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

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

On 7/7/26 2:06 AM, Yury Norov wrote:
> 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@linux.ibm.com>
>> ---
>>   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.

Sure.

> 
>> +	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