Re: [PATCH] sched/numa, mm: Skip page promotion if cpu pid is valid

[Donet Tom]

On 3/26/26 3:59 PM, David Hildenbrand (Arm) wrote:
> On 3/26/26 08:12, Donet Tom wrote:
> > If memory tiering is disabled, cpupid of slow memory pages may
> > contain a valid CPU and PID. If tiering is enabled at runtime,
> > there is a chance that in should_numa_migrate_memory(), this
> > valid CPU/PID is treated as a last access timestamp, leading
> > to unnecessary promotion.
> Is that measurable? Should we at least have a Fixes: ?
>
> > Prevent this by skipping promotion when cpupid is valid.
> >
> > Signed-off-by: Donet Tom <donettom@xxxxxxxxxxxxx>
> > ---
> >   kernel/sched/fair.c | 7 +++++++
> >   1 file changed, 7 insertions(+)
> >
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 4b43809a3fb1..f5830a5a94d5 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -2001,6 +2001,13 @@ bool should_numa_migrate_memory(struct task_struct *p, struct folio *folio,
> >   		unsigned int latency, th, def_th;
> >   		long nr = folio_nr_pages(folio);
> >   
> /*
>   * When ...
>
> > +		/* When tiering is enabled at runtime, last_cpupid may
> > +		 * hold a valid cpupid instead of an access timestamp.
> > +		 * If so, skip page promotion.
> > +		 */
> > +		if (cpupid_valid(folio_last_cpupid(folio)))
> > +			return false;
> > +
> IIUC, as timestamp we use jiffies_to_msecs(). So, soon after bootup,
> we would no longer get false positives for cpupid_valid().
> I suppose overflows are not a problem, correct?

Thank you, David, for guiding me in the right direction.

I initially thought that overflows would not occur, and therefore
cpupid_valid() would not produce false positives. However,
after looking into it further, it appears that overflow can
happen when storing the access time.

The last_cpupid field is used to store the last access time.
From the code, it appears that 21 bits are used for this
(#define LAST_CPUPID_SHIFT (LAST__PID_SHIFT + LAST__CPU_SHIFT)).

With 21 bits, the maximum value that can be stored is
2097151ms (35Hrs) . If the access time exceeds this
range, it can overflow, which may lead to cpupid_valid()
returning false positives.

I think we need a reliable way to determine cpupid_valid() that
does not produce false positives.

-Donet


> So what we're saying is that folio_use_access_time()==true does not
> imply that there is actually a valid time in there.
>
> In numa_migrate_check() we could still use the valid cpuid I guess and
> make that code a bit clearer?
>
> diff --git a/mm/memory.c b/mm/memory.c
> index 631205a384e1..ba68933a9e4a 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -6119,10 +6119,9 @@ int numa_migrate_check(struct folio *folio, struct vm_fault *vmf,
>           * For memory tiering mode, cpupid of slow memory page is used
>           * to record page access time.  So use default value.
>           */
> -       if (folio_use_access_time(folio))
> +       *last_cpupid = folio_last_cpupid(folio);
> +       if (!cpupid_valid(*last_cpupid))
>                  *last_cpupid = (-1 & LAST_CPUPID_MASK);
> -       else
> -               *last_cpupid = folio_last_cpupid(folio);
>   
>          /* Record the current PID accessing VMA */
>          vma_set_access_pid_bit(vma);
>
>
> The change itself here looks reasonable to me.
>
> Acked-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>