Hi Peter, Thanks for sharing your thoughts. I understand your point and I tend to agree with it. I originally designed this using watchdog as the example of a critical system health signal and in the context of mobile device memory pressure is critical but I agree that there are more important things in life. I checked and your proposal to change it to FIFO-1 should still work for our purposes. Will test to make sure and reply to your patch. Couple clarifications in-line. On Thu, Aug 1, 2019 at 2:51 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote: > > On Tue, Jul 30, 2019 at 10:44:51AM -0700, Suren Baghdasaryan wrote: > > On Tue, Jul 30, 2019 at 1:11 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote: > > > > > > On Mon, Jul 29, 2019 at 06:33:10PM -0700, Suren Baghdasaryan wrote: > > > > When a process creates a new trigger by writing into /proc/pressure/* > > > > files, permissions to write such a file should be used to determine whether > > > > the process is allowed to do so or not. Current implementation would also > > > > require such a process to have setsched capability. Setting of psi trigger > > > > thread's scheduling policy is an implementation detail and should not be > > > > exposed to the user level. Remove the permission check by using _nocheck > > > > version of the function. > > > > > > > > Suggested-by: Nick Kralevich <nnk@xxxxxxxxxx> > > > > Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx> > > > > --- > > > > kernel/sched/psi.c | 2 +- > > > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > > > > > diff --git a/kernel/sched/psi.c b/kernel/sched/psi.c > > > > index 7acc632c3b82..ed9a1d573cb1 100644 > > > > --- a/kernel/sched/psi.c > > > > +++ b/kernel/sched/psi.c > > > > @@ -1061,7 +1061,7 @@ struct psi_trigger *psi_trigger_create(struct psi_group *group, > > > > mutex_unlock(&group->trigger_lock); > > > > return ERR_CAST(kworker); > > > > } > > > > - sched_setscheduler(kworker->task, SCHED_FIFO, ¶m); > > > > + sched_setscheduler_nocheck(kworker->task, SCHED_FIFO, ¶m); > > > > > > ARGGH, wtf is there a FIFO-99!! thread here at all !? > > > > We need psi poll_kworker to be an rt-priority thread so that psi > > There is a giant difference between 'needs to be higher than OTHER' and > FIFO-99. > > > notifications are delivered to the userspace without delay even when > > the CPUs are very congested. Otherwise it's easy to delay psi > > notifications by running a simple CPU hogger executing "chrt -f 50 dd > > if=/dev/zero of=/dev/null". Because these notifications are > > So what; at that point that's exactly what you're asking for. Using RT > is for those who know what they're doing. > > > time-critical for reacting to memory shortages we can't allow for such > > delays. > > Furthermore, actual RT programs will have pre-allocated and locked any > memory they rely on. They don't give a crap about your pressure > nonsense. > This signal is used not to protect other RT tasks but to monitor overall system memory health for the sake of system responsiveness. > > Notice that this kworker is created only if userspace creates a psi > > trigger. So unless you are using psi triggers you will never see this > > kthread created. > > By marking it FIFO-99 you're in effect saying that your stupid > statistics gathering is more important than your life. It will preempt > the task that's in control of the band-saw emergency break, it will > preempt the task that's adjusting the electromagnetic field containing > this plasma flow. > > That's insane. IMHO an opt-in feature stops being "stupid" as soon as the user opted in to use it, therefore explicitly indicating interest in it. However I assume you are using "stupid" here to indicate that it's "less important" rather than it's "useless". > I'm going to queue a patch to reduce this to FIFO-1, that will preempt > regular OTHER tasks but will not perturb (much) actual RT bits. > Thanks for posting the fix. > -- > To unsubscribe from this group and stop receiving emails from it, send an email to kernel-team+unsubscribe@xxxxxxxxxxx. >
