Messages in this thread | | | Date | Tue, 11 Nov 2008 23:01:53 +0530 | From | Vaidyanathan Srinivasan <> | Subject | Re: [RFC PATCH v3 5/5] sched: activate active load balancing in new idle cpus |
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* Peter Zijlstra <a.p.zijlstra@chello.nl> [2008-11-11 18:21:50]:
> On Tue, 2008-11-11 at 22:34 +0530, Vaidyanathan Srinivasan wrote: > > * Peter Zijlstra <a.p.zijlstra@chello.nl> [2008-11-11 14:47:15]: > > > > > On Tue, 2008-11-11 at 00:03 +0530, Vaidyanathan Srinivasan wrote: > > > > Active load balancing is a process by which migration thread > > > > is woken up on the target CPU in order to pull current > > > > running task on another package into this newly idle > > > > package. > > > > > > > > This method is already in use with normal load_balance(), > > > > this patch introduces this method to new idle cpus when > > > > sched_mc is set to POWERSAVINGS_BALANCE_WAKEUP. > > > > > > > > This logic provides effective consolidation of short running > > > > daemon jobs in a almost idle system > > > > > > > > The side effect of this patch may be ping-ponging of tasks > > > > if the system is moderately utilised. May need to adjust the > > > > iterations before triggering. > > > > > > OK, I'm so not getting this patch.. > > > > > > if normal newly idle balancing fails that means the other runqueue has > > > only a single task on it (or some other really stubborn stuff), so then > > > you go move that one task that is already running, from one cpu to > > > another. > > > > > > _why_? > > > > > > The only answer I can come up with is that you prefer one cpu's > > > idle-ness over another - which makes sense, as you try to get whole > > > packages idle. > > > > Your answer is correct. We want to move that one task from a non-idle > > cpu to this cpu that is just going to be idle. > > > > This is the same method used to move task in load_balance(), I have > > extended it for load_balance_newidle() to make the consolidation > > faster at sched_mc=2. > > > > > > > But I'm not seeing where that package logic is hidden.. > > > > > > The package logic comes from find_busiest_group(). If there are no > > imbalance, then find_busiest_group() will return NULL. However when > > sched_mc={1,2} then find_busiest_group() will select a group > > from which a running task may be pulled to this cpu in order to make > > the other package idle. If there is no opportunity to make a package > > idle and if there are no imbalance, then find_busiest_group() will > > return NULL and no action will be taken in load_balance_newidle(). > > > > Under normal task pull operation due to imbalance, there will be more > > than one task in the source run queue and move_tasks() will succeed. > > ld_moved will be true and the active balance code will not be > > triggered. > > > > If we enter a scenario where we are moving the only running task from > > another cpu, then this should have been suggested by > > find_busiest_group's sched_mc balance logic and thus moving that task > > will potentially freeup the source package. > > > > Thanks for the careful review. > > Ah, right, thanks! > > Could you clarify this by adding a comment to this effect right before > the added code?
Sure. Will add detailed comments.
--Vaidy
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