Messages in this thread | | | Date | Mon, 5 Feb 2024 21:28:38 +0100 | From | Frederic Weisbecker <> | Subject | Re: [PATCH v10 18/20] timers: Implement the hierarchical pull model |
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Le Mon, Feb 05, 2024 at 04:59:45PM +0100, Anna-Maria Behnsen a écrit : > Frederic Weisbecker <frederic@kernel.org> writes: > > > Le Mon, Jan 15, 2024 at 03:37:41PM +0100, Anna-Maria Behnsen a écrit : > >> +static bool tmigr_handle_remote_up(struct tmigr_group *group, > >> + struct tmigr_group *child, > >> + void *ptr) > >> +{ > >> + struct tmigr_remote_data *data = ptr; > >> + u64 now, next = KTIME_MAX; > >> + struct tmigr_event *evt; > >> + unsigned long jif; > >> + u8 childmask; > >> + > >> + jif = data->basej; > >> + now = data->now; > >> + > >> + childmask = data->childmask; > >> + > >> +again: > >> + /* > >> + * Handle the group only if @childmask is the migrator or if the > >> + * group has no migrator. Otherwise the group is active and is > >> + * handled by its own migrator. > >> + */ > >> + if (!tmigr_check_migrator(group, childmask)) > >> + return true; > >> + > >> + raw_spin_lock_irq(&group->lock); > >> + > >> + evt = tmigr_next_expired_groupevt(group, now); > >> + > >> + if (evt) { > >> + unsigned int remote_cpu = evt->cpu; > >> + > >> + raw_spin_unlock_irq(&group->lock); > >> + > >> + next = tmigr_handle_remote_cpu(remote_cpu, now, jif); > >> + > >> + /* check if there is another event, that needs to be handled */ > >> + goto again; > >> + } else { > >> + raw_spin_unlock_irq(&group->lock); > >> + } > >> + > >> + /* > >> + * Update of childmask for the next level and keep track of the expiry > >> + * of the first event that needs to be handled > >> + */ > >> + data->childmask = group->childmask; > >> + data->firstexp = next; > > > > So assume we have: > > > > [GRP1:0] > > migrator = [GRP0:0] > > active = [GRP0:0] > > nextevt = TIMER3 > > / \ > > [GRP0:0] [GRP0:1] > > migrator = CPU0 migrator = NONE > > active = CPU0 active = NONE > > nextevt = KTIME_MAX nextevt = TIMER3 > > / \ / \ > > 0 1 2 3 > > idle idle idle idle (TIMER3) > > > > Then CPU 0 goes idle: > > > > [GRP1:0] > > migrator = NONE > > active = NONE > > nextevt = TIMER3 > > / \ > > [GRP0:0] [GRP0:1] > > migrator = NONE migrator = NONE > > active = NONE active = NONE > > nextevt = KTIME_MAX nextevt = TIMER3 > > / \ / \ > > 0 1 2 3 > > idle idle idle idle (TIMER3) > > > > CPU 0 is the idle migrator and its tmc->wakeup is TIMER3. > > But CPU 0 has a local timer that expires before TIMER3. > > > > When that timer interrupt fires, it raises the softirq, which > > executes on IRQ tail. So CPU0 eventually calls tmigr_handle_remote() > > before TIMER3 has expired. > > > > This leads to tmigr_next_expired_groupevt() to return NULL and then > > data->firstexp = KTIME_MAX and then tmc->wakeup = KTIME_MAX. > > > > Later on, tmigr_new_timer() is called with a KTIME_MAX global > > event and so tmc->wakeup stays with KTIME_MAX, ignoring TIMER3. > > > > It looks like you need to handle the tmigr_next_expired_groupevt() > > case returning NULL. > > Yes. You are right. group->next_expiry is updated via > tmigr_next_expired_groupevt(). So I should take this value to rely on > for the firstevt. > > But with this, we do not need the return value of > tmigr_handle_remote_cpu(). When the upperst level is inactive and there > is a timer (e.g. propagated there by tmigr_handle_remote_cpu()), the > return value of tmigr_handle_remote_cpu() is the next_expiry of the top > level group. But this value is also noticed by walking the hierarchy up > to the top level with tmigr_handle_remote_up().
Indeed that's a nice simplification!
This also cure an issue with KTIME_MAX overwriting data->firstexp in tmigr_handle_remote_up() while walking upper level after an expiration on below level.
Thanks.
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