Re: [PATCH -tip 14/32] sched: migration changes for core scheduling

[Balbir Singh]

On Thu, Nov 26, 2020 at 05:26:31PM +0800, Li, Aubrey wrote:
> On 2020/11/26 16:32, Balbir Singh wrote:
> > On Thu, Nov 26, 2020 at 11:20:41AM +0800, Li, Aubrey wrote:
> >> On 2020/11/26 6:57, Balbir Singh wrote:
> >>> On Wed, Nov 25, 2020 at 11:12:53AM +0800, Li, Aubrey wrote:
> >>>> On 2020/11/24 23:42, Peter Zijlstra wrote:
> >>>>> On Mon, Nov 23, 2020 at 12:36:10PM +0800, Li, Aubrey wrote:
> >>>>>>>> +#ifdef CONFIG_SCHED_CORE
> >>>>>>>> +		/*
> >>>>>>>> +		 * Skip this cpu if source task's cookie does not match
> >>>>>>>> +		 * with CPU's core cookie.
> >>>>>>>> +		 */
> >>>>>>>> +		if (!sched_core_cookie_match(cpu_rq(cpu), env->p))
> >>>>>>>> +			continue;
> >>>>>>>> +#endif
> >>>>>>>> +
> >>>>>>>
> >>>>>>> Any reason this is under an #ifdef? In sched_core_cookie_match() won't
> >>>>>>> the check for sched_core_enabled() do the right thing even when
> >>>>>>> CONFIG_SCHED_CORE is not enabed?> 
> >>>>>> Yes, sched_core_enabled works properly when CONFIG_SCHED_CORE is not
> >>>>>> enabled. But when CONFIG_SCHED_CORE is not enabled, it does not make
> >>>>>> sense to leave a core scheduler specific function here even at compile
> >>>>>> time. Also, for the cases in hot path, this saves CPU cycles to avoid
> >>>>>> a judgment.
> >>>>>
> >>>>> No, that's nonsense. If it works, remove the #ifdef. Less (#ifdef) is
> >>>>> more.
> >>>>>
> >>>>
> >>>> Okay, I pasted the refined patch here.
> >>>> @Joel, please let me know if you want me to send it in a separated thread.
> >>>>
> >>>
> >>> You still have a bunch of #ifdefs, can't we just do
> >>>
> >>> #ifndef CONFIG_SCHED_CORE
> >>> static inline bool sched_core_enabled(struct rq *rq)
> >>> {
> >>>         return false;
> >>> }
> >>> #endif
> >>>
> >>> and frankly I think even that is not needed because there is a jump
> >>> label __sched_core_enabled that tells us if sched_core is enabled or
> >>> not.
> >>
> >> Hmm..., I need another wrapper for CONFIG_SCHED_CORE specific variables.
> >> How about this one?
> >>
> > 
> > Much better :)
> >  
> >> Thanks,
> >> -Aubrey
> >>
> >> From 61dac9067e66b5b9ea26c684c8c8235714bab38a Mon Sep 17 00:00:00 2001
> >> From: Aubrey Li <aubrey.li@xxxxxxxxxxxxxxx>
> >> Date: Thu, 26 Nov 2020 03:08:04 +0000
> >> Subject: [PATCH] sched: migration changes for core scheduling
> >>
> >>  - Don't migrate if there is a cookie mismatch
> >>      Load balance tries to move task from busiest CPU to the
> >>      destination CPU. When core scheduling is enabled, if the
> >>      task's cookie does not match with the destination CPU's
> >>      core cookie, this task will be skipped by this CPU. This
> >>      mitigates the forced idle time on the destination CPU.
> >>
> >>  - Select cookie matched idle CPU
> >>      In the fast path of task wakeup, select the first cookie matched
> >>      idle CPU instead of the first idle CPU.
> >>
> >>  - Find cookie matched idlest CPU
> >>      In the slow path of task wakeup, find the idlest CPU whose core
> >>      cookie matches with task's cookie
> >>
> >>  - Don't migrate task if cookie not match
> >>      For the NUMA load balance, don't migrate task to the CPU whose
> >>      core cookie does not match with task's cookie
> >>
> >> Tested-by: Julien Desfossez <jdesfossez@xxxxxxxxxxxxxxxx>
> >> Signed-off-by: Aubrey Li <aubrey.li@xxxxxxxxxxxxxxx>
> >> Signed-off-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
> >> Signed-off-by: Vineeth Remanan Pillai <viremana@xxxxxxxxxxxxxxxxxxx>
> >> Signed-off-by: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
> >> ---
> >>  kernel/sched/fair.c  | 57 ++++++++++++++++++++++++++++++++++++++++----
> >>  kernel/sched/sched.h | 43 +++++++++++++++++++++++++++++++++
> >>  2 files changed, 95 insertions(+), 5 deletions(-)
> >>
> >> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> >> index de82f88ba98c..70dd013dff1d 100644
> >> --- a/kernel/sched/fair.c
> >> +++ b/kernel/sched/fair.c
> >> @@ -1921,6 +1921,13 @@ static void task_numa_find_cpu(struct task_numa_env *env,
> >>  		if (!cpumask_test_cpu(cpu, env->p->cpus_ptr))
> >>  			continue;
> >>  
> >> +		/*
> >> +		 * Skip this cpu if source task's cookie does not match
> >> +		 * with CPU's core cookie.
> >> +		 */
> >> +		if (!sched_core_cookie_match(cpu_rq(cpu), env->p))
> >> +			continue;
> >> +
> >>  		env->dst_cpu = cpu;
> >>  		if (task_numa_compare(env, taskimp, groupimp, maymove))
> >>  			break;
> >> @@ -5867,11 +5874,15 @@ find_idlest_group_cpu(struct sched_group *group, struct task_struct *p, int this
> >>  
> >>  	/* Traverse only the allowed CPUs */
> >>  	for_each_cpu_and(i, sched_group_span(group), p->cpus_ptr) {
> >> +		struct rq *rq = cpu_rq(i);
> >> +
> >> +		if (!sched_core_cookie_match(rq, p))
> >> +			continue;
> >> +
> >>  		if (sched_idle_cpu(i))
> >>  			return i;
> >>  
> >>  		if (available_idle_cpu(i)) {
> >> -			struct rq *rq = cpu_rq(i);
> >>  			struct cpuidle_state *idle = idle_get_state(rq);
> >>  			if (idle && idle->exit_latency < min_exit_latency) {
> >>  				/*
> >> @@ -6129,8 +6140,19 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
> >>  	for_each_cpu_wrap(cpu, cpus, target) {
> >>  		if (!--nr)
> >>  			return -1;
> >> -		if (available_idle_cpu(cpu) || sched_idle_cpu(cpu))
> >> -			break;
> >> +
> >> +		if (available_idle_cpu(cpu) || sched_idle_cpu(cpu)) {
> >> +			/*
> >> +			 * If Core Scheduling is enabled, select this cpu
> >> +			 * only if the process cookie matches core cookie.
> >> +			 */
> >> +			if (sched_core_enabled(cpu_rq(cpu))) {
> >> +				if (__cookie_match(cpu_rq(cpu), p))
> >> +					break;
> >> +			} else {
> >> +				break;
> >> +			}
> >> +		}
> > 
> > Isn't this better and equivalent?
> > 
> > 	if ((available_idle_cpu(cpu) || sched_idle_cpu(cpu)) &&
> > 		sched_core_cookie_match(cpu_rq(cpu), p))
> > 		break;
> >
> 
>  
> That's my previous implementation in the earlier version.
> But since here is the hot code path, we want to remove the idle
> core check in sched_core_cookie_match.

