Subject: Re: [PATCH] sched: Resolve sd_idle and first_idle_cpu Catch-22 - v1
From: Peter Zijlstra <peterz@infradead.org>
To: Suresh Siddha <suresh.b.siddha@intel.com>
Cc: Venkatesh Pallipadi <venki@google.com>, Ingo Molnar <mingo@elte.hu>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Paul Turner <pjt@google.com>, Mike Galbraith <efault@gmx.de>,
        Nick Piggin <npiggin@gmail.com>
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Date: Wed, 09 Feb 2011 16:55:28 +0100
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On Mon, 2011-02-07 at 11:53 -0800, Suresh Siddha wrote:
> 
> Peter, to answer your question of why SMT is treated different to cores
> sharing cache, performance improvements contributed by SMT is far less
> compared to the cores and any wrong decisions in SMT load balancing
> (especially in the presence of idle cores, packages) has a bigger
> impact.
> 
> I think in the tbench case referred by Nick, idle HT siblings in a busy
> package picked the load instead of the idle packages. And thus we
> probably had to wait for active load balance to kick in to distribute
> the load etc by which the damage would have been. Performance impact of
> this condition wouldn't be as severe in the cores sharing last level
> cache and other resources.
> 
> Also there are lot of changes in this area since 2005. So it would be
> nice to revisit the tbench case and see if the logic of propagating busy
> sibling status to the higher level load balances is still needed or not.
> 
> On the contrary, perhaps there might be some workloads which may benefit
> in performance/latency if we completely do away with this less
> aggressive SMT load balancing. 

Right, but our current capacity logic does exactly that and seems to
work for more than 2 smt siblings (it does the whole asymmetric power7
muck).

>From a quick glance at the sched.c state at the time of Nick's patch,
the capacity logic wasn't around then.

So I see no reason what so ever to keep this SMT exception.

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