On Mon, 15 May 2023 21:47:03 -0400 Vineeth Remanan Pillai <vineeth@xxxxxxxxxxxxxxx> wrote: > Hi Luca, > > On Mon, May 15, 2023 at 4:06 AM luca abeni > <luca.abeni@xxxxxxxxxxxxxxx> wrote: > > > > > this patch is giving me some headaches: > > > Sorry about that.. I was also stressing out on how to get the > reclaiming done right for the past couple of days ;-) Well, this math is hard... :) > > Vineeth Pillai <vineeth@xxxxxxxxxxxxxxx> wrote: > > [...] > > > * Uextra: Extra bandwidth not reserved: > > > - * = Umax - \Sum(u_i / #cpus in the root > > > domain) > > > + * = Umax - this_bw > > > > While I agree that this setting should be OK, it ends up with > > dq = -Uact / Umax * dt > > which I remember I originally tried, and gave some issues > > (I do not remember the details, but I think if you try N > > identical reclaiming tasks, with N > M, the reclaimed time > > is not distributed equally among them?) > > > I have noticed this behaviour where the reclaimed time is not equally > distributed when we have more tasks than available processors. But it > depended on where the task was scheduled. Within the same cpu, the > distribution seemed to be proportional. Yes, as far as I remember it is due to migrations. IIRC, the problem is related to the fact that using "dq = -Uact / Umax * dt" a task running on a core might end up trying to reclaim some idle time from other cores (which is obviously not possible). This is why m-GRUB used "1 - Uinact" instead of "Uact" [...] > > I need to think a little bit more about this... > > > Thanks for looking into this.. I have a basic idea why tasks with less > bandwidth reclaim less in SMP when number of tasks is less than number > of cpus, but do not yet have a verifiable fix for it. I think I can now understand at least part of the problem. In my understanding, the problem is due to using dq = -(max{u_i, (Umax - Uinact - Uextra)} / Umax) * dt It should really be dq = -(max{u_i, (1 - Uinact - Uextra)} / Umax) * dt (since we divide by Umax, using "Umax - ..." will lead to reclaiming up to "Umax / Umax" = 1) Did you try this equation? I'll write more about this later... And thanks for coping with all my comments! Luca > > If patches 1 and 4 looks good to you, we shall drop 2 and 3 and fix > the SMP issue with varying bandwidth separately.. Patch 4 would > differ a bit when I remove 2 and 3 so as to use the formula: > "dq = -(max{u, (Umax_reclaim - Uinact - Uextra)} / Umax_reclaim) dt" > > Thanks for your patience with all these brainstorming:-) > > Thanks, > Vineeth