Date:	Thu, 4 Sep 2014 09:31:23 +0800
From:	Yuyang Du <yuyang.du@...el.com>
To:	mingo@...hat.com, peterz@...radead.org,
	linux-kernel@...r.kernel.org
Cc:	pjt@...gle.com, bsegall@...gle.com, arjan.van.de.ven@...el.com,
	len.brown@...el.com, rafael.j.wysocki@...el.com,
	alan.cox@...el.com, mark.gross@...el.com, fengguang.wu@...el.com,
	umgwanakikbuti@...il.com
Subject: Re: [PATCH 0/3 v5] sched: Rewrite per entity runnable load average
 tracking

Ping Peter and Ingo, and Paul and Ben.

Yuyang

On Fri, Aug 01, 2014 at 08:30:59AM +0800, Yuyang Du wrote:
> v5 changes:
> 
> Thank Peter intensively for reviewing this patchset in detail and all his comments.
> And Mike for general and cgroup pipe-test. Morten, Ben, and Vincent in the discussion.
> 
> - Remove dead task and task group load_avg
> - Do not update trivial delta to task_group load_avg (threshold 1/64 old_contrib)
> - mul_u64_u32_shr() is used in decay_load, so on 64bit, load_sum can afford
>   about 4353082796 (=2^64/47742/88761) entities with the highest weight (=88761)
>   always runnable, greater than previous theoretical maximum 132845
> - Various code efficiency and style changes
> 
> We carried out some performance tests (thanks to Fengguang and his LKP). The results
> are shown as follows. The patchset (including threepatches) is on top of mainline
> v3.16-rc5. We may report more perf numbers later.
> 
> Overall, this rewrite has better performance, and reduced net overhead in load
> average tracking, flat efficiency in multi-layer cgroup pipe-test.
> 
> --------------------------------------------------------------------------------------
> 
> host: lkp-snb01
> model: Sandy Bridge-EP
> memory: 32G
> 
> host: lkp-hsx03
> model: Brickland Haswell-EX
> nr_cpu: 144
> memory: 128G
> 
> host: xps2
> model: Nehalem
> memory: 4G
> 
> Legend:
> 	[+-]XX% - change percent
> 	~XX%    - stddev percent
> 
>    v3.16-rc5       PATCH 1/3 + 2/3 + 3/3
> ---------------  -------------------------  
>     150854 ~ 2%     +53.3%     231234 ~ 0%  lkp-snb01/hackbench/1600%-process-pipe
>     150986 ~ 1%      +1.6%     153470 ~ 0%  lkp-snb01/hackbench/1600%-process-socket
>     174142 ~ 2%     +19.1%     207396 ~ 0%  lkp-snb01/hackbench/1600%-threads-pipe
>     156982 ~ 0%      -0.8%     155706 ~ 1%  lkp-snb01/hackbench/1600%-threads-socket
>      95201 ~ 0%      -0.7%      94492 ~ 0%  lkp-snb01/hackbench/50%-process-pipe
>      85279 ~ 0%     +78.7%     152428 ~ 1%  lkp-snb01/hackbench/50%-process-socket
>      89911 ~ 0%      +0.6%      90477 ~ 0%  lkp-snb01/hackbench/50%-threads-pipe
>      78145 ~ 0%     +87.5%     146505 ~ 0%  lkp-snb01/hackbench/50%-threads-socket
>     981503 ~ 1%     +25.5%    1231710 ~ 0%  TOTAL hackbench.throughput
> 
> ---------------  -------------------------  
>   75839119 ~ 0%      +0.1%   75922106 ~ 0%  xps2/pigz/100%-128K
>   77292677 ~ 0%      +0.1%   77399500 ~ 0%  xps2/pigz/100%-512K
>  153131796 ~ 0%      +0.1%  153321606 ~ 0%  TOTAL pigz.throughput
> 
> ---------------  -------------------------  
>   28868660 ~ 0%      +0.5%   29000332 ~ 0%  lkp-hsx03/vm-scalability/300s-anon-r-rand-mt
>   28760522 ~ 0%      +1.1%   29090639 ~ 0%  lkp-hsx03/vm-scalability/300s-anon-r-rand
>  3.351e+08 ~ 0%      +0.1%  3.353e+08 ~ 0%  lkp-hsx03/vm-scalability/300s-anon-r-seq-mt
>  3.346e+08 ~ 0%      +0.5%  3.364e+08 ~ 0%  lkp-hsx03/vm-scalability/300s-anon-r-seq
