RE: [PATCH 3/4 net-next] net: mana: add a function to spread IRQs per CPUs

[Haiyang Zhang]

> -----Original Message-----
> From: Michael Kelley <mhklinux@xxxxxxxxxxx>
> Sent: Tuesday, January 9, 2024 2:23 PM
> To: Souradeep Chakrabarti <schakrabarti@xxxxxxxxxxxxxxxxxxx>; KY Srinivasan
> <kys@xxxxxxxxxxxxx>; Haiyang Zhang <haiyangz@xxxxxxxxxxxxx>;
> wei.liu@xxxxxxxxxx; Dexuan Cui <decui@xxxxxxxxxxxxx>;
> davem@xxxxxxxxxxxxx; edumazet@xxxxxxxxxx; kuba@xxxxxxxxxx;
> pabeni@xxxxxxxxxx; Long Li <longli@xxxxxxxxxxxxx>; yury.norov@xxxxxxxxx;
> leon@xxxxxxxxxx; cai.huoqing@xxxxxxxxx; ssengar@xxxxxxxxxxxxxxxxxxx;
> vkuznets@xxxxxxxxxx; tglx@xxxxxxxxxxxxx; linux-hyperv@xxxxxxxxxxxxxxx;
> netdev@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; linux-
> rdma@xxxxxxxxxxxxxxx
> Cc: Souradeep Chakrabarti <schakrabarti@xxxxxxxxxxxxx>; Paul Rosswurm
> <paulros@xxxxxxxxxxxxx>
> Subject: RE: [PATCH 3/4 net-next] net: mana: add a function to spread IRQs per
> CPUs
> 
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> From: Souradeep Chakrabarti <schakrabarti@xxxxxxxxxxxxxxxxxxx> Sent:
> Tuesday, January 9, 2024 2:51 AM
> >
> > From: Yury Norov <yury.norov@xxxxxxxxx>
> >
> > Souradeep investigated that the driver performs faster if IRQs are
> > spread on CPUs with the following heuristics:
> >
> > 1. No more than one IRQ per CPU, if possible;
> > 2. NUMA locality is the second priority;
> > 3. Sibling dislocality is the last priority.
> >
> > Let's consider this topology:
> >
> > Node            0               1
> > Core        0       1       2       3
> > CPU       0   1   2   3   4   5   6   7
> >
> > The most performant IRQ distribution based on the above topology
> > and heuristics may look like this:
> >
> > IRQ     Nodes   Cores   CPUs
> > 0       1       0       0-1
> > 1       1       1       2-3
> > 2       1       0       0-1
> > 3       1       1       2-3
> > 4       2       2       4-5
> > 5       2       3       6-7
> > 6       2       2       4-5
> > 7       2       3       6-7
> 
> I didn't pay attention to the detailed discussion of this issue
> over the past 2 to 3 weeks during the holidays in the U.S., but
> the above doesn't align with the original problem as I understood
> it.  I thought the original problem was to avoid putting IRQs on
> both hyper-threads in the same core, and that the perf
> improvements are based on that configuration.  At least that's
> what the commit message for Patch 4/4 in this series says.
> 
> The above chart results in 8 IRQs being assigned to the 8 CPUs,
> probably with 1 IRQ per CPU.   At least on x86, if the affinity
> mask for an IRQ contains multiple CPUs, matrix_find_best_cpu()
> should balance the IRQ assignments between the CPUs in the mask.
> So the original problem is still present because both hyper-threads
> in a core are likely to have an IRQ assigned.
> 
> Of course, this example has 8 IRQs and 8 CPUs, so assigning an
> IRQ to every hyper-thread may be the only choice.  If that's the
> case, maybe this just isn't a good example to illustrate the
> original problem and solution.  But even with a better example
> where the # of IRQs is <= half the # of CPUs in a NUMA node,
> I don't think the code below accomplishes the original intent.
> 
> Maybe I've missed something along the way in getting to this
> version of the patch.  Please feel free to set me straight. :-)
> 
> Michael

I have the same question as Michael. Also, I'm asking Souradeep
in another channel: So, the algorithm still uses up all current 
NUMA node before moving on to the next NUMA node, right?

Except each IRQ is affinitized to 2 CPUs. 
For example, a system with 2 IRQs:
IRQ     Nodes   Cores  CPUs
0       1       0      0-1
1       1       1      2-3
 
Is this performing better than the algorithm in earlier patches? like below:
IRQ     Nodes   Cores  CPUs
0       1       0      0
1       1       1      2

Thanks,
- Haiyang