Hi, On Mon 05-09-11 20:06:04, kautuk.c @samsung.com wrote: > > OK, I don't care much whether we have there del_timer() or > > del_timer_sync(). Let me just say that the race you are afraid of is > > probably not going to happen in practice so I'm not sure it's valid to be > > afraid of CPU cycles being burned needlessly. The timer is armed when an > > dirty inode is first attached to default bdi's dirty list. Then the default > > bdi flusher thread would have to be woken up so that following happens: > > CPU1 CPU2 > > timer fires -> wakeup_timer_fn() > > bdi_forker_thread() > > del_timer(&me->wakeup_timer); > > wb_do_writeback(me, 0); > > ... > > set_current_state(TASK_INTERRUPTIBLE); > > wake_up_process(default_backing_dev_info.wb.task); > > > > Especially wb_do_writeback() is going to take a long time so just that > > single thing makes the race unlikely. Given del_timer_sync() is slightly > > more costly than del_timer() even for unarmed timer, it is questionable > > whether (chance race happens * CPU spent in extra loop) > (extra CPU spent > > in del_timer_sync() * frequency that code is executed in > > bdi_forker_thread())... > > > > Ok, so this means that we can compare the following 2 paths of code: > i) One extra iteration of the bdi_forker_thread loop, versus > ii) The amount of time it takes for the del_timer_sync to wait till the > timer_fn on the other CPU finishes executing + schedule resulting in a > guaranteed sleep. No, ii) is going to be as rare. But instead you should compare i) against: iii) The amount of time it takes del_timer_sync() to check whether the timer_fn is running on a different CPU (which is work del_timer() doesn't do). We are going to spend time in iii) each and every time if (wb_has_dirty_io(me) || !list_empty(&me->bdi->work_list)) evaluates to true. Now frequency of i) and iii) happening is hard to evaluate so it's not clear what's going to be better. Certainly I don't think such evaluation is worth my time... Honza -- Jan Kara <jack@xxxxxxx> SUSE Labs, CR -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>