Re: [PATCH 01/15] sched/fair: Add avg_vruntime

[Abel Wu]

On 5/31/23 7:58 PM, Peter Zijlstra wrote:
> +/*
> + * Compute virtual time from the per-task service numbers:
> + *
> + * Fair schedulers conserve lag:
> + *
> + *   \Sum lag_i = 0
> + *
> + * Where lag_i is given by:
> + *
> + *   lag_i = S - s_i = w_i * (V - v_i)

Since the ideal service time S is task-specific, should this be:

	lag_i = S_i - s_i = w_i * (V - v_i)

> + *
> + * Where S is the ideal service time and V is it's virtual time counterpart.
> + * Therefore:
> + *
> + *   \Sum lag_i = 0
> + *   \Sum w_i * (V - v_i) = 0
> + *   \Sum w_i * V - w_i * v_i = 0
> + *
> + * From which we can solve an expression for V in v_i (which we have in
> + * se->vruntime):
> + *
> + *       \Sum v_i * w_i   \Sum v_i * w_i
> + *   V = -------------- = --------------
> + *          \Sum w_i            W
> + *
> + * Specifically, this is the weighted average of all entity virtual runtimes.
> + *
> + * [[ NOTE: this is only equal to the ideal scheduler under the condition
> + *          that join/leave operations happen at lag_i = 0, otherwise the
> + *          virtual time has non-continguous motion equivalent to:
> + *
> + *	      V +-= lag_i / W
> + *
> + *	    Also see the comment in place_entity() that deals with this. ]]
> + *
> + * However, since v_i is u64, and the multiplcation could easily overflow
> + * transform it into a relative form that uses smaller quantities:
> + *
> + * Substitute: v_i == (v_i - v0) + v0
> + *
> + *     \Sum ((v_i - v0) + v0) * w_i   \Sum (v_i - v0) * w_i
> + * V = ---------------------------- = --------------------- + v0
> + *                  W                            W
> + *
> + * Which we track using:
> + *
> + *                    v0 := cfs_rq->min_vruntime
> + * \Sum (v_i - v0) * w_i := cfs_rq->avg_vruntime

IMHO 'sum_runtime' would be more appropriate? Since it actually is
the summed real time rather than virtual time. And also 'sum_load'
instead of 'avg_load'.

Thanks & Best,
	Abel