Messages in this thread | | | Date | Sun, 23 Sep 2007 23:21:26 -0700 (PDT) | From | Tong Li <> | Subject | Re: [git] CFS-devel, group scheduler, fixes |
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On Sun, 23 Sep 2007, Mike Galbraith wrote:
> On Sat, 2007-09-22 at 12:01 +0200, Mike Galbraith wrote: >> On Fri, 2007-09-21 at 20:27 -0700, Tong Li wrote: >>> Mike, >>> >>> Could you try this patch to see if it solves the latency problem? >> >> No, but it helps some when running two un-pinned busy loops, one at nice >> 0, and the other at nice 19. Yesterday I hit latencies of up to 1.2 >> _seconds_ doing this, and logging sched_debug and /proc/`pidof >> Xorg`/sched from SCHED_RR shells. > > Looking at a log (snippet attached) from this morning with the last hunk > of your patch still intact, it looks like min_vruntime is being modified > after a task is queued. If you look at the snippet, you'll see the nice > 19 bash busy loop on CPU1 with a vruntime of 3010385.345325, and one > second later on CPU1 with it's vruntime at 2814952.425082, but > min_vruntime is 3061874.838356.
I think this could be what was happening: between the two seconds, CPU 0 becomes idle and it pulls the nice 19 task over via pull_task(), which calls set_task_cpu(), which changes the task's vruntime to the current min_vruntime of CPU 0 (in my patch). Then, after set_task_cpu(), CPU 0 calls activate_task(), which calls enqueue_task() and in turn update_curr(). Now, nr_running on CPU 0 is 0, so sync_vruntime() gets called and CPU 0's min_vruntime gets synced to the system max. Thus, the nice 19 task now has a vruntime less than CPU 0's min_vruntime. I think this can be fixed by adding the following code in set_task_cpu() before we adjust p->vruntime:
if (!new_rq->cfs.nr_running) sync_vruntime(new_rq);
> static void sync_vruntime(struct cfs_rq *cfs_rq) > { > struct rq *rq; > - int cpu; > + int cpu, wrote = 0; > > for_each_online_cpu(cpu) { > rq = &per_cpu(runqueues, cpu); > + if (spin_is_locked(&rq->lock)) > + continue; > + smp_wmb(); > cfs_rq->min_vruntime = max_vruntime(cfs_rq->min_vruntime, > rq->cfs.min_vruntime); > + wrote++; > } > - schedstat_inc(cfs_rq, nr_sync_min_vruntime); > + if (wrote) > + schedstat_inc(cfs_rq, nr_sync_min_vruntime); > }
I think this rq->lock check works because it prevents the above scenario (CPU 0 is in pull_task so it must hold the rq lock). But my concern is that it may be too conservative, since sync_vruntime is called by update_curr, which mostly gets called in enqueue_task() and dequeue_task(), both of which are often invoked with the rq lock being held. Thus, if we don't allow sync_vruntime when rq lock is held, the sync will occur much less frequently and thus weaken cross-CPU fairness.
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