linux-kernel - Re: [PATCH] sched/fair: Skip wake

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Message-ID: <1443492214.3201.34.camel@gmail.com>
Date:	Tue, 29 Sep 2015 04:03:34 +0200
From:	Mike Galbraith <umgwanakikbuti@...il.com>
To:	Kirill Tkhai <ktkhai@...n.com>
Cc:	linux-kernel@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH] sched/fair: Skip wake_affine() for core siblings

On Mon, 2015-09-28 at 22:19 +0300, Kirill Tkhai wrote:
> >>  Imagine a situation, when we share a mutex
> >> with a task on another NUMA node. When the task is realising the mutex
> >> it is waking us, but we definitelly won't use affine logic in this case.
> > 
> > Why not?  A wakeup is a wakeup is a wakeup, they all do the same thing.
> > If wake_wide() doesn't NAK an affine wakeup, we ask wake_affine() for
> > its opinion, then look for an idle CPU near the waker's CPU if it says
> > OK, or near wakee's previous CPU if it says go away. 
> 
> But NUMA sd does not have SD_WAKE_AFFINE flag, so this case a new cpu won't
> be choosen from previous node. There will be choosen the highest domain
> of smp_processor_id(), which has SD_BALANCE_WAKE flag, and the cpu will
> be choosen from the idlest group/cpu. And we don't have a deal with old
> cache at all. This looks like a completely wrong behaviour...

SD_WAKE_AFFINE is enabled globally by default, and SD_BALANCE_WAKE is
disabled globally due to cost and whatnot.

wingenfelder:~/:[0]# tune-sched-domains
{cpu0/domain0:SMT} SD flag: 4783
+   1: SD_LOAD_BALANCE:          Do load balancing on this domain
+   2: SD_BALANCE_NEWIDLE:       Balance when about to become idle
+   4: SD_BALANCE_EXEC:          Balance on exec
+   8: SD_BALANCE_FORK:          Balance on fork, clone
-  16: SD_BALANCE_WAKE:          Wake to idle CPU on task wakeup
+  32: SD_WAKE_AFFINE:           Wake task to waking CPU
-  64:                           [unused]
+ 128: SD_SHARE_CPUCAPACITY:     Domain members share cpu power
- 256: SD_SHARE_POWERDOMAIN:     Domain members share power domain
+ 512: SD_SHARE_PKG_RESOURCES:   Domain members share cpu pkg resources
-1024: SD_SERIALIZE:             Only a single load balancing instance
-2048: SD_ASYM_PACKING:          Place busy groups earlier in the domain
+4096: SD_PREFER_SIBLING:        Prefer to place tasks in a sibling domain
-8192: SD_OVERLAP:               sched_domains of this level overlap
-16384: SD_NUMA:                 cross-node balancing
{cpu0/domain1:MC} SD flag: 4655
+   1: SD_LOAD_BALANCE:          Do load balancing on this domain
+   2: SD_BALANCE_NEWIDLE:       Balance when about to become idle
+   4: SD_BALANCE_EXEC:          Balance on exec
+   8: SD_BALANCE_FORK:          Balance on fork, clone
-  16: SD_BALANCE_WAKE:          Wake to idle CPU on task wakeup
+  32: SD_WAKE_AFFINE:           Wake task to waking CPU
-  64:                           [unused]
- 128: SD_SHARE_CPUCAPACITY:     Domain members share cpu power
- 256: SD_SHARE_POWERDOMAIN:     Domain members share power domain
+ 512: SD_SHARE_PKG_RESOURCES:   Domain members share cpu pkg resources
-1024: SD_SERIALIZE:             Only a single load balancing instance
-2048: SD_ASYM_PACKING:          Place busy groups earlier in the domain
+4096: SD_PREFER_SIBLING:        Prefer to place tasks in a sibling domain
-8192: SD_OVERLAP:               sched_domains of this level overlap
-16384: SD_NUMA:                 cross-node balancing
{cpu0/domain2:NUMA} SD flag: 25647
+   1: SD_LOAD_BALANCE:          Do load balancing on this domain
+   2: SD_BALANCE_NEWIDLE:       Balance when about to become idle
+   4: SD_BALANCE_EXEC:          Balance on exec
+   8: SD_BALANCE_FORK:          Balance on fork, clone
-  16: SD_BALANCE_WAKE:          Wake to idle CPU on task wakeup
+  32: SD_WAKE_AFFINE:           Wake task to waking CPU
-  64:                           [unused]
- 128: SD_SHARE_CPUCAPACITY:     Domain members share cpu power
- 256: SD_SHARE_POWERDOMAIN:     Domain members share power domain
- 512: SD_SHARE_PKG_RESOURCES:   Domain members share cpu pkg resources
+1024: SD_SERIALIZE:             Only a single load balancing instance
-2048: SD_ASYM_PACKING:          Place busy groups earlier in the domain
-4096: SD_PREFER_SIBLING:        Prefer to place tasks in a sibling domain
+8192: SD_OVERLAP:               sched_domains of this level overlap
+16384: SD_NUMA:                 cross-node balancing

	-Mike


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