Messages in this thread | | | Date | Tue, 23 Oct 2018 00:09:35 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH 00/10] steal tasks to improve CPU utilization |
| |
On Mon, Oct 22, 2018 at 03:07:10PM -0400, Steven Sistare wrote: > 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().
Fair enough. And yes, it might make sense to fully replace the current newidle balance with something along these lines.
> 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.
Yes, this would be a fairly simple change and make sense until we have a full replacement.
> 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.
Agreed; I don't think that'll be pretty.
| |