linux-kernel - Re: [PATCH?] Livelock in pick_next_task_fair() / idle

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Date:	Fri, 03 Jul 2015 06:42:32 +0200
From:	Mike Galbraith <umgwanakikbuti@...il.com>
To:	Yuyang Du <yuyang.du@...el.com>
Cc:	Morten Rasmussen <morten.rasmussen@....com>,
	Peter Zijlstra <peterz@...radead.org>,
	Rabin Vincent <rabin.vincent@...s.com>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Paul Turner <pjt@...gle.com>, Ben Segall <bsegall@...gle.com>
Subject: Re: [PATCH?] Livelock in pick_next_task_fair() / idle_balance()

On Fri, 2015-07-03 at 02:42 +0800, Yuyang Du wrote:

> But still, I think, even with the above, in idle balancing, pulling until the source
> rq's nr_running == 1 is not just "a short term fix", but should be there permanently
> acting like a last guard with no overhead, why not.

Yeah, seems so.  Searching for steal all samples...
(this is all with autogroup)

load_balance: idle - s_run: 0  d_run: 2 s_load: 0  d_load: 3  imb: 23  det_tasks: 2  det_load: 3  zeros: 1
load_balance: idle - s_run: 0  d_run: 2 s_load: 0  d_load: 0  imb: 32  det_tasks: 2  det_load: 0  zeros: 2
load_balance: idle - s_run: 0  d_run: 2 s_load: 0  d_load: 1  imb: 17  det_tasks: 2  det_load: 1  zeros: 1
load_balance: idle - s_run: 0  d_run: 2 s_load: 0  d_load: 37  imb: 22  det_tasks: 2  det_load: 37  zeros: 1
load_balance: idle - s_run: 0  d_run: 2 s_load: 0  d_load: 0  imb: 102  det_tasks: 2  det_load: 0  zeros: 2


load_balance: idle - s_run: 0  d_run: 1 s_load: 0  d_load: 93  imb: 47  det_tasks: 1  det_load: 93  zeros: 0
load_balance: idle - s_run: 0  d_run: 2 s_load: 0  d_load: 202  imb: 125  det_tasks: 2  det_load: 202  zeros: 0
load_balance: idle - s_run: 0  d_run: 2 s_load: 0  d_load: 243  imb: 188  det_tasks: 2  det_load: 243  zeros: 0
load_balance: idle - s_run: 0  d_run: 1 s_load: 0  d_load: 145  imb: 73  det_tasks: 1  det_load: 145  zeros: 0
load_balance: idle - s_run: 0  d_run: 1 s_load: 0  d_load: 46  imb: 24  det_tasks: 1  det_load: 46  zeros: 0

Both varieties of total pilferage (w/wo 0 load tasks involved) seem to
happen only during idle balance, never periodic (yet).

Oddity: make -j8 occasionally stacks/pulls piles of load=dinky.

homer:/sys/kernel/debug/tracing # for i in `seq 1 10`; do cat trace|grep "s_run: 1.*det_tasks: $i.*zeros: 0"|wc -l; done
71634
1567
79
15
1
3
0
2
3
0
homer:/sys/kernel/debug/tracing # cat trace|grep "s_run: 1.*det_tasks: 8.*zeros: 0"
          <idle>-0     [002] dNs.   594.973783: load_balance: norm - s_run: 1  d_run: 9 s_load: 67  d_load: 1110  imb: 86  det_tasks: 8  det_load: 86  zeros: 0
           <...>-10367 [007] d...  1456.477281: load_balance: idle - s_run: 1  d_run: 8 s_load: 805  d_load: 22  imb: 45  det_tasks: 8  det_load: 22  zeros: 0
homer:/sys/kernel/debug/tracing # cat trace|grep "s_run: 1.*det_tasks: 9.*zeros: 0"
           <...>-23317 [004] d...   486.677925: load_balance: idle - s_run: 1  d_run: 9 s_load: 888  d_load: 27  imb: 47  det_tasks: 9  det_load: 27  zeros: 0
           <...>-11485 [002] d...   573.411095: load_balance: idle - s_run: 1  d_run: 9 s_load: 124  d_load: 78  imb: 82  det_tasks: 9  det_load: 78  zeros: 0
           <...>-23286 [000] d...  1510.378740: load_balance: idle - s_run: 1  d_run: 9 s_load: 102  d_load: 58  imb: 63  det_tasks: 9  det_load: 58  zeros: 0

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