Re: [PATCH 0/7] slub: Fastpath optimization (especially for RT) V1

From: Jesper Dangaard Brouer
Date: Wed Dec 17 2014 - 07:09:12 EST

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In reply to: Joonsoo Kim: "Re: [PATCH 0/7] slub: Fastpath optimization (especially for RT) V1"
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On Wed, 17 Dec 2014 16:13:49 +0900 Joonsoo Kim <js1304@xxxxxxxxx> wrote:

> Ping... and I found another way to remove preempt_disable/enable
> without complex changes.
>
> What we want to ensure is getting tid and kmem_cache_cpu
> on the same cpu. We can achieve that goal with below condition loop.
>
> I ran Jesper's benchmark and saw 3~5% win in a fast-path loop over
> kmem_cache_alloc+free in CONFIG_PREEMPT.
>
> 14.5 ns -> 13.8 ns

Hi Kim,

I've tested you patch. Full report below patch.

Summary, I'm seeing 18.599 ns -> 17.523 ns (-1.076ns better).

For network overload tests:

Dropping packets in iptables raw, which is hitting the slub fast-path.
Here I'm seeing an improvement of 3ns.

For IP-forward, which is also invoking the slub slower path, I'm seeing
an improvement of 6ns (I were not expecting to see any improvement
here, the kmem_cache_alloc code is 24bytes smaller, so perhaps it's
saving some icache).

Full report below patch...

> See following patch.
>
> Thanks.
>
> ----------->8-------------
> diff --git a/mm/slub.c b/mm/slub.c
> index 95d2142..e537af5 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -2399,8 +2399,10 @@ redo:
> * on a different processor between the determination of the pointer
> * and the retrieval of the tid.
> */
> - preempt_disable();
> - c = this_cpu_ptr(s->cpu_slab);
> + do {
> + tid = this_cpu_read(s->cpu_slab->tid);
> + c = this_cpu_ptr(s->cpu_slab);
> + } while (IS_ENABLED(CONFIG_PREEMPT) && unlikely(tid != c->tid));
>
> /*
> * The transaction ids are globally unique per cpu and per operation on
> @@ -2408,8 +2410,6 @@ redo:
> * occurs on the right processor and that there was no operation on the
> * linked list in between.
> */
> - tid = c->tid;
> - preempt_enable();
>
> object = c->freelist;
> page = c->page;
> @@ -2655,11 +2655,10 @@ redo:
> * data is retrieved via this pointer. If we are on the same cpu
> * during the cmpxchg then the free will succedd.
> */
> - preempt_disable();
> - c = this_cpu_ptr(s->cpu_slab);
> -
> - tid = c->tid;
> - preempt_enable();
> + do {
> + tid = this_cpu_read(s->cpu_slab->tid);
> + c = this_cpu_ptr(s->cpu_slab);
> + } while (IS_ENABLED(CONFIG_PREEMPT) && unlikely(tid != c->tid));
>
> if (likely(page == c->page)) {
> set_freepointer(s, object, c->freelist);

SLUB evaluation 03
==================

Testing patch from Joonsoo Kim <iamjoonsoo.kim@xxxxxxx> slub fast-path
preempt_{disable,enable} avoidance.

Kernel
======
Compiler: GCC 4.9.1

Kernel config ::

$ grep PREEMPT .config
CONFIG_PREEMPT_RCU=y
CONFIG_PREEMPT_NOTIFIERS=y
# CONFIG_PREEMPT_NONE is not set
# CONFIG_PREEMPT_VOLUNTARY is not set
CONFIG_PREEMPT=y
CONFIG_PREEMPT_COUNT=y
# CONFIG_DEBUG_PREEMPT is not set

$ egrep -e "SLUB|SLAB" .config
# CONFIG_SLUB_DEBUG is not set
# CONFIG_SLAB is not set
CONFIG_SLUB=y
# CONFIG_SLUB_CPU_PARTIAL is not set
# CONFIG_SLUB_STATS is not set

On top of::

commit f96fe225677b3efb74346ebd56fafe3997b02afa
Merge: 5543798 eea3e8f
Author: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Date: Fri Dec 12 16:11:12 2014 -0800

Merge git://git.kernel.org/pub/scm/linux/kernel/git/davem/net

Setup
=====

netfilter_unload_modules.sh
netfilter_unload_modules.sh
sudo rmmod nf_reject_ipv4 nf_reject_ipv6

base_device_setup.sh eth4 # 10G sink/receiving interface (ixgbe)
base_device_setup.sh eth5
sudo ethtool --coalesce eth4 rx-usecs 30
sudo ip neigh add 192.168.21.66 dev eth5 lladdr 00:00:ba:d0:ba:d0
sudo ip route add 198.18.0.0/15 via 192.168.21.66 dev eth5

