On Mon, Oct 04, 2021 at 09:12:37AM +0100, Tvrtko Ursulin wrote:
On 01/10/2021 16:48, Peter Zijlstra wrote:
Hmm? That's for normalize_rt_tasks() only, right? Just don't have it call the notifier in that special case (that's a magic sysrq thing anyway).
You mean my talk about tasklist_lock? No, it is also on the syscall part I am interested in as well. Call chain looks like this:
Urgh, I alwys miss that because it lives outside of sched.. :/
sys_setpriority() { ... rcu_read_lock(); read_lock(&tasklist_lock); ... set_one_prio() set_user_nice() { ... task_rq_lock(); -> my notifier from this RFC [1] task_rq_unlock(); -> I can move the notifier here for _some_ improvement [2] } ... read_unlock(&tasklist_lock); rcu_read_unlock(); }
So this RFC had the notifier call chain at [1], which I understood was the thing you initially pointed was horrible, being under a scheduler lock.
I can trivially move it to [2] but that still leaves it under the tasklist lock. I don't have a good feel how much better that would be. If not good enough then I will look for a smarter solution with less opportunity for global impact.
So task_list lock is pretty terrible and effectively unbound already (just create more tasks etc..) so adding a notifier call there shouldn't really make it much worse.