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CFS Bandwidth Control v7.2

From:  Paul Turner <pjt@google.com>
To:  linux-kernel@vger.kernel.org
Subject:  [patch 00/18] CFS Bandwidth Control v7.2
Date:  Thu, 21 Jul 2011 09:43:25 -0700
Message-ID:  <20110721164325.231521704@google.com>
Cc:  Peter Zijlstra <a.p.zijlstra@chello.nl>, Bharata B Rao <bharata@linux.vnet.ibm.com>, Dhaval Giani <dhaval.giani@gmail.com>, Balbir Singh <bsingharora@gmail.com>, Vaidyanathan Srinivasan <svaidy@linux.vnet.ibm.com>, Srivatsa Vaddagiri <vatsa@in.ibm.com>, Kamalesh Babulal <kamalesh@linux.vnet.ibm.com>, Hidetoshi Seto <seto.hidetoshi@jp.fujitsu.com>, Ingo Molnar <mingo@elte.hu>, Pavel Emelyanov <xemul@openvz.org>, Jason Baron <jbaron@redhat.com>
Archive-link:  Article

Hi all,

Please find attached the incremental v7.2 for bandwidth control.

This release follows a fairly intensive period of scraping cycles across
various configurations.  Unfortunately we seem to be currently taking an IPC
hit for jump_labels (despite a savings in branches/instr. ret) which despite
fairly extensive digging I don't have a good explanation for.  The emitted
assembly /looks/ ok, but cycles/wall time is consistently higher across several
platforms.

As such I've demoted the jumppatch to [RFT] while these details are worked
out.  But there's no point in holding up the rest of the series any more.

[ Please find the specific discussion related to the above attached to patch 
17/18. ]

So -- without jump labels -- the current performance looks like:

                            instructions            cycles                  branches         
---------------------------------------------------------------------------------------------
clovertown [!BWC]           843695716               965744453               151224759        
+unconstrained              845934117 (+0.27)       974222228 (+0.88)       152715407 (+0.99)
+10000000000/1000:          855102086 (+1.35)       978728348 (+1.34)       154495984 (+2.16)
+10000000000/1000000:       853981660 (+1.22)       976344561 (+1.10)       154287243 (+2.03)

barcelona [!BWC]            810514902               761071312               145351489        
+unconstrained              820573353 (+1.24)       748178486 (-1.69)       148161233 (+1.93)
+10000000000/1000:          827963132 (+2.15)       757829815 (-0.43)       149611950 (+2.93)
+10000000000/1000000:       827701516 (+2.12)       753575001 (-0.98)       149568284 (+2.90)

westmere [!BWC]             792513879               702882443               143267136        
+unconstrained              802533191 (+1.26)       694415157 (-1.20)       146071233 (+1.96)
+10000000000/1000:          809861594 (+2.19)       701781996 (-0.16)       147520953 (+2.97)
+10000000000/1000000:       809752541 (+2.18)       705278419 (+0.34)       147502154 (+2.96)

Under the workload:
  mkdir -p /cgroup/cpu/test
  echo $$ > /dev/cgroup/cpu/test (only cpu,cpuacct mounted)
  (W1) taskset -c 0 perf stat --repeat 50 -e instructions,cycles,branches bash -c "for ((i=0;i<5;i++)); do $(dirname $0)/pipe-test 20000; done"

This may seem a strange work-load but it works around some bizarro overheads
currently introduced by perf.  Comparing for example with::w
  (W2)taskset -c 0 perf stat --repeat 50 -e instructions,cycles,branches bash -c "$(dirname $0)/pipe-test 100000;true"
  (W3)taskset -c 0 perf stat --repeat 50 -e instructions,cycles,branches bash -c "$(dirname $0)/pipe-test 100000;"


We see: 
 (W1)  westmere [!BWC]             792513879               702882443               143267136             0.197246943  
 (W2)  westmere [!BWC]             912241728               772576786               165734252             0.214923134  
 (W3)  westmere [!BWC]             904349725               882084726               162577399             0.748506065  

vs an 'ideal' total exec time of (approximately):
$ time taskset -c 0 ./pipe-test 100000
 real    0m0.198 user    0m0.007s ys     0m0.095s

The overhead in W2 is explained by that invoking pipe-test directly, one of
the siblings is becoming the perf_ctx parent, invoking lots of pain every time
we switch.  I do not have a reasonable explantion as to why (W1) is so much
cheaper than (W2), I stumbled across it by accident when I was trying some
combinations to reduce the <perf stat>-to-<perf stat> variance.

v7.2
-----------
- Build errors in !CGROUP_SCHED case fixed
- !CONFIG_SMP now 'supported' (#ifdef munging)
- gcc was failing to inline account_cfs_rq_runtime, affecting performance
- checks in expire_cfs_rq_runtime() and check_enqueue_throttle() re-organized
  to save branches.
- jump labels introduced in the case BWC is not being used system-wide to
  reduce inert overhead.
- branch saved in expiring runtime (reorganize conditonals)

Hidetoshi, the following patchsets have changed enough to necessitate tweaking
of your Reviewed-by:
[patch 09/18] sched: add support for unthrottling group entities (extensive)
[patch 11/18] sched: prevent interactions with throttled entities (update_cfs_shares)
[patch 12/18] sched: prevent buddy interactions with throttled entities (new)


Previous postings:
-----------------
v7.1: https://lkml.org/lkml/2011/7/7/24
v7: http://lkml.org/lkml/2011/6/21/43
v6: http://lkml.org/lkml/2011/5/7/37
v5: http://lkml.org/lkml/2011/3 /22/477
v4: http://lkml.org/lkml/2011/2/23/44
v3: http://lkml.org/lkml/2010/10/12/44
v2: http://lkml.org/lkml/2010/4/28/88
Original posting: http://lkml.org/lkml/2010/2/12/393

Prior approaches: http://lkml.org/lkml/2010/1/5/44 ["CFS Hard limits v5"]

Thanks,

- Paul

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