On Mon, Oct 24, 2022 at 01:03:11PM +0200, Frederic Weisbecker wrote:
> Lines: 94
>
> On Sat, Sep 24, 2022 at 04:22:25PM +0100, Aaron Tomlin wrote:
> > From: Marcelo Tosatti <mtosatti@xxxxxxxxxx>
> >
> > From the vmstat shepherd, for CPUs that have the tick stopped, do not
> > queue local work to flush the per-CPU vmstats, since in that case the
> > flush is performed on return to userspace or when entering idle. Also
> > cancel any delayed work on the local CPU, when entering idle on nohz
> > full CPUs. Per-CPU pages can be freed remotely from housekeeping CPUs.
> >
> > Signed-off-by: Marcelo Tosatti <mtosatti@xxxxxxxxxx>
> > ---
> > mm/vmstat.c | 18 +++++++++++++-----
> > 1 file changed, 13 insertions(+), 5 deletions(-)
> >
> > diff --git a/mm/vmstat.c b/mm/vmstat.c
> > index 472175642bd9..3b9a497965b4 100644
> > --- a/mm/vmstat.c
> > +++ b/mm/vmstat.c
> > @@ -29,6 +29,7 @@
> > #include <linux/page_ext.h>
> > #include <linux/page_owner.h>
> > #include <linux/migrate.h>
> > +#include <linux/tick.h>
> >
> > #include "internal.h"
> >
> > @@ -1990,19 +1991,23 @@ static void vmstat_update(struct work_struct *w)
> > */
> > void quiet_vmstat(void)
> > {
> > + struct delayed_work *dw;
> > +
> > if (system_state != SYSTEM_RUNNING)
> > return;
> >
> > if (!is_vmstat_dirty())
> > return;
> >
> > + refresh_cpu_vm_stats(false);
> > +
> > /*
> > - * Just refresh counters and do not care about the pending delayed
> > - * vmstat_update. It doesn't fire that often to matter and canceling
> > - * it would be too expensive from this path.
> > - * vmstat_shepherd will take care about that for us.
> > + * If the tick is stopped, cancel any delayed work to avoid
> > + * interruptions to this CPU in the future.
> > */
> > - refresh_cpu_vm_stats(false);
> > + dw = &per_cpu(vmstat_work, smp_processor_id());
> > + if (delayed_work_pending(dw) && tick_nohz_tick_stopped())
> > + cancel_delayed_work(dw);
> > }
> >
> > /*
> > @@ -2024,6 +2029,9 @@ static void vmstat_shepherd(struct work_struct *w)
> > for_each_online_cpu(cpu) {
> > struct delayed_work *dw = &per_cpu(vmstat_work, cpu);
> >
> > + if (tick_nohz_tick_stopped_cpu(cpu))
> > + continue;
> > +
> > if (!delayed_work_pending(dw) && per_cpu(vmstat_dirty, cpu))
> > queue_delayed_work_on(cpu, mm_percpu_wq, dw, 0);
>
> All these checks are racy though. You may well eventually:
>
> 1) Arm the timer after the CPU has entered in userspace
> 2) Not arm the timer when the CPU has entered the kernel
>
> How about converting that to an IPI instead? This should be a good candidate
> for the future IPI deferment.
>
> Another possible way to go is this:
>
> 1) vmstat_shepherd completely ignores nohz_full CPUs
> 2) vmstat_work is only ever armed locally
> 3) A nohz_full CPU turning its local vmstat as dirty checks if vmstat_work is
> pending. If not, queue it, possibly through a self IPI (IRQ_WORK) to get
> away with current locking context.
I'm afraid there might be workloads where local vmstat touch is a
hot-path.
> 3) Fold on idle if dirty
> 4) Fold on user enter and disarm vmstat_work if pending
>
> Does that sound possible?
>
> Thanks.
I guess so, but proper barriers would also work.
Do you have any particular reason for the 1-4 sequence above
instead of barriers?
