Messages in this thread | | | From | Vineeth Remanan Pillai <> | Date | Sun, 14 May 2023 22:56:55 -0400 | Subject | Re: [PATCH 1/2] sched/deadline: accurate reclaim bandwidth for GRUB |
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Hi Luca,
> Sure, I shall split the patch. Joel also suggested splitting the patch > and I was probably wrong to think that the patch was simple to be a > single patch :-). > > Since equation (3) has theoretical backing, I am perfectly fine with > using it for our fix. Will have 2 patches as you suggested. > > Will get the v3 out soon.. > Sorry for the delay. I was testing the fixes and noticed one other issue and was working on a fix.
This is with the extra_bw for tracking the unused bandwidth in the system. As you have shown me: "extra_bw = Umax - ((Sum of bw of all tasks) / #cpus)" But I noticed that tracking the extra_bw globally while rest of the values as per-cpu causes issues with reclaiming especially when we have tasks with small bandwidth. This is because GRUB tries to proportionately divide unused bandwidth based on running bw on a cpu. So even if there is only a single task in a cpu, it reclaims less if its bandwidth is less.
I think removing the extra_bw and tracking unused bandwidth using "max_bw - this_bw" should be okay. Since admission control will guarantee that we don't admit more than the capacity, we should be good with allowing tasks on a cpu to use max_bw while reclaiming.
With the above theory, the code becomes simpler and is the same as the v2 patch. max{u_i, (max_bw - Uinact - Uextra)} = max {u_i, (max_bw - (this_bw - running_bw) - (max_bw - this_bw))} = max {u_i, running_bw} = running_bw So, "dq = -(running_bw / max_bw) dt"
v2 had passed all my tests. But having Uextra broke couple of those when it came to multiprocessors and after identifying the root cause and re-writing the equation, everything works now and passes all my tests. I have more confidence in the above equation as its derived from the SMP GRUB rule using our max_bw.
Please have a look and let me know what you think about this.
I have the v3 ready with patches split into 5(including doc patch). I shall post v3 soon after this so that you can see the code changes as well to have a better look.
Thanks, Vineeth
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