value if we return?

The guarantee we need on 64bit for the check w/o seqcount is:

CPU0                                         CPU1

 if (ktime_before(now, tick_next_period))
 	return;

 raw_spin_lock(&jiffies_lock);
 ....
 jiffies_64 += ticks;                           
 
 tick_next_period = next;                   if (ktime_before(now, tick_next_period))
  	                                           return;

When CPU1 returns because it observes the new value in tick_next_period
then it has to be guaranteed that jiffies_64 is observable as well.

I might have gotten it completely wrong again.

Thanks,

        tglx