Re: [PATCH v6 17/23] virt/steal_monitor: Provide default method to get systemwide steal time

[Shrikanth Hegde]

On 7/4/26 12:46 AM, Yury Norov wrote:
> On Wed, Jul 01, 2026 at 07:46:48PM +0530, Shrikanth Hegde wrote:
> > steal monitor takes global view of steal time instead of individual
> > vCPU. For this collect overall steal values across all the vCPUs or
> > vCPUs of interest.
> >
> > Default implementation chooses steal time across all active CPUs.
> >
> > Signed-off-by: Shrikanth Hegde <sshegde@xxxxxxxxxxxxx>
> > ---
> > v5->v6:
> > - Add cpus_read_lock() for hotplug safety
> >
> >   drivers/virt/steal_monitor/Makefile   |  2 +-
> >   drivers/virt/steal_monitor/defaults.c | 28 +++++++++++++++++++++++++++
> >   drivers/virt/steal_monitor/sm_core.h  |  3 +++
> >   3 files changed, 32 insertions(+), 1 deletion(-)
> >   create mode 100644 drivers/virt/steal_monitor/defaults.c
> >
> > diff --git a/drivers/virt/steal_monitor/Makefile b/drivers/virt/steal_monitor/Makefile
> > index 24cee55342ce..7c16f8cf9583 100644
> > --- a/drivers/virt/steal_monitor/Makefile
> > +++ b/drivers/virt/steal_monitor/Makefile
> > @@ -11,4 +11,4 @@
> >   #
> >   obj-$(subst y,m,$(CONFIG_PREFERRED_CPU)) += steal_monitor.o
> >   
> > -steal_monitor-y := sm_core.o
> > +steal_monitor-y := sm_core.o defaults.o
> > diff --git a/drivers/virt/steal_monitor/defaults.c b/drivers/virt/steal_monitor/defaults.c
> > new file mode 100644
> > index 000000000000..6681f9938f6a
> > --- /dev/null
> > +++ b/drivers/virt/steal_monitor/defaults.c
> > @@ -0,0 +1,28 @@
> > +// SPDX-License-Identifier: GPL-2.0-only
> > +/*
> > + * Base file contains the default implementations.
> > + * These are defined as __weak so that arch may define
> > + * strong symbols to override.
>
> But there's no arch doing that, right? It looks like overcomplication
> with no benefit.
>
> There's just ~50 drivers defining weak symbols, so this is not a
> common practice. I think it would be much simpler for the arch people
> to simply write their own driver controlling cpu_preferred_mask,
> rather than tweaking your code.

s390 folks asked for explicit hooks since they have some of the info
coming from HW w.r.t to vertical high/vertical low etc. So i have designed it that way.
(I know code is not there yet)

If each arch writes its own driver, wouldn't that lead to
- code duplication as each arch more or less may have to do same.
- each arch has to catch up any improvements that happens to default monitor.
- each arch has to honor design construct and not violate it.
- kconfig puzzles, to choose generic or arch specific ones etc.


If it makes sense,
- I can make it as strong symbols for now.
- when the arch specific implementations arrive, can make it as
  __weak to accommodate that.