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

[Shrikanth Hegde <sshegde@linux.ibm.com>]

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@foss.st.com>
>>   L:	linux-sound@vger.kernel.org
>> 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@linux.ibm.com>
>> + */
>> +
>> +#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