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








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