On 10/22/2018 1:04 PM, Peter Zijlstra wrote: > On Mon, Oct 22, 2018 at 07:59:31AM -0700, Steve Sistare wrote: >> When a CPU has no more CFS tasks to run, and idle_balance() fails to >> find a task, then attempt to steal a task from an overloaded CPU in the >> same LLC. Maintain and use a bitmap of overloaded CPUs to efficiently >> identify candidates. To minimize search time, steal the first migratable >> task that is found when the bitmap is traversed. For fairness, search >> for migratable tasks on an overloaded CPU in order of next to run. >> >> This simple stealing yields a higher CPU utilization than idle_balance() >> alone, because the search is cheap, so it may be called every time the CPU >> is about to go idle. idle_balance() does more work because it searches >> widely for the busiest queue, so to limit its CPU consumption, it declines >> to search if the system is too busy. Simple stealing does not offload the >> globally busiest queue, but it is much better than running nothing at all. > > Why I don't dislike the idea; I feel it is unfortunate to have two > different mechanisms to do effectively the same thing. > > Can't we improve idle_balance() instead of building this parallel > functionality?
We could delete idle_balance() and use stealing exclusively for handling new idle. For each sd level, stealing would look for an overloaded CPU in the overloaded bitmap(s) that overlap that level. I played with that a little but it is not ready for prime time, and I did not want to hold the patch series for it. Also, I would like folks to get some production experience with stealing on a variety of architectures before considering a radical step like replacing idle_balance(). We could merge the stealing code into the idle_balance() code to get a union of the two, but IMO that would be less readable. We could remove the core and socket levels from idle_balance() and let stealing handle those levels. I think that makes sense after stealing performance is validated on more architectures, but we would still have two different mechanisms. - Steve
