linux-kernel - Re: [PATCH v10 13/20] timers: Add get next timer interrupt functionality for remote CPUs

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Date: Mon, 19 Feb 2024 17:57:16 +0100
From: Anna-Maria Behnsen <anna-maria@...utronix.de>
To: Frederic Weisbecker <frederic@...nel.org>
Cc: linux-kernel@...r.kernel.org, Peter Zijlstra <peterz@...radead.org>,
 John Stultz <jstultz@...gle.com>, Thomas Gleixner <tglx@...utronix.de>,
 Eric Dumazet <edumazet@...gle.com>, "Rafael J . Wysocki"
 <rafael.j.wysocki@...el.com>, Arjan van de Ven <arjan@...radead.org>,
 "Paul E . McKenney" <paulmck@...nel.org>, Rik van Riel <riel@...riel.com>,
 Steven Rostedt <rostedt@...dmis.org>, Sebastian Siewior
 <bigeasy@...utronix.de>, Giovanni Gherdovich <ggherdovich@...e.cz>, Lukasz
 Luba <lukasz.luba@....com>, "Gautham R . Shenoy" <gautham.shenoy@....com>,
 Srinivas Pandruvada <srinivas.pandruvada@...el.com>, K Prateek Nayak
 <kprateek.nayak@....com>, Frederic Weisbecker <frederic@...nel.org>
Subject: Re: [PATCH v10 13/20] timers: Add get next timer interrupt
 functionality for remote CPUs

Frederic Weisbecker <frederic@...nel.org> writes:

> Le Mon, Jan 15, 2024 at 03:37:36PM +0100, Anna-Maria Behnsen a écrit :
>> +# ifdef CONFIG_SMP
>> +/**
>> + * fetch_next_timer_interrupt_remote() - Store next timers into @tevt
>> + * @basej:	base time jiffies
>> + * @basem:	base time clock monotonic
>> + * @tevt:	Pointer to the storage for the expiry values
>> + * @cpu:	Remote CPU
>> + *
>> + * Stores the next pending local and global timer expiry values in the
>> + * struct pointed to by @tevt. If a queue is empty the corresponding
>> + * field is set to KTIME_MAX. If local event expires before global
>> + * event, global event is set to KTIME_MAX as well.
>> + *
>> + * Caller needs to make sure timer base locks are held (use
>> + * timer_lock_remote_bases() for this purpose).
>> + */
>> +void fetch_next_timer_interrupt_remote(unsigned long basej, u64 basem,
>> +				       struct timer_events *tevt,
>> +				       unsigned int cpu)
>> +{
>> +	struct timer_base *base_local, *base_global;
>> +
>> +	/* Preset local / global events */
>> +	tevt->local = tevt->global = KTIME_MAX;
>> +
>> +	base_local = per_cpu_ptr(&timer_bases[BASE_LOCAL], cpu);
>> +	base_global = per_cpu_ptr(&timer_bases[BASE_GLOBAL], cpu);
>> +
>> +	lockdep_assert_held(&base_local->lock);
>> +	lockdep_assert_held(&base_global->lock);
>> +
>> +	fetch_next_timer_interrupt(basej, basem, base_local, base_global, tevt);
>
> If the next timer is global and it is <= jiffies + 1, the result will be
> returned in tevt.local only and not on tevt.global. So a remote fetch may miss it.

Oh no. But yes, sounds reasonable.

> For this to work on both local and remote fetch, you may need:
>
> diff --git a/kernel/time/timer.c b/kernel/time/timer.c
> index 320eb4ceafa2..64ce9a7760f5 100644
> --- a/kernel/time/timer.c
> +++ b/kernel/time/timer.c
> @@ -2004,6 +2007,8 @@ static unsigned long fetch_next_timer_interrupt(unsigned long basej, u64 basem,
>  		if (time_before(nextevt, basej))
>  			nextevt = basej;
>  		tevt->local = basem + (u64)(nextevt - basej) * TICK_NSEC;
> +		if (!local_first)
> +			tevt->global = tevt->local;
>  		return nextevt;
>  	}
>  

Will fix it - with a big comment explaining why this is required for
remote call sites and will not hurt when executed on the local cpu.

Thanks a lot!