On Wed, 2010-03-31 at 22:32 -0700, Mike Galbraith wrote:
Ok. This is not a big issue with the new idle cpu change, as atleast we
don't endup calling wake_affine() with the wrong sd. I have never tried
to understand any code surrounded by CONFIG_FAIR_GROUP_SCHED so can't
comment if the using affine_sd for updating group scheduler shares is
correct or not. But please look below for the issues with selecting the
idle sibling right away.
Just like the current code, my patch is doing the idle checks only once.
Current code is doing idle checks for the woken-up cpu and my code is
first selecting woken-up vs previously-ran and then doing idle sibling
checks . So don't expect to see much cost increase.
Last level cache sharing is much more important than small L1 and mid
level caches. Also performance impact of keeping both the threads on a
core busy in the context of an idle core and then periodic balancer
coming in and correcting this is more costly.
For not wrecking balance we should do the wake_balance() and based on
that decision, do the select_idle_sibling() for selecting an idle cpu in
that cache affinity. Current code in -tip is opposite of this.
Consider this scenario. Today we do balance on fork() and exec(). This
will cause the tasks to start far away. On systems like NHM-EP, tasks
will start on two different sockets/nodes(as each socket is a numa node)
and allocate their memory locally etc. Task A starting on Node-0 and
Task B starting on Node-1. Once task B sleeps and if Task A or something
else wakes up task B on Node-0, (with the recent change) just because
there is an idle HT sibling on node-0 we endup waking the task on
node-0. This is wrong. We should first atleast go through wake_affine()
and if wake_affine() says ok to move the task to node-0, then we can
look at the cache siblings for node-0 and select an appropriate cpu.
thanks,
suresh
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