Re: [PATCH v7 08/12] virt: Introduce steal monitor driver

[Shrikanth Hegde]

Hi Yury,

On 7/11/26 2:23 AM, Yury Norov wrote:
> On Fri, Jul 10, 2026 at 03:26:44AM +0530, Shrikanth Hegde wrote:
> > Introduce a new driver in virt named steal_monitor. This driver
> > will compute the steal time and drive the policy decisions of preferred
> > CPU state.

[...]

> > +
> >   STI AUDIO (ASoC) DRIVERS
> >   M:	Arnaud Pouliquen <arnaud.pouliquen@xxxxxxxxxxx>
> >   L:	linux-sound@xxxxxxxxxxxxxxx
> > diff --git a/drivers/virt/Kconfig b/drivers/virt/Kconfig
> > index 52eb7e4ba71f..a52233b2502e 100644
> > --- a/drivers/virt/Kconfig
> > +++ b/drivers/virt/Kconfig
> > @@ -47,6 +47,8 @@ source "drivers/virt/nitro_enclaves/Kconfig"
> >   
> >   source "drivers/virt/acrn/Kconfig"
> >   
> > +source "drivers/virt/steal_monitor/Kconfig"
> > +
> >   endif
> >   
> >   source "drivers/virt/coco/Kconfig"
> > diff --git a/drivers/virt/Makefile b/drivers/virt/Makefile
> > index f29901bd7820..b67fd8968ec3 100644
> > --- a/drivers/virt/Makefile
> > +++ b/drivers/virt/Makefile
> > @@ -9,4 +9,5 @@ obj-y				+= vboxguest/
> >   
> >   obj-$(CONFIG_NITRO_ENCLAVES)	+= nitro_enclaves/
> >   obj-$(CONFIG_ACRN_HSM)		+= acrn/
> > +obj-$(CONFIG_STEAL_MONITOR)	+= steal_monitor/
> >   obj-y				+= coco/
> > diff --git a/drivers/virt/steal_monitor/Kconfig b/drivers/virt/steal_monitor/Kconfig
> > new file mode 100644
> > index 000000000000..c7d7599c30ce
> > --- /dev/null
> > +++ b/drivers/virt/steal_monitor/Kconfig
> > @@ -0,0 +1,18 @@
> > +# SPDX-License-Identifier: GPL-2.0-only
> > +config STEAL_MONITOR
>
> The config should go in the last patch of the series. Otherwise,
> in case of bisection, you'll have half-written driver enabled by
> default.

Ok. I will keep it a patch at the end.

> > +	tristate "Dynamic vCPU management based on steal time"
> > +	depends on PARAVIRT && SMP
> > +	select PREFERRED_CPU
> > +	default m
> > +	help
> > +	  This driver helps to reduce the steal time in paravirtualised
> > +	  environment, thereby reducing vCPU preemption. Reducing vCPU
> > +	  preemption provides improved lock holder preemption and reduces
> > +	  cost of vCPU preemption in the host.
> > +
> > +	  By default preferred CPUs will be same as active CPUs. Depending
> > +	  on the steal time when steal_monitor driver is enabled,
> > +	  preferred CPUs could become subset of active CPUs.
> > +
> > +	  It is recommended to build it as module and load the module
> > +	  to enable it.
> > diff --git a/drivers/virt/steal_monitor/Makefile b/drivers/virt/steal_monitor/Makefile
> > new file mode 100644
> > index 000000000000..bd7d120a79b5
> > --- /dev/null
> > +++ b/drivers/virt/steal_monitor/Makefile
> > @@ -0,0 +1,6 @@
> > +# SPDX-License-Identifier: GPL-2.0-only
> > +#
> > +# Steal time monitor to alter preferred CPU state.
> > +obj-$(CONFIG_STEAL_MONITOR) += steal_monitor.o
> > +
> > +steal_monitor-y := sm_core.o
> > diff --git a/drivers/virt/steal_monitor/sm_core.c b/drivers/virt/steal_monitor/sm_core.c
> > new file mode 100644
> > index 000000000000..180db424846c
> > --- /dev/null
> > +++ b/drivers/virt/steal_monitor/sm_core.c
> > @@ -0,0 +1,38 @@
> > +// SPDX-License-Identifier: GPL-2.0-only
> > +/*
> > + * Steal time Monitor.
> > + *
> > + * Periodically compute steal time. Based on the thresholds either
> > + * reduce/increase the preferred CPUs which can be used
> > + * by the workload to avoid vCPU preemption to an extent possible.
> > + *
> > + * Available as module with CONFIG_STEAL_MONITOR=m
> > + *
> > + * Copyright (C) 2026 IBM
> > + * Author: Shrikanth Hegde <sshegde@xxxxxxxxxxxxx>
> > + */
> > +
> > +#include "sm_core.h"
> > +
> > +struct steal_monitor sm_core_ctx;
> > +
> > +static int __init steal_monitor_init(void)
> > +{
> > +	pr_info("steal_monitor is enabled\n");
> > +	return 0;
> > +}
> > +
> > +static void __exit steal_monitor_exit(void)
> > +{
> > +	guard(cpus_read_lock)();
> > +	cpumask_copy(&__cpu_preferred_mask, cpu_active_mask);
> > +
> > +	pr_info("steal_monitor is disabled\n");
> > +}
> > +
> > +module_init(steal_monitor_init);
> > +module_exit(steal_monitor_exit);
> > +
> > +MODULE_LICENSE("GPL");
> > +MODULE_AUTHOR("IBM Corporation");
> > +MODULE_DESCRIPTION("Virtualization Steal Time Monitor");
> > diff --git a/drivers/virt/steal_monitor/sm_core.h b/drivers/virt/steal_monitor/sm_core.h
> > new file mode 100644
> > index 000000000000..8bbb606add99
> > --- /dev/null
> > +++ b/drivers/virt/steal_monitor/sm_core.h
> > @@ -0,0 +1,27 @@
> > +/* SPDX-License-Identifier: GPL-2.0-only */
> > +#ifndef __VIRT_STEAL_CORE_H
> > +#define __VIRT_STEAL_CORE_H
> > +
> > +#include <linux/types.h>
> > +
> > +#include <linux/module.h>
> > +#include <linux/kernel.h>
> > +#include <linux/init.h>
> > +#include <linux/cpuhplock.h>
> > +#include <linux/cpumask.h>
> > +#include <linux/workqueue.h>
> > +#include <linux/ktime.h>
> > +
> > +struct steal_monitor {
> > +	struct delayed_work	work;
> > +	u64			prev_steal;
> > +	int			prev_direction;
>
> Did you run pahole on it?

Ah my bad. Will do.

> > +	unsigned int		interval_ms;
> > +	unsigned int		high_threshold;
> > +	unsigned int		low_threshold;
> > +	ktime_t			prev_time;
>
> This 'prev_' prefix is useless and distracting. Just drop it.
>
> > +};

Ok. will keep it as time, steal. (prev_direction is being removed in v8)

> > +
> > +extern struct steal_monitor sm_core_ctx;
> > +
> > +#endif /* __VIRT_STEAL_CORE_H */
> > --
> > 2.47.3