On Thu, Jul 23, 2020 at 10:32:54AM +0200, Thomas Gleixner wrote: > Frederic Weisbecker <[email protected]> writes: > > On Fri, Jul 17, 2020 at 12:50:34AM +0200, Peter Zijlstra wrote: > >> On Thu, Jul 16, 2020 at 10:19:26PM +0200, Thomas Gleixner wrote: > >> > +static void __run_posix_cpu_timers(struct task_struct *tsk) > >> > +{ > >> > + struct posix_cputimers *pct = &tsk->posix_cputimers; > >> > + > >> > + if (!test_and_set_bit(CPUTIMERS_WORK_SCHEDULED, &pct->flags)) > >> > + task_work_add(tsk, &pct->task_work, true); > >> > +} > >> > + > >> > +static inline void posix_cpu_timers_enable_work(struct task_struct *tsk) > >> > +{ > >> > + clear_bit(CPUTIMERS_WORK_SCHEDULED, > >> > &tsk->posix_cputimers.flags); > >> /* > >> * Ensure we observe everything before a failing test_and_set() > >> * in __run_posix_cpu_timers(). > >> */ > >> smp_mb__after_atomic(); > >> > +} > >> > >> Such that when another timer interrupt happens while we run this, we're > >> guaranteed to either see it, or get re-queued and thus re-run the > >> function. > > > > But each thread in the process enqueues its own task work and flips its > > own flags. So if task A runs the task work and task B runs > > __run_posix_cpu_timers(), > > they wouldn't be ordering against the same flags. > > If two tasks queue work independent of each other then one of them will > find it done already, which is the same as if two tasks of the same > process execute run_posix_cpu_timers() in parallel. > > I really don't want to go into the rathole of making the work or the > synchronization process wide. That's a guarantee for disaster. > > Handling task work strictly per task is straight forward and simple. The > eventually resulting contention on sighand lock in task work is > unavoidable, but that's a reasonable tradeoff vs. the complexity you > need to handle task work process wide.
Definetly! I was only commenting on the barrier suggestion. But I believe it shouldn't be needed in the end. If we were to have a per task work for thread timers and a per process work for process timers, that means we would need to cut down the whole thing, and also take care about timers firing after exit_task_work(), which isn't an issue in the thread case as the work will simply be ignored for an exiting task but it's a big issue in the case of process wide handling. Anyway, the current layout is simple enough.
