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[Bug 1585850] Re: [Bug]KNL:Spread MWAIT cache lines over all nodes

To: kernel-packages@xxxxxxxxxxxxxxxxxxx
From: Kamal Mostafa <kamal@xxxxxxxxxxxxx>
Date: Tue, 31 May 2016 17:55:01 -0000
Reply-to: Bug 1585850 <1585850@xxxxxxxxxxxxxxxxxx>
Sender: bounces@xxxxxxxxxxxxx

** Changed in: linux (Ubuntu Xenial)
       Status: In Progress => Fix Committed

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https://bugs.launchpad.net/bugs/1585850

Title:
  [Bug]KNL:Spread MWAIT cache lines over all nodes

Status in intel:
  New
Status in linux package in Ubuntu:
  Fix Committed
Status in linux source package in Xenial:
  Fix Committed
Status in linux source package in Yakkety:
  Fix Committed

Bug description:
  KNL sees severely degraded memory bandwidth on MCDRAM with C-states deeper than C1 and with sub numa clustering enabled. This has been root caused to all idle task's MWAIT cache line placed in node 0. Node 0's CHA goes into degraded mode because the many MWAIT lines overflow an internal hash table.
  Fix is to allocate the idle task's task structs on their respective nodes.

  The fix patch is merged into linux-next and will be part of v4.7,
  please back port into 16.04 kernel:

  725fc62 kernek/fork.c: allocate idle task for a CPU always on its
  local node

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