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Message-ID: <20201130093333.GD473773@balbir-desktop>
Date: Mon, 30 Nov 2020 20:33:33 +1100
From: Balbir Singh <bsingharora@...il.com>
To: "Li, Aubrey" <aubrey.li@...ux.intel.com>
Cc: Peter Zijlstra <peterz@...radead.org>,
"Joel Fernandes (Google)" <joel@...lfernandes.org>,
Nishanth Aravamudan <naravamudan@...italocean.com>,
Julien Desfossez <jdesfossez@...italocean.com>,
Tim Chen <tim.c.chen@...ux.intel.com>,
Vineeth Pillai <viremana@...ux.microsoft.com>,
Aaron Lu <aaron.lwe@...il.com>,
Aubrey Li <aubrey.intel@...il.com>, tglx@...utronix.de,
linux-kernel@...r.kernel.org, mingo@...nel.org,
torvalds@...ux-foundation.org, fweisbec@...il.com,
keescook@...omium.org, kerrnel@...gle.com,
Phil Auld <pauld@...hat.com>,
Valentin Schneider <valentin.schneider@....com>,
Mel Gorman <mgorman@...hsingularity.net>,
Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
Paolo Bonzini <pbonzini@...hat.com>, vineeth@...byteword.org,
Chen Yu <yu.c.chen@...el.com>,
Christian Brauner <christian.brauner@...ntu.com>,
Agata Gruza <agata.gruza@...el.com>,
Antonio Gomez Iglesias <antonio.gomez.iglesias@...el.com>,
graf@...zon.com, konrad.wilk@...cle.com, dfaggioli@...e.com,
pjt@...gle.com, rostedt@...dmis.org, derkling@...gle.com,
benbjiang@...cent.com,
Alexandre Chartre <alexandre.chartre@...cle.com>,
James.Bottomley@...senpartnership.com, OWeisse@...ch.edu,
Dhaval Giani <dhaval.giani@...cle.com>,
Junaid Shahid <junaids@...gle.com>, jsbarnes@...gle.com,
chris.hyser@...cle.com, Ben Segall <bsegall@...gle.com>,
Josh Don <joshdon@...gle.com>, Hao Luo <haoluo@...gle.com>,
Tom Lendacky <thomas.lendacky@....com>,
Aubrey Li <aubrey.li@...el.com>,
"Paul E. McKenney" <paulmck@...nel.org>,
Tim Chen <tim.c.chen@...el.com>
Subject: Re: [PATCH -tip 14/32] sched: migration changes for core scheduling
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@...ux.intel.com>
> >> 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@...italocean.com>
> >> Signed-off-by: Aubrey Li <aubrey.li@...ux.intel.com>
> >> Signed-off-by: Tim Chen <tim.c.chen@...ux.intel.com>
> >> Signed-off-by: Vineeth Remanan Pillai <viremana@...ux.microsoft.com>
> >> Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
> >> ---
> >> 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
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