Re: [PATCH v3 02/14] sched_ext: Use shorter slice in bypass mode

[Andrea Righi <arighi@nvidia.com>]

On Tue, Nov 11, 2025 at 07:43:19AM -1000, Tejun Heo wrote:
> There have been reported cases of bypass mode not making forward progress fast
> enough. The 20ms default slice is unnecessarily long for bypass mode where the
> primary goal is ensuring all tasks can make forward progress.
> 
> Introduce SCX_SLICE_BYPASS set to 5ms and make the scheduler automatically
> switch to it when entering bypass mode. Also make the bypass slice value
> tunable through the slice_bypass_us module parameter (adjustable between 100us
> and 100ms) to make it easier to test whether slice durations are a factor in
> problem cases.
> 
> v3: Use READ_ONCE/WRITE_ONCE for scx_slice_dfl access (Dan).
> 
> v2: Removed slice_dfl_us module parameter. Fixed typos (Andrea).
> 
> Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
> Cc: Dan Schatzberg <schatzberg.dan@gmail.com>
> Cc: Andrea Righi <andrea.righi@linux.dev>
> Signed-off-by: Tejun Heo <tj@kernel.org>

Looks good.

Reviewed-by: Andrea Righi <arighi@nvidia.com>

Thanks,
-Andrea

> ---
>  include/linux/sched/ext.h |   11 +++++++++++
>  kernel/sched/ext.c        |   34 +++++++++++++++++++++++++++++++---
>  2 files changed, 42 insertions(+), 3 deletions(-)
> 
> --- a/include/linux/sched/ext.h
> +++ b/include/linux/sched/ext.h
> @@ -17,7 +17,18 @@
>  enum scx_public_consts {
>  	SCX_OPS_NAME_LEN	= 128,
>  
> +	/*
> +	 * %SCX_SLICE_DFL is used to refill slices when the BPF scheduler misses
> +	 * to set the slice for a task that is selected for execution.
> +	 * %SCX_EV_REFILL_SLICE_DFL counts the number of times the default slice
> +	 * refill has been triggered.
> +	 *
> +	 * %SCX_SLICE_BYPASS is used as the slice for all tasks in the bypass
> +	 * mode. As making forward progress for all tasks is the main goal of
> +	 * the bypass mode, a shorter slice is used.
> +	 */
>  	SCX_SLICE_DFL		= 20 * 1000000,	/* 20ms */
> +	SCX_SLICE_BYPASS	=  5 * 1000000, /*  5ms */
>  	SCX_SLICE_INF		= U64_MAX,	/* infinite, implies nohz */
>  };
>  
> --- a/kernel/sched/ext.c
> +++ b/kernel/sched/ext.c
> @@ -143,6 +143,32 @@ static struct scx_dump_data scx_dump_dat
>  /* /sys/kernel/sched_ext interface */
>  static struct kset *scx_kset;
>  
> +/*
> + * Parameters that can be adjusted through /sys/module/sched_ext/parameters.
> + * There usually is no reason to modify these as normal scheduler operation
> + * shouldn't be affected by them. The knobs are primarily for debugging.
> + */
> +static u64 scx_slice_dfl = SCX_SLICE_DFL;
> +static unsigned int scx_slice_bypass_us = SCX_SLICE_BYPASS / NSEC_PER_USEC;
> +
> +static int set_slice_us(const char *val, const struct kernel_param *kp)
> +{
> +	return param_set_uint_minmax(val, kp, 100, 100 * USEC_PER_MSEC);
> +}
> +
> +static const struct kernel_param_ops slice_us_param_ops = {
> +	.set = set_slice_us,
> +	.get = param_get_uint,
> +};
> +
> +#undef MODULE_PARAM_PREFIX
> +#define MODULE_PARAM_PREFIX	"sched_ext."
> +
> +module_param_cb(slice_bypass_us, &slice_us_param_ops, &scx_slice_bypass_us, 0600);
> +MODULE_PARM_DESC(slice_bypass_us, "bypass slice in microseconds, applied on [un]load (100us to 100ms)");
> +
> +#undef MODULE_PARAM_PREFIX
> +
>  #define CREATE_TRACE_POINTS
>  #include <trace/events/sched_ext.h>
>  
> @@ -919,7 +945,7 @@ static void dsq_mod_nr(struct scx_dispat
>  
>  static void refill_task_slice_dfl(struct scx_sched *sch, struct task_struct *p)
>  {
> -	p->scx.slice = SCX_SLICE_DFL;
> +	p->scx.slice = READ_ONCE(scx_slice_dfl);
>  	__scx_add_event(sch, SCX_EV_REFILL_SLICE_DFL, 1);
>  }
>  
> @@ -2896,7 +2922,7 @@ void init_scx_entity(struct sched_ext_en
>  	INIT_LIST_HEAD(&scx->runnable_node);
>  	scx->runnable_at = jiffies;
>  	scx->ddsp_dsq_id = SCX_DSQ_INVALID;
> -	scx->slice = SCX_SLICE_DFL;
> +	scx->slice = READ_ONCE(scx_slice_dfl);
>  }
>  
>  void scx_pre_fork(struct task_struct *p)
> @@ -3774,6 +3800,7 @@ static void scx_bypass(bool bypass)
>  		WARN_ON_ONCE(scx_bypass_depth <= 0);
>  		if (scx_bypass_depth != 1)
>  			goto unlock;
> +		WRITE_ONCE(scx_slice_dfl, scx_slice_bypass_us * NSEC_PER_USEC);
>  		bypass_timestamp = ktime_get_ns();
>  		if (sch)
>  			scx_add_event(sch, SCX_EV_BYPASS_ACTIVATE, 1);
> @@ -3782,6 +3809,7 @@ static void scx_bypass(bool bypass)
>  		WARN_ON_ONCE(scx_bypass_depth < 0);
>  		if (scx_bypass_depth != 0)
>  			goto unlock;
> +		WRITE_ONCE(scx_slice_dfl, SCX_SLICE_DFL);
>  		if (sch)
>  			scx_add_event(sch, SCX_EV_BYPASS_DURATION,
>  				      ktime_get_ns() - bypass_timestamp);
> @@ -4780,7 +4808,7 @@ static int scx_enable(struct sched_ext_o
>  			queue_flags |= DEQUEUE_CLASS;
>  
>  		scoped_guard (sched_change, p, queue_flags) {
> -			p->scx.slice = SCX_SLICE_DFL;
> +			p->scx.slice = READ_ONCE(scx_slice_dfl);
>  			p->sched_class = new_class;
>  		}
>  	}