On Mon, 10 Feb 2025 12:46:44 +0100 Frederic Weisbecker
> Le Mon, Feb 10, 2025 at 06:50:26PM +0800, Hillf Danton
> > On Sun, 9 Feb 2025 23:30:04 +0100 Frederic Weisbecker <frederic@xxxxxxxxxx>
> > > @@ -769,6 +772,9 @@ static bool cpu_needs_drain(unsigned int cpu)
> > > {
> > > struct cpu_fbatches *fbatches = &per_cpu(cpu_fbatches, cpu);
> > >
> > > + if (!housekeeping_cpu(cpu, HK_TYPE_KERNEL_NOISE))
> > > + return false;
> > > +
> > > /* Check these in order of likelihood that they're not zero */
> > > return folio_batch_count(&fbatches->lru_add) ||
> > > folio_batch_count(&fbatches->lru_move_tail) ||
> > > --
> > > 2.46.0
> >
> > Nit, I'd like to add a debug line to test your assumption that
> > isolated tasks are pinned to a single nohz_full CPU.
> >
> > --- x/mm/swap.c
> > +++ y/mm/swap.c
> > @@ -767,9 +767,10 @@ static void lru_add_drain_per_cpu(struct
> > static bool cpu_needs_drain(unsigned int cpu)
> > {
> > struct cpu_fbatches *fbatches = &per_cpu(cpu_fbatches, cpu);
> > + bool yes;
> >
> > /* Check these in order of likelihood that they're not zero */
> > - return folio_batch_count(&fbatches->lru_add) ||
> > + yes = folio_batch_count(&fbatches->lru_add) ||
> > folio_batch_count(&fbatches->lru_move_tail) ||
> > folio_batch_count(&fbatches->lru_deactivate_file) ||
> > folio_batch_count(&fbatches->lru_deactivate) ||
> > @@ -777,6 +778,12 @@ static bool cpu_needs_drain(unsigned int
> > folio_batch_count(&fbatches->lru_activate) ||
> > need_mlock_drain(cpu) ||
> > has_bh_in_lru(cpu, NULL);
> > +
> > + if (!housekeeping_cpu(cpu, HK_TYPE_KERNEL_NOISE)) {
> > + VM_BUG_ON(yes);
> > + return false;
> > + }
> > + return yes;
>
> If the task isn't pinned then the guarantees of nohz_full are broken anyway.
> Also if the task migrates it will simply execute the work elsewhere.
>
Coding in kernel depends on the smart/stupid activity in user space, but
the dependence sounds no good.
> My only worry is kernel threads. Those are simply ignored in this patchset but
> this is not right as they can do allocations. Yet they can't execute anything
> on return to userspace...
>
> Thoughts?
