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Date: Thu, 2 Dec 2021 18:13:20 +0100
From: Frederic Weisbecker <frederic@...nel.org>
To: Marcelo Tosatti <mtosatti@...hat.com>
Cc: linux-kernel@...r.kernel.org, Nitesh Lal <nilal@...hat.com>,
Nicolas Saenz Julienne <nsaenzju@...hat.com>,
Christoph Lameter <cl@...ux.com>,
Juri Lelli <juri.lelli@...hat.com>,
Peter Zijlstra <peterz@...radead.org>,
Alex Belits <abelits@...its.com>, Peter Xu <peterx@...hat.com>,
Thomas Gleixner <tglx@...utronix.de>,
Daniel Bristot de Oliveira <bristot@...hat.com>
Subject: Re: [patch v7 02/10] add prctl task isolation prctl docs and samples
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?
Thanks.
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