Re: [PATCH v8 4/5] tick/nohz_full: Ensure quiet_vmstat() is called on exit to user-mode when the idle tick is stopped

[Frederic Weisbecker <frederic@kernel.org>]

On Tue, Sep 27, 2022 at 01:17:02PM -0300, Rafael Folco wrote:
> Tested this patch w/ nohz_full setup and oslat 8h run on isolated cpus, max
> latency is 7us versus 15us without the patch.
> Thanks.

What about the added overhead upon user enter?
That is my main worry. For people who want nohz_full for lowest latency
(or rather lowest noise) then it's a good add. But what if some people want
to use nohz_full for HPC and prefer faster syscalls over avoiding an interrupt
once in a while? (although arguably I never heard from users of such
workloads...)

Thanks.


> 
> On Mon, Sep 26, 2022 at 8:02 PM Aaron Tomlin <atomlin@redhat.com> wrote:
> 
> > This patch ensures CPU-specific vmstat differentials do not remain
> > when the scheduling-tick is stopped and before exiting to user-mode
> > in the context of nohz_full only.
> >
> > A trivial test program was used to determine the impact of the proposed
> > changes and under vanilla. The mlock(2) and munlock(2) system calls was
> > used solely to modify vmstat item 'NR_MLOCK'. The following is an average
> > count of CPU-cycles across the aforementioned system calls:
> >
> >                                   Vanilla                 Modified
> >
> >   Cycles per syscall              8461                    8690    (+2.6%)
> >
> > Signed-off-by: Aaron Tomlin <atomlin@redhat.com>
> > ---
> >  include/linux/tick.h     |  5 +++--
> >  kernel/time/tick-sched.c | 15 +++++++++++++++
> >  2 files changed, 18 insertions(+), 2 deletions(-)
> >
> > diff --git a/include/linux/tick.h b/include/linux/tick.h
> > index bfd571f18cfd..a2bbd6d32e33 100644
> > --- a/include/linux/tick.h
> > +++ b/include/linux/tick.h
> > @@ -11,7 +11,6 @@
> >  #include <linux/context_tracking_state.h>
> >  #include <linux/cpumask.h>
> >  #include <linux/sched.h>
> > -#include <linux/rcupdate.h>
> >
> >  #ifdef CONFIG_GENERIC_CLOCKEVENTS
> >  extern void __init tick_init(void);
> > @@ -272,6 +271,7 @@ static inline void tick_dep_clear_signal(struct
> > signal_struct *signal,
> >
> >  extern void tick_nohz_full_kick_cpu(int cpu);
> >  extern void __tick_nohz_task_switch(void);
> > +void __tick_nohz_user_enter_prepare(void);
> >  extern void __init tick_nohz_full_setup(cpumask_var_t cpumask);
> >  #else
> >  static inline bool tick_nohz_full_enabled(void) { return false; }
> > @@ -296,6 +296,7 @@ static inline void tick_dep_clear_signal(struct
> > signal_struct *signal,
> >
> >  static inline void tick_nohz_full_kick_cpu(int cpu) { }
> >  static inline void __tick_nohz_task_switch(void) { }
> > +static inline void __tick_nohz_user_enter_prepare(void) { }
> >  static inline void tick_nohz_full_setup(cpumask_var_t cpumask) { }
> >  #endif
> >
> > @@ -308,7 +309,7 @@ static inline void tick_nohz_task_switch(void)
> >  static inline void tick_nohz_user_enter_prepare(void)
> >  {
> >         if (tick_nohz_full_cpu(smp_processor_id()))
> > -               rcu_nocb_flush_deferred_wakeup();
> > +               __tick_nohz_user_enter_prepare();
> >  }
> >
> >  #endif
> > diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> > index b0e3c9205946..634cd0fac267 100644
> > --- a/kernel/time/tick-sched.c
> > +++ b/kernel/time/tick-sched.c
> > @@ -26,6 +26,7 @@
> >  #include <linux/posix-timers.h>
> >  #include <linux/context_tracking.h>
> >  #include <linux/mm.h>
> > +#include <linux/rcupdate.h>
> >
> >  #include <asm/irq_regs.h>
> >
> > @@ -519,6 +520,20 @@ void __tick_nohz_task_switch(void)
> >         }
> >  }
> >
> > +void __tick_nohz_user_enter_prepare(void)
> > +{
> > +       struct tick_sched *ts;
> > +
> > +       if (tick_nohz_full_cpu(smp_processor_id())) {
> > +               ts = this_cpu_ptr(&tick_cpu_sched);
> > +
> > +               if (ts->tick_stopped)
> > +                       quiet_vmstat();
> > +               rcu_nocb_flush_deferred_wakeup();
> > +       }
> > +}
> > +EXPORT_SYMBOL_GPL(__tick_nohz_user_enter_prepare);
> > +
> >  /* Get the boot-time nohz CPU list from the kernel parameters. */
> >  void __init tick_nohz_full_setup(cpumask_var_t cpumask)
> >  {
> > --
> > 2.37.1
> >
> >
> >
> 
> -- 
> Folco