lmbench ctxsw regression with CFS

[Nick Piggin]

Hi,

I didn't follow all of the scheduler debates and flamewars, so apologies
if this was already covered. Anyway.

lmbench 3 lat_ctx context switching time with 2 processes bound to a
single core increases by between 25%-35% on my Core2 system (didn't do
enough runs to get more significance, but it is around 30%). The problem
bisected to the main CFS commit.

I was really hoping that a smaller runqueue data structure could actually
increase performance with the common case of small numbers of tasks :(

I assume this was a known issue before CFS was merged. Do you know what is
causing the slowdown? Any plans to fix it?

Thanks,
Nick
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