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Message-ID: <20210621122553.GA32028@e120325.cambridge.arm.com>
Date: Mon, 21 Jun 2021 13:25:53 +0100
From: Beata Michalska <beata.michalska@....com>
To: Joel Fernandes <joel@...lfernandes.org>
Cc: LKML <linux-kernel@...r.kernel.org>,
Dietmar Eggemann <dietmar.eggemann@....com>,
Ingo Molnar <mingo@...hat.com>, Josh Don <joshdon@...gle.com>,
Juri Lelli <juri.lelli@...hat.com>,
Luis Chamberlain <mcgrof@...nel.org>,
Mel Gorman <mgorman@...e.de>,
Peter Zijlstra <peterz@...radead.org>,
Qais Yousef <qais.yousef@....com>,
"Rafael J. Wysocki" <rjw@...ysocki.net>,
Vincent Guittot <vincent.guittot@...aro.org>,
Viresh Kumar <viresh.kumar@...aro.org>,
Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [RFC] schedutil: Fix iowait boost issues for slow I/O devices
Hi Joel,
On Fri, Jun 18, 2021 at 12:07:16AM -0400, Joel Fernandes wrote:
> Forgot to CC +Beata Michalska , sorry.
>
> On Fri, Jun 18, 2021 at 12:06 AM Joel Fernandes (Google)
> <joel@...lfernandes.org> wrote:
> >
> > The iowait boost code is currently broken. Following are the issues and
> > possible solitions:
> >
> > Issue #1: If a CPU requests iowait boost in a cluster, another CPU can
> > go ahead and decay it very quickly if it thinks there's no new request
> > for the iowait boosting CPU in the meanwhile. To fix this, track when
> > the iowait boost was last applied to a policy. This happens when
> > should_update_freq() returns true. I have made the code wait for at
> > least 10 ticks between 2 different iowait_boost_apply() for any decay to
> > happen, and made it configurable via sysctl.
> >
> > Issue #2: If the iowait is longer than a tick, then successive iowait
> > boost doubling does not happen. So I/O waiting tasks for slow devices
> > never gets a boost. This is much worse if the tick rate is high since we
> > use ticks to measure if no new I/O completion happened. To workaround
> > this, be liberal about how many ticks should elapse before resetting the
> > boost. I have chosen a conservative number of 20, and made it
> > configurable via sysctl.
> >
> > Tested on a 6+2 ARM64 device, running dd:
> > dd if=zeros of=/dev/null bs=1M count=64 iflag=direct
> > Throughput improves from 180MB/s to 200MB/s (~5 percent).
> >
> > Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
> >
> > ---
> > NOTE: This RFC patch is for discussion of the issues and I am posting for
> > comments. Beata and Vince are also working on an alternate solution.
> >
> > include/linux/sched/sysctl.h | 3 +++
> > kernel/sched/core.c | 3 +++
> > kernel/sched/cpufreq_schedutil.c | 22 +++++++++++++++++++---
> > kernel/sched/sched.h | 3 +++
> > kernel/sysctl.c | 14 ++++++++++++++
> > 5 files changed, 42 insertions(+), 3 deletions(-)
> >
> > diff --git a/include/linux/sched/sysctl.h b/include/linux/sched/sysctl.h
> > index db2c0f34aaaf..03ac66b45406 100644
> > --- a/include/linux/sched/sysctl.h
> > +++ b/include/linux/sched/sysctl.h
> > @@ -53,6 +53,9 @@ extern int sysctl_resched_latency_warn_ms;
> > extern int sysctl_resched_latency_warn_once;
> > #endif
> >
> > +extern unsigned int sysctl_iowait_reset_ticks;
> > +extern unsigned int sysctl_iowait_apply_ticks;
> > +
> > /*
> > * control realtime throttling:
> > *
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index adea0b1e8036..e44985fb6a93 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -76,6 +76,9 @@ __read_mostly int sysctl_resched_latency_warn_once = 1;
> > */
> > const_debug unsigned int sysctl_sched_nr_migrate = 32;
> >
> > +unsigned int sysctl_iowait_reset_ticks = 20;
> > +unsigned int sysctl_iowait_apply_ticks = 10;
> > +
> > /*
> > * period over which we measure -rt task CPU usage in us.
