Re: [patch 5/5] sched: consolidate sbe sbf

[Nick Piggin]

Ingo Molnar wrote:
> * Nick Piggin <nickpiggin@xxxxxxxxxxxx> wrote:
>
>
> > 5/5
> >
> > Any ideas about what to do with schedstats?
> > Do we really need balance on exec and fork as seperate
> > statistics?
>
>
> > Consolidate balance-on-exec with balance-on-fork. This is made easy
> > by the sched-domains RCU patches.
> >
> > As well as the general goodness of code reduction, this allows
> > the runqueues to be unlocked during balance-on-fork.
> >
> > schedstats is a problem. Maybe just have balance-on-event instead
> > of distinguishing fork and exec?
> >
> > Signed-off-by: Nick Piggin <nickpiggin@xxxxxxxxxxxx>
>
>
> looks good.

One problem I just noticed, sorry. This is doing set_cpus_allowed
without holding the runqueue lock and without checking the hard
affinity mask either.

We could just do a set_cpus_allowed, or take the lock, set_cpus_allowed,
and take the new lock, but that's probably a bit heavy if we can avoid it.
In the interests of speed in this fast path, do you think we can do this
in sched_fork, before the task has even been put on the tasklist?

That would avoid all locking problems. Passing clone_flags into sched_fork
would not be a problem if we want to distinguish fork() and clone(CLONE_VM).

Yes? I'll cut a new patch to do just that.

>  Acked-by: Ingo Molnar <mingo@xxxxxxx>
>
> while the code is now consolidated, i think we still need the separate 
> fork/exec stats for schedstat.

This makes it a bit harder then, to get good stats in the sched-domain
(which is really what we want). It would basically mean doing
if (balance fork)
	schedstat_inc(sbf_cnt);
else if (balance exec)
	schedstat_inc(sbe_cnt);
etc.

That should all get optimised out by the compiler, but still a bit ugly.
Any ideas?


--
SUSE Labs, Novell Inc.

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