Re: [PATCH v6 21/23] virt/steal_monitor: Add direction control

[Yury Norov]

On Wed, Jul 01, 2026 at 07:46:52PM +0530, Shrikanth Hegde wrote:
> Cache the previous direction on steal time. So two consecutive values of
> high values or low values are taken for decrease/increase of preferred
> CPUs. This helps to avoid oscillations.
> 
> Signed-off-by: Shrikanth Hegde <sshegde@xxxxxxxxxxxxx>
> ---
>  drivers/virt/steal_monitor/sm_core.c | 20 ++++++++++++++++++--
>  1 file changed, 18 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/virt/steal_monitor/sm_core.c b/drivers/virt/steal_monitor/sm_core.c
> index 7b7435f79b85..4810bad96818 100644
> --- a/drivers/virt/steal_monitor/sm_core.c
> +++ b/drivers/virt/steal_monitor/sm_core.c
> @@ -20,6 +20,12 @@ struct steal_monitor sm_core_ctx = {
>  	.low_threshold = 200,	/* 2% */
>  };
>  
> +enum sm_direction {
> +	SM_DIR_INCREASE = -1,
> +	SM_DIR_NONE	=  0,
> +	SM_DIR_DECREASE	=  1,
> +};
> +
>  module_param_named(interval_ms, sm_core_ctx.interval_ms, uint, 0644);
>  MODULE_PARM_DESC(interval_ms,
>  		 "Sampling frequency for steal values in milliseconds (default: 1000)");
> @@ -59,12 +65,22 @@ static void compute_preferred_cpus_work(struct work_struct *work)
>  
>  	steal_ratio = div64_u64(delta_steal, delta_ns);
>  	/* If the steal time values are high, reduce preferred CPUs */
> -	if (steal_ratio > sm_core_ctx.high_threshold)
> +	if (sm_core_ctx.prev_direction == SM_DIR_DECREASE &&
> +	    steal_ratio > sm_core_ctx.high_threshold)
>  		decrease_preferred_cpus(&sm_core_ctx);
>  	/* If the steal time values are low, increase preferred CPUs */
> -	if (steal_ratio <= sm_core_ctx.low_threshold)
> +	if (sm_core_ctx.prev_direction == SM_DIR_INCREASE &&
> +	    steal_ratio <= sm_core_ctx.low_threshold)
>  		increase_preferred_cpus(&sm_core_ctx);
>  
> +	/* mark the direction. This helps to avoid ping-pongs */

Increasing the gap between hi and lo_threshold helps to avoid
ping-pongs.

> +	if (steal_ratio > sm_core_ctx.high_threshold)
> +		sm_core_ctx.prev_direction = SM_DIR_DECREASE;
> +	else if (steal_ratio <= sm_core_ctx.low_threshold)
> +		sm_core_ctx.prev_direction = SM_DIR_INCREASE;
> +	else
> +		sm_core_ctx.prev_direction = SM_DIR_NONE;
> +
>  	/* At least one core is kept as preferred */
>  	WARN_ON(cpumask_empty(cpu_preferred_mask));
>  
> -- 
> 2.47.3