Re: [RFC] schedutil: Fix iowait boost issues for slow I/O devices

[Beata Michalska <beata.michalska@arm.com>]

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@joelfernandes.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@joelfernandes.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
> >