Messages in this thread | | | Date | Sat, 8 Jan 2022 01:03:08 +0100 | From | Frederic Weisbecker <> | Subject | Re: [patch v8 02/10] add prctl task isolation prctl docs and samples |
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On Fri, Jan 07, 2022 at 08:30:01AM -0300, Marcelo Tosatti wrote: > On Fri, Jan 07, 2022 at 12:49:56AM +0100, Frederic Weisbecker wrote: > > On Wed, Dec 08, 2021 at 01:09:08PM -0300, Marcelo Tosatti wrote: > > > Add documentation and userspace sample code for prctl > > > task isolation interface. > > > > > > Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com> > > > > Acked-by: Frederic Weisbecker <frederic@kernel.org> > > > > Thanks a lot! Time for me to look at the rest of the series. > > > > Would be nice to have Thomas's opinion as well at least on > > the interface (this patch). > > Yes. AFAIAW most of his earlier comments on what the > interface should look like have been addressed (or at > least i've tried to)... including the ability for > the system admin to configure the isolation options. > > The one thing missing is to attempt to enter nohz_full > on activation (which Christoph asked for). > > Christoph, have a question on that. At > https://lkml.org/lkml/2021/12/14/346, you wrote: > > "Applications running would ideally have no performance penalty and there > is no issue with kernel activity unless the application is in its special > low latency loop. NOHZ is currently only activated after spinning in that > loop for 2 seconds or so. Would be best to be able to trigger that > manually somehow." > > So was thinking of something similar to what the full task isolation > patchset does (with the behavior of returning an error as option...): > > +int try_stop_full_tick(void) > +{ > + int cpu = smp_processor_id(); > + struct tick_sched *ts = this_cpu_ptr(&tick_cpu_sched); > + > + /* For an unstable clock, we should return a permanent error code. */ > + if (atomic_read(&tick_dep_mask) & TICK_DEP_MASK_CLOCK_UNSTABLE) > + return -EINVAL; > + > + if (!can_stop_full_tick(cpu, ts)) > + return -EAGAIN; > + > + tick_nohz_stop_sched_tick(ts, cpu); > + return 0; > +} > > Is that sufficient? (note it might still be possible > for a failure to enter nohz_full due to a number of > reasons), see tick_nohz_stop_sched_tick.
Well, I guess we can simply make tick_nohz_full_update_tick() an API, then it could be a QUIESCE feature.
But keep in mind we may not only fail to enter into nohz_full mode, we may also enter it but, instead of completely stopping the tick, it can be delayed to some future if there is still a timer callback queued somewhere.
Make sure you test "ts->next_tick == KTIME_MAX" after stopping the tick.
This raise the question: what do we do if a quiescing fails? At least if it's a oneshot, we can return an -EBUSY from the prctl() but otherwise, subsequent kernel entry/exit are a problem.
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