I see, so we basically need a jump label, if sched_core_cookie_match
can be inlined with a check for sched_core_enabled() upfront, it might
solve a lot of the concern, readability of this section of code is not
the best.

> 
> >>  	}
> >>  
> >>  	time = cpu_clock(this) - time;
> >> @@ -7530,8 +7552,9 @@ int can_migrate_task(struct task_struct *p, struct lb_env *env)
> >>  	 * We do not migrate tasks that are:
> >>  	 * 1) throttled_lb_pair, or
> >>  	 * 2) cannot be migrated to this CPU due to cpus_ptr, or
> >> -	 * 3) running (obviously), or
> >> -	 * 4) are cache-hot on their current CPU.
> >> +	 * 3) task's cookie does not match with this CPU's core cookie
> >> +	 * 4) running (obviously), or
> >> +	 * 5) are cache-hot on their current CPU.
> >>  	 */
> >>  	if (throttled_lb_pair(task_group(p), env->src_cpu, env->dst_cpu))
> >>  		return 0;
> >> @@ -7566,6 +7589,13 @@ int can_migrate_task(struct task_struct *p, struct lb_env *env)
> >>  		return 0;
> >>  	}
> >>  
> >> +	/*
> >> +	 * Don't migrate task if the task's cookie does not match
> >> +	 * with the destination CPU's core cookie.
> >> +	 */
> >> +	if (!sched_core_cookie_match(cpu_rq(env->dst_cpu), p))
> >> +		return 0;
> >> +
> >>  	/* Record that we found atleast one task that could run on dst_cpu */
> >>  	env->flags &= ~LBF_ALL_PINNED;
> >>  
> >> @@ -8792,6 +8822,23 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p, int this_cpu)
> >>  					p->cpus_ptr))
> >>  			continue;
> >>  
> >> +		if (sched_core_enabled(cpu_rq(this_cpu))) {
> >> +			int i = 0;
> >> +			bool cookie_match = false;
> >> +
> >> +			for_each_cpu(i, sched_group_span(group)) {
> >> +				struct rq *rq = cpu_rq(i);
> >> +
> >> +				if (sched_core_cookie_match(rq, p)) {
> >> +					cookie_match = true;
> >> +					break;
> >> +				}
> >> +			}
> >> +			/* Skip over this group if no cookie matched */
> >> +			if (!cookie_match)
> >> +				continue;
> >> +		}
> >> +
> > 
> > Again, I think this can be refactored because sched_core_cookie_match checks
> > for sched_core_enabled()
> > 
> > 	int i = 0;
> > 	bool cookie_match = false;
> > 	for_each_cpu(i, sched_group_span(group)) {
> > 		if (sched_core_cookie_match(cpu_rq(i), p))
> > 			break;
> > 	}
> > 	if (i >= nr_cpu_ids)
> > 		continue;
> 
> There is a loop here when CONFIG_SCHED_CORE=n, which is unwanted I guess.
> 

Yes, potentially, may be abstract the for_each_cpu into a function and then
optimize out the case for SCHED_CORE=n, I feel all the extra checks in the
various places make the code hard to read.

Balbir