Re: [PATCH RFC 0/2] Optimize the snmp stat aggregation for large cpus

From: Eric Dumazet
Date: Wed Aug 26 2015 - 10:09:50 EST

On Wed, 2015-08-26 at 15:55 +0530, Raghavendra K T wrote:
> On 08/26/2015 04:37 AM, David Miller wrote:
> > From: Raghavendra K T <raghavendra.kt@xxxxxxxxxxxxxxxxxx>
> > Date: Tue, 25 Aug 2015 13:24:24 +0530
> >
> >> Please let me know if you have suggestions/comments.
> >
> > Like Eric Dumazet said the idea is good but needs some adjustments.
> >
> > You might want to see whether a per-cpu work buffer works for this.
> sure, Let me know if I understood correctly,
> we allocate the temp buffer,
> we will have a "add_this_cpu_data" function and do
> for_each_online_cpu(cpu)
> smp_call_function_single(cpu, add_this_cpu_data, buffer, 1)
> if not could you please point to an example you had in mind.

Sorry I do not think it is a good idea.

Sending an IPI is way more expensive and intrusive than reading 4 or 5
cache lines from memory (per cpu)

Definitely not something we want.

> >
> > It's extremely unfortunately that we can't depend upon the destination
> > buffer being properly aligned, because we wouldn't need a temporary
> > scratch area if it were aligned properly.
> True, But I think for 64 bit cpus when (pad == 0) we can go ahead and
> use stats array directly and get rid of put_unaligned(). is it correct?

Nope. We have no alignment guarantee. It could be 0x............04
pointer value. (ie not a multiple of 8)

> (my internal initial patch had this version but thought it is ugly to
> have ifdef BITS_PER_LONG==64)

This has nothing to do with arch having 64bit per long. It is about
alignment of a u64.

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