# sudo tuned-adm active
Current active profile: latency-performance

Drop in raw
-----------
alias iptables='sudo iptables'
iptables -t raw -N simple || iptables -t raw -F simple
iptables -t raw -I simple -d 198.18.0.0/15 -j DROP
iptables -t raw -D PREROUTING -j simple
iptables -t raw -I PREROUTING -j simple

Generator
---------
./pktgen02_burst.sh -d 198.18.0.2 -i eth8 -m 90:E2:BA:0A:56:B4 -b 8 -t 3 -s 64

Patch by Joonsoo Kim to avoid preempt in slub
=============================================

baseline: without patch
-----------------------

baseline kernel v3.18-7016-gf96fe22 at commit f96fe22567

Type:kmem fastpath reuse Per elem: 46 cycles(tsc) 18.599 ns
- (measurement period time:1.859917529 sec time_interval:1859917529)
- (invoke count:100000000 tsc_interval:4649791431)

alloc N-pattern before free with 256 elements

Type:kmem alloc+free N-pattern Per elem: 100 cycles(tsc) 40.077 ns
- (measurement period time:1.025993290 sec time_interval:1025993290)
- (invoke count:25600000 tsc_interval:2564981743)

single flow/CPU
* IP-forward
- instant rx:0 tx:1165376 pps n:60 average: rx:0 tx:1165928 pps
(instant variation TX -0.407 ns (min:-0.828 max:0.507) RX 0.000 ns)
* Drop in RAW (slab fast-path test)
- instant rx:3245248 tx:0 pps n:60 average: rx:3245325 tx:0 pps
(instant variation TX 0.000 ns (min:0.000 max:0.000) RX -0.007 ns)

Christoph's slab_test, baseline kernel (at commit f96fe22567)::

Single thread testing
=====================
1. Kmalloc: Repeatedly allocate then free test
10000 times kmalloc(8) -> 49 cycles kfree -> 62 cycles
10000 times kmalloc(16) -> 48 cycles kfree -> 64 cycles
10000 times kmalloc(32) -> 53 cycles kfree -> 70 cycles
10000 times kmalloc(64) -> 64 cycles kfree -> 77 cycles
10000 times kmalloc(128) -> 74 cycles kfree -> 84 cycles
10000 times kmalloc(256) -> 84 cycles kfree -> 114 cycles
10000 times kmalloc(512) -> 83 cycles kfree -> 116 cycles
10000 times kmalloc(1024) -> 81 cycles kfree -> 120 cycles
10000 times kmalloc(2048) -> 104 cycles kfree -> 136 cycles
10000 times kmalloc(4096) -> 142 cycles kfree -> 165 cycles
10000 times kmalloc(8192) -> 238 cycles kfree -> 226 cycles
10000 times kmalloc(16384) -> 403 cycles kfree -> 264 cycles
2. Kmalloc: alloc/free test
10000 times kmalloc(8)/kfree -> 68 cycles
10000 times kmalloc(16)/kfree -> 68 cycles
10000 times kmalloc(32)/kfree -> 69 cycles
10000 times kmalloc(64)/kfree -> 68 cycles
10000 times kmalloc(128)/kfree -> 68 cycles
10000 times kmalloc(256)/kfree -> 68 cycles
10000 times kmalloc(512)/kfree -> 74 cycles
10000 times kmalloc(1024)/kfree -> 75 cycles
10000 times kmalloc(2048)/kfree -> 74 cycles
10000 times kmalloc(4096)/kfree -> 74 cycles
10000 times kmalloc(8192)/kfree -> 75 cycles
10000 times kmalloc(16384)/kfree -> 510 cycles

$ nm --print-size vmlinux | egrep -e 'kmem_cache_alloc|kmem_cache_free|is_pointer_to_page'
ffffffff81163bd0 00000000000000e1 T kmem_cache_alloc
ffffffff81163ac0 000000000000010c T kmem_cache_alloc_node
ffffffff81162cb0 000000000000013b T kmem_cache_free

with patch
----------

single flow/CPU
* IP-forward
- instant rx:0 tx:1174652 pps n:60 average: rx:0 tx:1174222 pps
(instant variation TX 0.311 ns (min:-0.230 max:1.018) RX 0.000 ns)
* compare against baseline:
- 1174222-1165928 = +8294pps
- (1/1174222*10^9)-(1/1165928*10^9) = -6.058ns

* Drop in RAW (slab fast-path test)
- instant rx:3277440 tx:0 pps n:74 average: rx:3277737 tx:0 pps
(instant variation TX 0.000 ns (min:0.000 max:0.000) RX -0.028 ns)
* compare against baseline:
- 3277737-3245325 = +32412 pps
- (1/3277737*10^9)-(1/3245325*10^9) = -3.047ns

SLUB fast-path test: time_bench_kmem_cache1
* modprobe time_bench_kmem_cache1 ; rmmod time_bench_kmem_cache1; sudo dmesg -c

