Messages in this thread Patch in this message | | | Date | Mon, 9 Feb 2015 12:22:37 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH] sched, autogroup: Fix failure when writing to cpu.rt_runtime_us |
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On Sat, Feb 07, 2015 at 03:02:11PM +0800, Zefan Li wrote: > Before 8323f26ce342 "sched: Fix race in task_group()", task_group() of > those RT tasks always return root_task_group, but the escape can't happen.
Ah, yes, I'm an idiot. I'm not sure what I was thinking, but I seemed to have confused myself very well indeed.
> After commit 8323f26ce342, task_group always return root_task_group except > for the case I showed: > > 1. Change scheduling policy before setsid(): > > # cat /proc/sched_debug | grep test > R test 4194 24851.893077 945 120 24851.893077 11196.482331 0.000000 / > > 2. Change policy after setsid(): > > R test 4142 4962.517723 420 120 4962.517723 4974.126149 0.000000 /autogroup-44
Yes, which of course is inconsistent as well, it really is in the autogroup, regardless of its class.
> I think we can fix it with: > > diff --git a/kernel/sched/auto_group.c b/kernel/sched/auto_group.c > index 8a2e230..8c3a169 100644 > --- a/kernel/sched/auto_group.c > +++ b/kernel/sched/auto_group.c > @@ -115,9 +115,6 @@ bool task_wants_autogroup(struct task_struct *p, struct task_group *tg) > if (tg != &root_task_group) > return false; > > - if (p->sched_class != &fair_sched_class) > - return false; > -
Yes.
> This is exactly what I did at first, but besides the issue described above, > seems it might lead to starving RT tasks. > > If there's some rt task in autogroups but none in root cgroup, it's allowed > to set rt_runtime to 0, so I think we have to disallow this setting, like what > we already do with global rt_runtime.
> @@ -7540,6 +7543,9 @@ static int __rt_schedulable(struct task_group *tg, u64 period, u64 runtime) > .rt_runtime = runtime, > }; > > + if (tg == &root_task_group && runtime == 0) > + return -EINVAL; > +
Indeed, setting runtime=0 for the root group is a very bad thing regardless of this patch. It would disallow the kernel from creating RT threads, which it needs for 'correct' operation in a number of cases.
But lets make that a separate patch.
So how about this?
--- Subject: sched, autogroup: Fix failure to set cpu.rt_runtime_us From: Peter Zijlstra <peterz@infradead.org> Date: Mon Feb 9 11:53:18 CET 2015
Because task_group() uses a cache of autogroup_task_group(), whoes output depends on sched_class, switching classes can generate problems.
In particular, when started as fair, the cache points to the autogroup, so when switching to RT the tg_rt_schedulable() test fails for every cpu.rt_{runtime,period}_us change because now the autogroup has tasks and no runtime.
Furthermore, going back to the previous semantics of varying task_group() with sched_class has the down-side that the sched_debug output varies as well, even though the task really is in the autogroup.
Therefore add an autogroup exception to tg_has_rt_tasks() -- such that both (all) task_group() usages in sched/core now have one. And remove all the remnants of the variable task_group() output.
Cc: Mike Galbraith <umgwanakikbuti@gmail.com> Cc: Stefan Bader <stefan.bader@canonical.com> Reported-by: Zefan Li <lizefan@huawei.com> Fixes: 8323f26ce342 ("sched: Fix race in task_group()") Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> --- kernel/sched/auto_group.c | 6 +----- kernel/sched/core.c | 6 ++++++ 2 files changed, 7 insertions(+), 5 deletions(-)
--- a/kernel/sched/auto_group.c +++ b/kernel/sched/auto_group.c @@ -87,8 +87,7 @@ static inline struct autogroup *autogrou * so we don't have to move tasks around upon policy change, * or flail around trying to allocate bandwidth on the fly. * A bandwidth exception in __sched_setscheduler() allows - * the policy change to proceed. Thereafter, task_group() - * returns &root_task_group, so zero bandwidth is required. + * the policy change to proceed. */ free_rt_sched_group(tg); tg->rt_se = root_task_group.rt_se; @@ -115,9 +114,6 @@ bool task_wants_autogroup(struct task_st if (tg != &root_task_group) return false; - if (p->sched_class != &fair_sched_class) - return false; - /* * We can only assume the task group can't go away on us if * autogroup_move_group() can see us on ->thread_group list. --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -7644,6 +7644,12 @@ static inline int tg_has_rt_tasks(struct { struct task_struct *g, *p; + /* + * Autogroups do not have RT tasks; see autogroup_create(). + */ + if (task_group_is_autogroup(tg)) + return 0; + for_each_process_thread(g, p) { if (rt_task(p) && task_group(p) == tg) return 1;
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