>   33537242 ~ 1%      +0.2%   33592010 ~ 0%  lkp-hsx03/vm-scalability/300s-anon-rx-rand-mt
>  3.358e+08 ~ 0%      +0.7%   3.38e+08 ~ 0%  lkp-hsx03/vm-scalability/300s-anon-rx-seq-mt
>    1805110 ~ 0%      -0.0%    1804723 ~ 0%  lkp-hsx03/vm-scalability/300s-lru-file-mmap-read-rand
>   13024108 ~ 0%      +8.8%   14171706 ~ 0%  lkp-hsx03/vm-scalability/300s-lru-file-mmap-read
>  1.112e+09 ~ 0%      +0.5%  1.117e+09 ~ 0%  TOTAL vm-scalability.throughput
> 
> --------------------------------------------------------------------------------------
> 
> v4 changes:
> 
> Thanks to Morten, Ben, and Fengguang for v4 revision.
> 
> - Insert memory barrier before writing cfs_rq->load_last_update_copy.
> - Fix typos.
> 
> v3 changes:
> 
> Many thanks to Ben for v3 revision.
> 
> Regarding the overflow issue, we now have for both entity and cfs_rq:
> 
> struct sched_avg {
>     .....
>     u64 load_sum;
>     unsigned long load_avg;
>     .....
> };
> 
> Given the weight for both entity and cfs_rq is:
> 
> struct load_weight {
>     unsigned long weight;
>     .....
> };
> 
> So, load_sum's max is 47742 * load.weight (which is unsigned long), then on 32bit,
> it is absolutly safe. On 64bit, with unsigned long being 64bit, but we can afford
> about 4353082796 (=2^64/47742/88761) entities with the highest weight (=88761)
> always runnable, even considering we may multiply 1<<15 in decay_load64, we can
> still support 132845 (=4353082796/2^15) always runnable, which should be acceptible.
> 
> load_avg = load_sum / 47742 = load.weight (which is unsigned long), so it should be
> perfectly safe for both entity (even with arbitrary user group share) and cfs_rq on
> both 32bit and 64bit. Originally, we saved this division, but have to get it back
> because of the overflow issue on 32bit (actually load average itself is safe from
> overflow, but the rest of the code referencing it always uses long, such as cpu_load,
> etc., which prevents it from saving).
> 
> - Fix overflow issue both for entity and cfs_rq on both 32bit and 64bit.
> - Track all entities (both task and group entity) due to group entity's clock issue.
>   This actually improves code simplicity.
> - Make a copy of cfs_rq sched_avg's last_update_time, to read an intact 64bit
>   variable on 32bit machine when in data race (hope I did it right).
> - Minor fixes and code improvement.
> 
> v2 changes:
> 
> Thanks to PeterZ and Ben for their help in fixing the issues and improving
> the quality, and Fengguang and his 0Day in finding compile errors in different
> configurations for version 2.
> 
> - Batch update the tg->load_avg, making sure it is up-to-date before update_cfs_shares
> - Remove migrating task from the old CPU/cfs_rq, and do so with atomic operations
> 
> 
> Yuyang Du (3):
>   sched: Remove update_rq_runnable_avg
>   sched: Rewrite per entity runnable load average tracking
>   sched: Remove task and group entity load_avg when they are dead
> 
>  include/linux/sched.h |   21 +-
>  kernel/sched/debug.c  |   30 +--
>  kernel/sched/fair.c   |  594 ++++++++++++++++---------------------------------
>  kernel/sched/proc.c   |    2 +-
>  kernel/sched/sched.h  |   22 +-
>  5 files changed, 218 insertions(+), 451 deletions(-)
> 
> -- 
> 1.7.9.5
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