> > * default: 1s
> > diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> > index 4f09afd2f321..4e4e1b0aec6c 100644
> > --- a/kernel/sched/cpufreq_schedutil.c
> > +++ b/kernel/sched/cpufreq_schedutil.c
> > @@ -27,6 +27,7 @@ struct sugov_policy {
> > struct list_head tunables_hook;
> >
> > raw_spinlock_t update_lock;
> > + u64 last_update;
> > u64 last_freq_update_time;
> > s64 freq_update_delay_ns;
> > unsigned int next_freq;
> > @@ -186,9 +187,13 @@ static bool sugov_iowait_reset(struct sugov_cpu *sg_cpu, u64 time,
> > bool set_iowait_boost)
> > {
> > s64 delta_ns = time - sg_cpu->last_update;
> > + unsigned int ticks = TICK_NSEC;
> > +
> > + if (sysctl_iowait_reset_ticks)
> > + ticks = sysctl_iowait_reset_ticks * TICK_NSEC;
> >
I am not sure how that would play with power vs performance balance.
And what about sporadic I/O wake-ups ? Wouldn't that mess too much allowing
freq spikes for smht which is actually not I/O heavy ?
I guess same would apply to the changes to sugov_iowait_apply.
---
BR
B.
> > - /* Reset boost only if a tick has elapsed since last request */
> > - if (delta_ns <= TICK_NSEC)
> > + /* Reset boost only if enough ticks has elapsed since last request. */
> > + if (delta_ns <= ticks)
> > return false;
> >
> > sg_cpu->iowait_boost = set_iowait_boost ? IOWAIT_BOOST_MIN : 0;
> > @@ -260,6 +265,7 @@ static void sugov_iowait_boost(struct sugov_cpu *sg_cpu, u64 time,
> > */
> > static void sugov_iowait_apply(struct sugov_cpu *sg_cpu, u64 time)
> > {
> > + struct sugov_policy *sg_policy = sg_cpu->sg_policy;
> > unsigned long boost;
> >
> > /* No boost currently required */
> > @@ -270,7 +276,9 @@ static void sugov_iowait_apply(struct sugov_cpu *sg_cpu, u64 time)
> > if (sugov_iowait_reset(sg_cpu, time, false))
> > return;
> >
> > - if (!sg_cpu->iowait_boost_pending) {
> > + if (!sg_cpu->iowait_boost_pending &&
> > + (!sysctl_iowait_apply_ticks ||
> > + (time - sg_policy->last_update > (sysctl_iowait_apply_ticks * TICK_NSEC)))) {
> > /*
> > * No boost pending; reduce the boost value.
> > */
> > @@ -449,6 +457,14 @@ sugov_update_shared(struct update_util_data *hook, u64 time, unsigned int flags)
> > if (!sugov_update_next_freq(sg_policy, time, next_f))
> > goto unlock;
> >
> > + /*
> > + * Required for ensuring iowait decay does not happen too
> > + * quickly. This can happen, for example, if a neighboring CPU
> > + * does a cpufreq update immediately after a CPU that just
> > + * completed I/O.
> > + */
> > + sg_policy->last_update = time;
> > +
> > if (sg_policy->policy->fast_switch_enabled)
> > cpufreq_driver_fast_switch(sg_policy->policy, next_f);
> > else
> > diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> > index 8f0194cee0ba..2b9c6d5091f7 100644
> > --- a/kernel/sched/sched.h
> > +++ b/kernel/sched/sched.h
> > @@ -2381,6 +2381,9 @@ extern void check_preempt_curr(struct rq *rq, struct task_struct *p, int flags);
> > extern const_debug unsigned int sysctl_sched_nr_migrate;
> > extern const_debug unsigned int sysctl_sched_migration_cost;
> >
> > +extern unsigned int sysctl_iowait_reset_ticks;
> > +extern unsigned int sysctl_iowait_apply_ticks;
> > +
> > #ifdef CONFIG_SCHED_HRTICK
> >
> > /*
> > diff --git a/kernel/sysctl.c b/kernel/sysctl.c
> > index 0afbfc83157a..83f9c5223ba4 100644
> > --- a/kernel/sysctl.c
> > +++ b/kernel/sysctl.c
> > @@ -1717,6 +1717,20 @@ static struct ctl_table kern_table[] = {
> > .mode = 0644,
> > .proc_handler = proc_dointvec,
> > },
> > + {
> > + .procname = "iowait_reset_ticks",
> > + .data = &sysctl_iowait_reset_ticks,
> > + .maxlen = sizeof(unsigned int),
> > + .mode = 0644,
> > + .proc_handler = proc_dointvec,
> > + },
> > + {
> > + .procname = "iowait_apply_ticks",
> > + .data = &sysctl_iowait_apply_ticks,
> > + .maxlen = sizeof(unsigned int),
> > + .mode = 0644,
> > + .proc_handler = proc_dointvec,
> > + },
> > #ifdef CONFIG_SCHEDSTATS
> > {
> > .procname = "sched_schedstats",
> > --
> > 2.32.0.288.g62a8d224e6-goog
> >
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