Re: [patch v7 02/10] add prctl task isolation prctl docs and samples

[Marcelo Tosatti <mtosatti@redhat.com>]

On Thu, Dec 02, 2021 at 06:13:20PM +0100, Frederic Weisbecker wrote:
> On Mon, Nov 29, 2021 at 12:13:25PM -0300, Marcelo Tosatti wrote:
> > On Tue, Nov 23, 2021 at 01:36:20PM +0100, Frederic Weisbecker wrote:
> > > On Fri, Nov 12, 2021 at 09:35:33AM -0300, Marcelo Tosatti wrote:
> > > > +**PR_ISOL_CFG_SET**:
> > > > +
> > > > +        Set task isolation configuration.
> > > > +        The general format is::
> > > > +
> > > > +                prctl(PR_ISOL_CFG_SET, what, arg3, arg4, arg5);
> > > > +
> > > > +        The 'what' argument specifies what to configure. Possible values are:
> > > > +
> > > > +        - ``I_CFG_FEAT``:
> > > > +
> > > > +                Set configuration of task isolation features. 'arg3' specifies
> > > > +                the feature. Possible values are:
> > > > +
> > > > +                - ``ISOL_F_QUIESCE``:
> > > > +
> > > > +                        If arg4 is QUIESCE_CONTROL, set the control structure
> > > > +                        for quiescing of background kernel activities, from
> > > > +                        the location pointed to by ``(int *)arg5``::
> > > > +
> > > > +                         struct task_isol_quiesce_control {
> > > > +                                __u64 flags;
> > > > +                                __u64 quiesce_mask;
> > > > +                                __u64 quiesce_oneshot_mask;
> > > > +                                __u64 pad[5];
> > > > +                         };
> > > > +
> > > > +                        Where:
> > > > +
> > > > +                        *flags*: Additional flags (should be zero).
> > > > +
> > > > +                        *quiesce_mask*: A bitmask containing which kernel
> > > > +                        activities to quiesce.
> > > > +
> > > > +                        *quiesce_oneshot_mask*: A bitmask indicating which kernel
> > > > +                        activities should behave in oneshot mode, that is, quiescing
> > > > +                        will happen on return from prctl(PR_ISOL_ACTIVATE_SET), but not
> > > > +                        on return of subsequent system calls. The corresponding bit(s)
> > > > +                        must also be set at quiesce_mask.
> > > > +
> > > > +                        *pad*: Additional space for future enhancements.
> > > > +
> > > > +                        For quiesce_mask (and quiesce_oneshot_mask), possible bit sets are:
> > > > +
> > > > +                        - ``ISOL_F_QUIESCE_VMSTATS``
> > > > +
> > > > +                        VM statistics are maintained in per-CPU counters to
> > > > +                        improve performance. When a CPU modifies a VM statistic,
> > > > +                        this modification is kept in the per-CPU counter.
> > > > +                        Certain activities require a global count, which
> > > > +                        involves requesting each CPU to flush its local counters
> > > > +                        to the global VM counters.
> > > > +
> > > > +                        This flush is implemented via a workqueue item, which
> > > > +                        might schedule a workqueue on isolated CPUs.
> > > > +
> > > > +                        To avoid this interruption, task isolation can be
> > > > +                        configured to, upon return from system calls, synchronize
> > > > +                        the per-CPU counters to global counters, thus avoiding
> > > > +                        the interruption.
> > > 
> > > Sorry I know this is already v7 but we really don't want to screw up this interface.
> > 
> > No problem.
> > 
> > > What would be more simple and flexible for individual features to quiesce:
> > > 
> > >    arg3 = ISOL_F_QUIESCE
> > >    arg4 = which feature to quiesce (eg: ISOL_F_QUIESCE_VMSTATS)
> > 
> > arg4 is QUIESCE_CONTROL today so one can expand the interface
> > (by adding new commands).
> > 
> > >    arg5 =
> > > 
> > >        struct task_isol_quiesce_control {
> > >            __u64 flags; //with ONESHOT as the first and only possible flag for now
> > >            __u64 pad[5];
> > >        };
> > 
> > So your idea is to allow expansion at this level ? Say while adding
> > a new
> > 
> > ISOL_F_QUIESCE_NEWITEM
> > 
> > one can add
> > 
> > 	struct task_isol_quiesce_control_newitem {
> > 		__u64 flags;
> > 		__u64 pad[5];
> > 	};
> > 
> > And add new fields to "struct task_isol_quiesce_control_newitem".
> > 
> > One downside of this suggestion is that for use-cases of the task_isol_computation.c type,
> > (see patch 2 "add prctl task isolation prctl docs and samples"), this might need
> > multiple system calls for each enable/disable cycle. Is that OK?
> > 
> > See more below.
> > 
> > > This way we can really do a finegrained control over each feature to quiesce.
> > 
> > With the patchset above, one can add new values to arg4 
> > (at the same level of QUIESCE_CONTROL). Your suggestion does not save
> > room for that.
> > 
> > One could add new values to the same space of I_CFG_FEAT:
> > 
> >           prctl(PR_ISOL_CFG_SET, I_CFG_FEAT_xxx, ...);
> > 
> > But that sounds awkward.
> > 
> > Or change the current ioctl to:
> > 
> >           prctl(PR_ISOL_CFG, I_CFG_FEAT_CONTROL, ...);
> > 
> > Which makes it less awkward.
> > 
> > What do you say?
> > 
> > --- 
> > 
> > And then, what about keeping the bitmaps with enabled/one-shot mode
> > per feature per bit (to avoid multiple system calls)
> > but having (in the future) additional per-quiesce instance commands ?
> 
> Ok got your points.
> 
> I guess we can then simply rename ISOL_F_QUIESCE to ISOL_F_QUIESCE_MULTIPLE
> for simple all-in-one configuration. Then if the need ever arise in the future,
> we can always add ISOL_F_QUIESCE (or ISOL_F_QUIESCE_ONE) to do finegrained
> control over a single quiescing feature.
> 
> Does that sound ok?

Yep, it does, will change that.