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Date: Fri, 18 Jun 2021 00:07:16 -0400
From: Joel Fernandes <joel@...lfernandes.org>
To: LKML <linux-kernel@...r.kernel.org>,
Beata Michalska <beata.michalska@....com>
Cc: 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
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;
>
> - /* 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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