Re: [PATCH 04/17] sched/fair: Add avg_vruntime

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Mar 29, 2023 at 09:50:51AM +0200, Peter Zijlstra wrote:
> On Tue, Mar 28, 2023 at 04:57:49PM -0700, Josh Don wrote:
> > On Tue, Mar 28, 2023 at 4:06 AM Peter Zijlstra <peterz@infradead.org> wrote:
> > [...]
> > > +/*
> > > + * Compute virtual time from the per-task service numbers:
> > > + *
> > > + * Fair schedulers conserve lag: \Sum lag_i = 0
> > > + *
> > > + * lag_i = S - s_i = w_i * (V - v_i)
> > > + *
> > > + * \Sum lag_i = 0 -> \Sum w_i * (V - v_i) = V * \Sum w_i - \Sum w_i * v_i = 0
> > 
> > Small note: I think it would be helpful to label these symbols
> > somewhere :) Weight  and vruntime are fairly obvious, but I don't
> > think 'S' and 'V' are as clear. Are these non-virtual ideal service
> > time, and average vruntime, respectively?
> 
> Yep, they are. I'll see what I can do with the comments.


/*
 * 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)
 *
 * 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
 *              \Sum w_i := cfs_rq->avg_load
 *
 * Since min_vruntime is a monotonic increasing variable that closely tracks
 * the per-task service, these deltas: (v_i - v), will be in the order of the
 * maximal (virtual) lag induced in the system due to quantisation.
 *
 * Also, we use scale_load_down() to reduce the size.
 *
 * As measured, the max (key * weight) value was ~44 bits for a kernel build.
 */


And the comment in place_entity() (slightly updated since this morning):


		/*
		 * If we want to place a task and preserve lag, we have to
		 * consider the effect of the new entity on the weighted
		 * average and compensate for this, otherwise lag can quickly
		 * evaporate.
		 *
		 * Lag is defined as:
		 *
		 *   lag_i = S - s_i = w_i * (V - v_i)
		 *
		 * To avoid the 'w_i' term all over the place, we only track
		 * the virtual lag:
		 *
		 *   vl_i = V - v_i <=> v_i = V - vl_i
		 *
		 * And we take V to be the weighted average of all v:
		 *
		 *   V = (\Sum w_j*v_j) / W
		 *
		 * Where W is: \Sum w_j
		 *
		 * Then, the weighted average after adding an entity with lag
		 * vl_i is given by:
		 *
		 *   V' = (\Sum w_j*v_j + w_i*v_i) / (W + w_i)
		 *      = (W*V + w_i*(V - vl_i)) / (W + w_i)
		 *      = (W*V + w_i*V - w_i*vl_i) / (W + w_i)
		 *      = (V*(W + w_i) - w_i*l) / (W + w_i)
		 *      = V - w_i*vl_i / (W + w_i)
		 *
		 * And the actual lag after adding an entity with vl_i is:
		 *
		 *   vl'_i = V' - v_i
		 *         = V - w_i*vl_i / (W + w_i) - (V - vl_i)
		 *         = vl_i - w_i*vl_i / (W + w_i)
		 *
		 * Which is strictly less than vl_i. So in order to preserve lag
		 * we should inflate the lag before placement such that the
		 * effective lag after placement comes out right.
		 *
		 * As such, invert the above relation for vl'_i to get the vl_i
		 * we need to use such that the lag after placement is the lag
		 * we computed before dequeue.
		 *
		 *   vl'_i = vl_i - w_i*vl_i / (W + w_i)
		 *         = ((W + w_i)*vl_i - w_i*vl_i) / (W + w_i)
		 *
		 *   (W + w_i)*vl'_i = (W + w_i)*vl_i - w_i*vl_i
		 *                   = W*vl_i
		 *
		 *   vl_i = (W + w_i)*vl'_i / W
		 */