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Date:   Wed, 28 Apr 2021 11:25:19 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Christian Borntraeger <borntraeger@de.ibm.com>
Cc:     bristot@redhat.com, bsegall@google.com, dietmar.eggemann@arm.com,
        greg@kroah.com, gregkh@linuxfoundation.org, joshdon@google.com,
        juri.lelli@redhat.com, linux-kernel@vger.kernel.org,
        linux@rasmusvillemoes.dk, mgorman@suse.de, mingo@kernel.org,
        rostedt@goodmis.org, valentin.schneider@arm.com,
        vincent.guittot@linaro.org, linux-s390@vger.kernel.org,
        kvm@vger.kernel.org
Subject: Re: sched: Move SCHED_DEBUG sysctl to debugfs
Message-ID: <YIkp/6/NDL7KsvpY@hirez.programming.kicks-ass.net>
References: <20210412102001.287610138@infradead.org>
 <20210427145925.5246-1-borntraeger@de.ibm.com>
 <YIkgzUWEPaXQTCOv@hirez.programming.kicks-ass.net>
 <cf2a6c6c-21ea-df7b-94d1-940a344b8d26@de.ibm.com>
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On Wed, Apr 28, 2021 at 10:54:37AM +0200, Christian Borntraeger wrote:
> 
> 
> On 28.04.21 10:46, Peter Zijlstra wrote:
> > On Tue, Apr 27, 2021 at 04:59:25PM +0200, Christian Borntraeger wrote:
> > > Peter,
> > > 
> > > I just realized that we moved away sysctl tunabled to debugfs in next.
> > > We have seen several cases where it was benefitial to set
> > > sched_migration_cost_ns to a lower value. For example with KVM I can
> > > easily get 50% more transactions with 50000 instead of 500000.
> > > Until now it was possible to use tuned or /etc/sysctl.conf to set
> > > these things permanently.
> > > 
> > > Given that some people do not want to have debugfs mounted all the time
> > > I would consider this a regression. The sysctl tunable was always
> > > available.
> > > 
> > > I am ok with the "informational" things being in debugfs, but not
> > > the tunables. So how do we proceed here?
> > 
> > It's all SCHED_DEBUG; IOW you're relying on DEBUG infrastructure for
> > production performance, and that's your fail.
> 
> No its not. sched_migration_cost_ns was NEVER protected by CONFIG_SCHED_DEBUG.
> It was available on all kernels with CONFIG_SMP.

The relevant section from origin/master:kernel/sysctl.c:

#ifdef CONFIG_SCHED_DEBUG
	{
		.procname	= "sched_min_granularity_ns",
		.data		= &sysctl_sched_min_granularity,
		.maxlen		= sizeof(unsigned int),
		.mode		= 0644,
		.proc_handler	= sched_proc_update_handler,
		.extra1		= &min_sched_granularity_ns,
		.extra2		= &max_sched_granularity_ns,
	},
	{
		.procname	= "sched_latency_ns",
		.data		= &sysctl_sched_latency,
		.maxlen		= sizeof(unsigned int),
		.mode		= 0644,
		.proc_handler	= sched_proc_update_handler,
		.extra1		= &min_sched_granularity_ns,
		.extra2		= &max_sched_granularity_ns,
	},
	{
		.procname	= "sched_wakeup_granularity_ns",
		.data		= &sysctl_sched_wakeup_granularity,
		.maxlen		= sizeof(unsigned int),
		.mode		= 0644,
		.proc_handler	= sched_proc_update_handler,
		.extra1		= &min_wakeup_granularity_ns,
		.extra2		= &max_wakeup_granularity_ns,
	},
#ifdef CONFIG_SMP
	{
		.procname	= "sched_tunable_scaling",
		.data		= &sysctl_sched_tunable_scaling,
		.maxlen		= sizeof(enum sched_tunable_scaling),
		.mode		= 0644,
		.proc_handler	= sched_proc_update_handler,
		.extra1		= &min_sched_tunable_scaling,
		.extra2		= &max_sched_tunable_scaling,
	},
	{
		.procname	= "sched_migration_cost_ns",
		.data		= &sysctl_sched_migration_cost,
		.maxlen		= sizeof(unsigned int),
		.mode		= 0644,
		.proc_handler	= proc_dointvec,
	},
	{
		.procname	= "sched_nr_migrate",
		.data		= &sysctl_sched_nr_migrate,
		.maxlen		= sizeof(unsigned int),
		.mode		= 0644,
		.proc_handler	= proc_dointvec,
	},
#ifdef CONFIG_SCHEDSTATS
	{
		.procname	= "sched_schedstats",
		.data		= NULL,
		.maxlen		= sizeof(unsigned int),
		.mode		= 0644,
		.proc_handler	= sysctl_schedstats,
		.extra1		= SYSCTL_ZERO,
		.extra2		= SYSCTL_ONE,
	},
#endif /* CONFIG_SCHEDSTATS */
#endif /* CONFIG_SMP */
#ifdef CONFIG_NUMA_BALANCING
	{
		.procname	= "numa_balancing_scan_delay_ms",
		.data		= &sysctl_numa_balancing_scan_delay,
		.maxlen		= sizeof(unsigned int),
		.mode		= 0644,
		.proc_handler	= proc_dointvec,
	},
	{
		.procname	= "numa_balancing_scan_period_min_ms",
		.data		= &sysctl_numa_balancing_scan_period_min,
		.maxlen		= sizeof(unsigned int),
		.mode		= 0644,
		.proc_handler	= proc_dointvec,
	},
	{
		.procname	= "numa_balancing_scan_period_max_ms",
		.data		= &sysctl_numa_balancing_scan_period_max,
		.maxlen		= sizeof(unsigned int),
		.mode		= 0644,
		.proc_handler	= proc_dointvec,
	},
	{
		.procname	= "numa_balancing_scan_size_mb",
		.data		= &sysctl_numa_balancing_scan_size,
		.maxlen		= sizeof(unsigned int),
		.mode		= 0644,
		.proc_handler	= proc_dointvec_minmax,
		.extra1		= SYSCTL_ONE,
	},
	{
		.procname	= "numa_balancing",
		.data		= NULL, /* filled in by handler */
		.maxlen		= sizeof(unsigned int),
		.mode		= 0644,
		.proc_handler	= sysctl_numa_balancing,
		.extra1		= SYSCTL_ZERO,
		.extra2		= SYSCTL_ONE,
	},
#endif /* CONFIG_NUMA_BALANCING */
#endif /* CONFIG_SCHED_DEBUG */

How is migration_cost not under SCHED_DEBUG? The bigger problem is that
world+dog has SCHED_DEBUG=y in their .config.