Messages in this thread | | | Date | Sat, 12 Jul 2008 12:19:22 -0700 | From | Max Krasnyansky <> | Subject | Re: current linux-2.6.git: cpusets completely broken |
| |
Dmitry Adamushko wrote: > 2008/7/12 Linus Torvalds <torvalds@linux-foundation.org>: >> >> On Sat, 12 Jul 2008, Vegard Nossum wrote: >>> Can somebody else please test/ack/review it too? This should eventually >>> go into 2.6.26 if it doesn't break anything else. >> And Dmitry, _please_ also explain what was going on. Why did things break >> from calling common_cpu_mem_hotplug_unplug() too much? That function is >> called pretty randomly anyway (for just about any random CPU event), so >> why did it fail in some circumstances? > > Upon CPU_DOWN_PREPARE, update_sched_domains() -> > detach_destroy_domains(&cpu_online_map) ; > does the following: > > /* > * Force a reinitialization of the sched domains hierarchy. The domains > * and groups cannot be updated in place without racing with the balancing > * code, so we temporarily attach all running cpus to the NULL domain > * which will prevent rebalancing while the sched domains are recalculated. > */ > > The sched-domains should be rebuilt when a CPU_DOWN ops. is completed, > effectivelly either upon CPU_DEAD{_FROZEN} (upon success) or > CPU_DOWN_FAILED{_FROZEN} (upon failure -- restore the things to their > initial state). That's what update_sched_domains() also does but only > for !CPUSETS case. > > With Max's patch, sched-domains' reinitialization is delegated to CPUSETS code: > > cpuset_handle_cpuhp() -> common_cpu_mem_hotplug_unplug() -> > rebuild_sched_domains() > > which as you've said "called pretty randomly anyway", e.g. for CPU_UP_PREPARE. > > [ ah, then rebuild_sched_domains() should not be there. It should be > nop for MEMPLUG events I presume - should make another patch. ] > > Being called for CPU_UP_PREPARE (and if its callback is called after > update_sched_domains()), it just negates all the work done by > update_sched_domains() -- i.e. a soon-to-be-offline cpu is included in > the sched-domains and that makes it visible for the load-balancer > while the CPU_DOWN ops. is in progress. > > __migrate_live_tasks() moves the tasks off a 'dead' cpu (it's already > "offline" when this function is called). > > try_to_wake_up() is called for one of these tasks from another CPU -> > the load-balancer (wake_idle()) picks up a "dead" CPU and places the > task on it. Then e.g. BUG_ON(rq->nr_running) detects this a bit later > -> oops. Ah, makes sense. Thanx for the explanation.
> Now another funny thing is that we probably have a memory leak with > common_cpu_mem_hotplug_unplug() "randomly" calling > rebuild_sched_domains() and sometimes re-allocating domains when they > already exist. I beleive that part is ok. We used to have a leak in the scheduler code where arch_init_sched_domains() just allocated new masks without freeing the old ones. I fixed that. rebuild_sched_domains() -> partition_sched_domains() is clean (I think). partition_sched_domains() first does the cleanup and then takes ownership of the domain masks.
btw It's perfectly ok (or at least it has be ok) to call rebuild_sched_domains() randomly because it's need to run every time sched_load_balance flags in the cpuset change, and on any other even that affects domains. As Paul J explained currently that's the only sane way to reconstruct the domains based on the cpuset settings.
Max
| |