Re: Re: [PATCH v3] sched/numa: add per-process numa_balancing

[Gang Li]

On 2022/1/12 22:43, Peter Zijlstra wrote:
> On Mon, Dec 06, 2021 at 10:45:28AM +0800, Gang Li wrote:
> > This patch add a new api PR_NUMA_BALANCING in prctl.
> >
> > A large number of page faults will cause performance loss when numa
> > balancing is performing. Thus those processes which care about worst-case
> > performance need numa balancing disabled. Others, on the contrary, allow a
> > temporary performance loss in exchange for higher average performance, so
> > enable numa balancing is better for them.
> >
> > Numa balancing can only be controlled globally by
> > /proc/sys/kernel/numa_balancing. Due to the above case, we want to
> > disable/enable numa_balancing per-process instead.
> >
> > Add numa_balancing under mm_struct. Then use it in task_tick_fair.
> >
> > Set per-process numa balancing:
> > 	prctl(PR_NUMA_BALANCING, PR_SET_NUMAB_DISABLE); //disable
> > 	prctl(PR_NUMA_BALANCING, PR_SET_NUMAB_ENABLE);  //enable
> > 	prctl(PR_NUMA_BALANCING, PR_SET_NUMAB_DEFAULT); //follow global
>
> This seems to imply you can prctl(ENABLE) even if the global is
> disabled, IOW sched_numa_balancing is off.

Of course, this semantic has been discussed here FYI.
 https://lore.kernel.org/all/20211118085819.GD3301@xxxxxxx/

On 11/18/21 4:58 PM, Mel Gorman wrote:
> On Thu, Nov 18, 2021 at 11:26:30AM +0800, Gang Li wrote:
>> 3. prctl(PR_NUMA_BALANCING, PR_SET_NUMAB_ENABLE);  //enable
>
> If PR_SET_NUMAB_ENABLE enables numa balancing for a task when
> kernel.numa_balancing == 0 instead of returning an error then sure.


> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 884f29d07963..2980f33ac61f 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -11169,8 +11169,12 @@ static void task_tick_fair(struct rq *rq, struct task_struct *curr, int queued)
> >   		entity_tick(cfs_rq, se, queued);
> >   	}
> >   
> > -	if (static_branch_unlikely(&sched_numa_balancing))
> > +#ifdef CONFIG_NUMA_BALANCING
> > +	if (curr->mm && (curr->mm->numab_enabled == NUMAB_ENABLED
> > +	    || (static_branch_unlikely(&sched_numa_balancing)
> > +	    && curr->mm->numab_enabled == NUMAB_DEFAULT)))
> >   		task_tick_numa(rq, curr);
> > +#endif
> >   
> >   	update_misfit_status(curr, rq);
> >   	update_overutilized_status(task_rq(curr));
>
> There's just about everything wrong there... not least of all the
> horrific coding style.

horrible code, yes.
I'll do some code clean.
--
Thanks
Gang Li