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Re: [PATCH RFC V2 2/2] net: Optimize snmp stat aggregation by walking all the percpu data at once

From Eric Dumazet <eric.dumazet@gmail.com>
Newsgroups linux.kernel
Subject Re: [PATCH RFC V2 2/2] net: Optimize snmp stat aggregation by walking all the percpu data at once
Date 2015-08-29 02:10 +0200
Message-ID <q2BSx-20S-5@gated-at.bofh.it> (permalink)
References (8 earlier) <q2z4m-6rs-11@gated-at.bofh.it> <q2ze2-6CP-11@gated-at.bofh.it> <q2znJ-6NW-23@gated-at.bofh.it> <q2AjL-89a-5@gated-at.bofh.it> <q2B6a-Rf-11@gated-at.bofh.it>
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On Fri, 2015-08-28 at 16:12 -0700, Joe Perches wrote:

> Generally true.  It's always difficult to know how much
> stack has been consumed though and smaller stack frames
> are generally better.

Calling kmalloc(288, GFP_KERNEL) might use way more than 288 bytes in
kernel stack on 64 bit arch.

__slab_alloc() itself for example uses 208 bytes on stack, so add all
others, and you might go above 500 bytes.
 
So for a _leaf_ function, it is better to declare an automatic variable,
as you in fact reduce max stack depth.

Not only it uses less kernel stack, it is also way faster, as you avoid
kmalloc()/kfree() overhead and reuse probably already hot cache lines in
kernel stack.



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[PATCH RFC V2 0/2]  Optimize the snmp stat aggregation for large cpus  Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com> - 2015-08-26 19:50 +0200
  [PATCH RFC V2 2/2] net: Optimize snmp stat aggregation by walking all the percpu data at once Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com> - 2015-08-26 19:50 +0200
    Re: [PATCH RFC V2 2/2] net: Optimize snmp stat aggregation by  walking all the percpu data at once David Miller <davem@davemloft.net> - 2015-08-27 20:40 +0200
      Re: [PATCH RFC V2 2/2] net: Optimize snmp stat aggregation by walking  all the percpu data at once Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com> - 2015-08-28 08:40 +0200
        Re: [PATCH RFC V2 2/2] net: Optimize snmp stat aggregation by  walking all the percpu data at once David Miller <davem@davemloft.net> - 2015-08-28 20:30 +0200
          Re: [PATCH RFC V2 2/2] net: Optimize snmp stat aggregation by  walking all the percpu data at once Joe Perches <joe@perches.com> - 2015-08-28 21:30 +0200
            Re: [PATCH RFC V2 2/2] net: Optimize snmp stat aggregation by  walking all the percpu data at once Eric Dumazet <eric.dumazet@gmail.com> - 2015-08-28 22:40 +0200
              Re: [PATCH RFC V2 2/2] net: Optimize snmp stat aggregation by  walking all the percpu data at once Eric Dumazet <eric.dumazet@gmail.com> - 2015-08-28 23:00 +0200
                Re: [PATCH RFC V2 2/2] net: Optimize snmp stat aggregation by  walking all the percpu data at once Joe Perches <joe@perches.com> - 2015-08-28 23:10 +0200
                Re: [PATCH RFC V2 2/2] net: Optimize snmp stat aggregation by  walking all the percpu data at once Eric Dumazet <eric.dumazet@gmail.com> - 2015-08-28 23:20 +0200
                Re: [PATCH RFC V2 2/2] net: Optimize snmp stat aggregation by  walking all the percpu data at once Joe Perches <joe@perches.com> - 2015-08-28 23:30 +0200
                Re: [PATCH RFC V2 2/2] net: Optimize snmp stat aggregation by  walking all the percpu data at once Eric Dumazet <eric.dumazet@gmail.com> - 2015-08-29 00:30 +0200
                Re: [PATCH RFC V2 2/2] net: Optimize snmp stat aggregation by  walking all the percpu data at once Joe Perches <joe@perches.com> - 2015-08-29 01:20 +0200
                Re: [PATCH RFC V2 2/2] net: Optimize snmp stat aggregation by  walking all the percpu data at once Eric Dumazet <eric.dumazet@gmail.com> - 2015-08-29 02:10 +0200
                Re: [PATCH RFC V2 2/2] net: Optimize snmp stat aggregation by  walking all the percpu data at once Joe Perches <joe@perches.com> - 2015-08-29 02:40 +0200
                Re: [PATCH RFC V2 2/2] net: Optimize snmp stat aggregation by  walking all the percpu data at once Eric Dumazet <eric.dumazet@gmail.com> - 2015-08-29 03:10 +0200
              Re: [PATCH RFC V2 2/2] net: Optimize snmp stat aggregation by  walking all the percpu data at once Joe Perches <joe@perches.com> - 2015-08-28 23:00 +0200
          Re: [PATCH RFC V2 2/2] net: Optimize snmp stat aggregation by  walking all the percpu data at once Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com> - 2015-08-29 05:00 +0200
            Re: [PATCH RFC V2 2/2] net: Optimize snmp stat aggregation by  walking all the percpu data at once Eric Dumazet <eric.dumazet@gmail.com> - 2015-08-29 05:30 +0200
              Re: [PATCH RFC V2 2/2] net: Optimize snmp stat aggregation by walking  all the percpu data at once Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com> - 2015-08-29 10:00 +0200
            Re: [PATCH RFC V2 2/2] net: Optimize snmp stat aggregation by  walking all the percpu data at once David Miller <davem@davemloft.net> - 2015-08-29 07:20 +0200
              Re: [PATCH RFC V2 2/2] net: Optimize snmp stat aggregation by walking  all the percpu data at once Raghavendra K T <raghavendra.kt@linux.vnet.ibm.com> - 2015-08-29 10:00 +0200

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