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

Anna-Maria Behnsen
2024-02-19T16:57:20Z

Frederic Weisbecker <frederic@kernel.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!