Type:kmem fastpath reuse Per elem: 43 cycles(tsc) 17.523 ns (step:0)
- (measurement period time:1.752338378 sec time_interval:1752338378)
- (invoke count:100000000 tsc_interval:4380843588)
* difference: 17.523 - 18.599 = -1.076ns

alloc N-pattern before free with 256 elements

Type:kmem alloc+free N-pattern Per elem: 100 cycles(tsc) 40.369 ns (step:0)
- (measurement period time:1.033447112 sec time_interval:1033447112)
- (invoke count:25600000 tsc_interval:2583616203)
* difference: 40.369 - 40.077 = +0.292ns

Christoph's slab_test::

Single thread testing
=====================
1. Kmalloc: Repeatedly allocate then free test
10000 times kmalloc(8) -> 46 cycles kfree -> 61 cycles
10000 times kmalloc(16) -> 46 cycles kfree -> 63 cycles
10000 times kmalloc(32) -> 49 cycles kfree -> 69 cycles
10000 times kmalloc(64) -> 57 cycles kfree -> 76 cycles
10000 times kmalloc(128) -> 66 cycles kfree -> 83 cycles
10000 times kmalloc(256) -> 84 cycles kfree -> 110 cycles
10000 times kmalloc(512) -> 77 cycles kfree -> 114 cycles
10000 times kmalloc(1024) -> 80 cycles kfree -> 116 cycles
10000 times kmalloc(2048) -> 102 cycles kfree -> 131 cycles
10000 times kmalloc(4096) -> 135 cycles kfree -> 163 cycles
10000 times kmalloc(8192) -> 238 cycles kfree -> 218 cycles
10000 times kmalloc(16384) -> 399 cycles kfree -> 262 cycles
2. Kmalloc: alloc/free test
10000 times kmalloc(8)/kfree -> 65 cycles
10000 times kmalloc(16)/kfree -> 66 cycles
10000 times kmalloc(32)/kfree -> 65 cycles
10000 times kmalloc(64)/kfree -> 66 cycles
10000 times kmalloc(128)/kfree -> 66 cycles
10000 times kmalloc(256)/kfree -> 71 cycles
10000 times kmalloc(512)/kfree -> 72 cycles
10000 times kmalloc(1024)/kfree -> 71 cycles
10000 times kmalloc(2048)/kfree -> 71 cycles
10000 times kmalloc(4096)/kfree -> 71 cycles
10000 times kmalloc(8192)/kfree -> 65 cycles
10000 times kmalloc(16384)/kfree -> 511 cycles

$ nm --print-size vmlinux | egrep -e 'kmem_cache_alloc|kmem_cache_free|is_pointer_to_page'
ffffffff81163ba0 00000000000000c9 T kmem_cache_alloc
ffffffff81163aa0 00000000000000f8 T kmem_cache_alloc_node
ffffffff81162cb0 0000000000000133 T kmem_cache_free

Kernel size change
------------------

$ scripts/bloat-o-meter vmlinux vmlinux-kim-preempt-avoid
add/remove: 0/0 grow/shrink: 0/8 up/down: 0/-248 (-248)
function old new delta
kmem_cache_free 315 307 -8
kmem_cache_alloc_node 268 248 -20
kmem_cache_alloc 225 201 -24
kfree 274 250 -24
__kmalloc_node_track_caller 356 324 -32
__kmalloc_node 340 308 -32
__kmalloc 324 273 -51
__kmalloc_track_caller 343 286 -57

Qmempool notes:
---------------

On baseline kernel:

Type:qmempool fastpath reuse SOFTIRQ Per elem: 33 cycles(tsc) 13.287 ns
- (measurement period time:0.398628965 sec time_interval:398628965)
- (invoke count:30000000 tsc_interval:996571541)

Type:qmempool fastpath reuse BH-disable Per elem: 47 cycles(tsc) 19.180 ns
- (measurement period time:0.575425927 sec time_interval:575425927)
- (invoke count:30000000 tsc_interval:1438563781)

qmempool_bench: N-pattern with 256 elements

Type:qmempool alloc+free N-pattern Per elem: 62 cycles(tsc) 24.955 ns (step:0)
- (measurement period time:0.638871008 sec time_interval:638871008)
- (invoke count:25600000 tsc_interval:1597176303)

--
Best regards,
Jesper Dangaard Brouer
MSc.CS, Sr. Network Kernel Developer at Red Hat
Author of http://www.iptv-analyzer.org
LinkedIn: http://www.linkedin.com/in/brouer
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Next message: Geert Uytterhoeven: "Re: [PATCH] clocksource: sh_tmu: Set cpu_possible_mask to fix SMP broadcast"
Previous message: Michal Simek: "[GIT PULL] arch/microblaze fixes for 3.19-rc1"
In reply to: Joonsoo Kim: "Re: [PATCH 0/7] slub: Fastpath optimization (especially for RT) V1"
Next in thread: Joonsoo Kim: "Re: [PATCH 0/7] slub: Fastpath optimization (especially for RT) V1"
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