Re: [PATCH V2 Resend 4/4] timer: Migrate running timer

[Viresh Kumar]

On 22 May 2013 14:04, Viresh Kumar <viresh.kumar@xxxxxxxxxx> wrote:
> Sorry for being late in replying to your queries.
>
> On 13 May 2013 16:05, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
>> Which mechanism is migrating the timer away?
>
> It will be the same: get_nohz_timer_target() which will decide target
> cpu for migration.
>
>> I have no objections to the functionality per se, but the proposed
>> solution is not going to fly.
>>
>> Aside of bloating the data structure you're changing the semantics of
>> __mod_timer(). No __mod_timer() caller can deal with -EBUSY. So you'd
>> break the world and some more.
>
> Ahh.. That idea was dropped already.
>
>> Here is a list of questions:
>>
>>       - Which mechanism migrates timers?
>>
>>       - How is that mechanism triggered?
>
> The mechanism remains the same as is for non-rearmed timers.
> i.e. get_nohz_timer_target()..
>
> We are just trying to give a approach with which we can migrate
> running timers. i.e. those which re-arm themselves from their
> handlers.
>
>>       - How does that deal with CPU bound timers?
>
> We will still check 'Pinned' for this timer as is done for any other
> normal timer. So, we don't migrate them.
>
> So, this is the clean draft for the idea I had.. (Naming is poor for
> now):
>
> diff --git a/include/linux/timer.h b/include/linux/timer.h
> index 8c5a197..ad00ebe 100644
> --- a/include/linux/timer.h
> +++ b/include/linux/timer.h
> @@ -20,6 +20,7 @@ struct timer_list {
>
>         void (*function)(unsigned long);
>         unsigned long data;
> +       int wait_for_migration_to_complete;
>
>         int slack;
>
> diff --git a/kernel/timer.c b/kernel/timer.c
> index a860bba..7791f28 100644
> --- a/kernel/timer.c
> +++ b/kernel/timer.c
> @@ -746,21 +746,15 @@ __mod_timer(struct timer_list *timer, unsigned
> long expires,
>         new_base = per_cpu(tvec_bases, cpu);
>
>         if (base != new_base) {
> -               /*
> -                * We are trying to schedule the timer on the local CPU.
> -                * However we can't change timer's base while it is running,
> -                * otherwise del_timer_sync() can't detect that the timer's
> -                * handler yet has not finished. This also guarantees that
> -                * the timer is serialized wrt itself.
> -                */
> -               if (likely(base->running_timer != timer)) {
> -                       /* See the comment in lock_timer_base() */
> -                       timer_set_base(timer, NULL);
> -                       spin_unlock(&base->lock);
> -                       base = new_base;
> -                       spin_lock(&base->lock);
> -                       timer_set_base(timer, base);
> -               }
> +               if (base->running_timer == timer)
> +                       timer->wait_for_migration_to_complete = 1;
> +
> +               /* See the comment in lock_timer_base() */
> +               timer_set_base(timer, NULL);
> +               spin_unlock(&base->lock);
> +               base = new_base;
> +               spin_lock(&base->lock);
> +               timer_set_base(timer, base);
>         }
>
>         timer->expires = expires;
> @@ -990,7 +984,8 @@ int try_to_del_timer_sync(struct timer_list *timer)
>
>         base = lock_timer_base(timer, &flags);
>
> -       if (base->running_timer != timer) {
> +       if ((base->running_timer != timer) &&
> +                       !timer->wait_for_migration_to_complete) {
>                 timer_stats_timer_clear_start_info(timer);
>                 ret = detach_if_pending(timer, base, true);
>         }
> @@ -1183,6 +1178,8 @@ static inline void __run_timers(struct tvec_base *base)
>                                 call_timer_fn(timer, fn, data);
>                                 spin_lock_irq(&base->lock);
>                         }
> +                       if (timer->wait_for_migration_to_complete)
> +                               timer->wait_for_migration_to_complete = 0;
>                 }
>         }
>         base->running_timer = NULL;
>
>
> Please see if it a junk idea or has some light of hope :)

Ping!!
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