Re: [Xen-devel] [PATCH v2] xen/sched: rework and rename vcpu_force_reschedule()

[Juergen Gross <jgross@xxxxxxxx>]

On 16.09.19 16:39, Jan Beulich wrote:
> On 16.09.2019 14:49, Juergen Gross wrote:
> > On 16.09.19 11:20, Jan Beulich wrote:
> > > On 14.09.2019 08:42, Juergen Gross wrote:
> > > > vcpu_force_reschedule() is only used for modifying the periodic timer
> > > > of a vcpu. Forcing a vcpu to give up the physical cpu for that purpose
> > > > is kind of brutal.
> > > >
> > > > So instead of doing the reschedule dance just operate on the timer
> > > > directly. By protecting periodic timer modifications against concurrent
> > > > timer activation via a per-vcpu lock it is even no longer required to
> > > > bother the target vcpu at all for updating its timer.
> > > >
> > > > Rename the function to vcpu_set_periodic_timer() as this now reflects
> > > > the functionality.
> > > >
> > > > Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
> > > I continue to be unhappy about there being no word at all about ...
> > >
> > > > @@ -724,24 +725,6 @@ static void vcpu_migrate_finish(struct vcpu *v)
> > > >        vcpu_wake(v);
> > > >    }
> > > >    
> > > > -/*
> > > > - * Force a VCPU through a deschedule/reschedule path.
> > > > - * For example, using this when setting the periodic timer period means that
> > > > - * most periodic-timer state need only be touched from within the scheduler
> > > > - * which can thus be done without need for synchronisation.
> > > > - */
> > > > -void vcpu_force_reschedule(struct vcpu *v)
> > > ... the originally intended synchronization-free handling. Forcing
> > > the vCPU through the scheduler may seem harsh (and quite some
> > > overhead), yes, but I don't think the above was written (and
> > > decided) without consideration. One effect of this can be seen by
> > > you ...
> > >
> > > > +void vcpu_set_periodic_timer(struct vcpu *v, s_time_t value)
> > > > +{
> > > > +    spin_lock(&v->periodic_timer_lock);
> > > > +
> > > > +    stop_timer(&v->periodic_timer);
> > > ... introducing a new stop_timer() here, i.e. which doesn't replace
> > > any existing one. The implication is that other than before the
> > > periodic timer may now not run (for a brief moment) despite it
> > > being supposed to run - after all it has been active so far
> > > whenever a vCPU was running.
> > >
> > > Then again, looking at the involved code paths yet again, I wonder
> > > whether this has been working right at all: There's an early exit
> > > from schedule() when prev == next, which bypasses
> > > vcpu_periodic_timer_work(). And I can't seem to be able to spot
> > > anything on the vcpu_force_reschedule() path which would guarantee
> > > this shortcut to not be taken.
> > First, the current "synchronization-free" handling is not existing. The
> > synchronization is just hidden in the calls of vcpu_migrate_*() and it
> > is done via the scheduler lock.
> Sure, but the scheduler lock needs to be taken during scheduling
> of the vCPU anyway. There was no "extra" synchronization involved.
Of course there was. The scheduling path was forced to happen, this
resulted in the extra synchronization (twice in fact, once for
de-scheduling and once for scheduling in again).

> > Yes, I'm adding a stop_timer(), but the related stop_timer() call in
> > the old code was in schedule(). So statically you are right, but
> > dynamically there is no new stop_timer() call involved.
> I did specifically check that my comment is not just about the
> "static" part (as you call it). As said - there was no stop_timer()
> before behind a running vCPU's back. This is what worries me.
There is a stop_timer() for the periodic timer happening each time the
vcpu is de-scheduled (look into schedule()). And as setting the periodic
timer today results in a de-scheduling of the vcpu the stop_timer() is
happening. I assume here that the de-scheduling would not have happened
without setting the timer, of course. In case the vcpu is not running
when the timer is being set, my patch will call stop_timer() for a
timer already being stopped.

> > And last: the case prev == next would not occur today, as the migrate
> > flag being set in vcpu->pause_flags would cause the vcpu to be taken
> > away from the cpu. So it is working today, but setting the periodic
> > timer requires two scheduling events in case the target vcpu is
> > currently running.
> I'm not going to claim I fully understood the code when looking at
> it in the morning, but I couldn't find the place(s) guaranteeing
> that by the time the migration of the vCPU is over it wouldn't be
> runnable again right away, and hence potentially re-chosen as the
> vCPU to run on the pCPU is was running on before.
vcpu_migrate_start() is setting _VPF_migrating in the vcpu's
pause_flags and then initiates a scheduling event via
vcpu_sleep_nosync(). vcpu_migrate_finish() will only reset the flag
if the vcpu is not running (so either it wasn't running before, or the
scheduling event already has happened). If it is still running it will
not reset the flag, but this will only be done via context_saved(),
which is called when the vcpus has been de-scheduled. The vcpu can
only be selected to be running again when the migrated flag is not set.


Juergen

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