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Date: Mon, 19 Feb 2024 17:04:56 +0100
From: Frederic Weisbecker <frederic@kernel.org>
To: Anna-Maria Behnsen <anna-maria@linutronix.de>
Cc: linux-kernel@vger.kernel.org, Peter Zijlstra <peterz@infradead.org>,
	John Stultz <jstultz@google.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Eric Dumazet <edumazet@google.com>,
	"Rafael J . Wysocki" <rafael.j.wysocki@intel.com>,
	Arjan van de Ven <arjan@infradead.org>,
	"Paul E . McKenney" <paulmck@kernel.org>,
	Rik van Riel <riel@surriel.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Sebastian Siewior <bigeasy@linutronix.de>,
	Giovanni Gherdovich <ggherdovich@suse.cz>,
	Lukasz Luba <lukasz.luba@arm.com>,
	"Gautham R . Shenoy" <gautham.shenoy@amd.com>,
	Srinivas Pandruvada <srinivas.pandruvada@intel.com>,
	K Prateek Nayak <kprateek.nayak@amd.com>,
	Frederic Weisbecker <frederic@kernel.org>
Subject: Re: [PATCH v10 13/20] timers: Add get next timer interrupt
 functionality for remote CPUs
Message-ID: <ZdN8KCTB2smSZb88@localhost.localdomain>
References: <20240115143743.27827-1-anna-maria@linutronix.de>
 <20240115143743.27827-14-anna-maria@linutronix.de>
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In-Reply-To: <20240115143743.27827-14-anna-maria@linutronix.de>

Le Mon, Jan 15, 2024 at 03:37:36PM +0100, Anna-Maria Behnsen a ?crit :
> +# 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.

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;
 	}