Messages in this thread | | | Date | Mon, 24 Jan 2022 15:20:25 -0300 | From | Marcelo Tosatti <> | Subject | Re: [patch v8 02/10] add prctl task isolation prctl docs and samples |
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On Mon, Jan 24, 2022 at 03:10:42PM -0300, Marcelo Tosatti wrote: > On Sat, Jan 08, 2022 at 01:03:08AM +0100, Frederic Weisbecker wrote: > > 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. > > Well, maybe two modes can be specified for the NOHZ_FULL task isolation > feature. On activation of task isolation: > > - Hint (default). Attempt to enter nohz_full mode, > continue if unable to do so. > > - Mandatory. Return an error if unable to enter nohz_full mode > (tracing required to determine actual reason. is that OK?)
This mode is poorly defined. What happens if some event after task isolation activation causes nohz_full mode to be disabled ?
Or an alternative is to let the verification of nohz_full mode to take place at a different location, for example a BPF tool. This works for our usecase, i believe.
> > static bool check_tick_dependency(atomic_t *dep) > { > int val = atomic_read(dep); > > if (val & TICK_DEP_MASK_POSIX_TIMER) { > trace_tick_stop(0, TICK_DEP_MASK_POSIX_TIMER); > return true; > } > > if (val & TICK_DEP_MASK_PERF_EVENTS) { > trace_tick_stop(0, TICK_DEP_MASK_PERF_EVENTS); > return true; > } > > if (val & TICK_DEP_MASK_SCHED) { > trace_tick_stop(0, TICK_DEP_MASK_SCHED); > return true; > } > > if (val & TICK_DEP_MASK_CLOCK_UNSTABLE) { > trace_tick_stop(0, TICK_DEP_MASK_CLOCK_UNSTABLE); > return true; > } > > if (val & TICK_DEP_MASK_RCU) { > trace_tick_stop(0, TICK_DEP_MASK_RCU); > return true; > } > > return false; > } > > One thing that can be done on the handlers is to execute any pending irq_work, which > would fix: > > https://lkml.org/lkml/2021/6/18/1174 > > How about that ? >
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