Re: [RFC][PATCH 05/13] mm/numa: automatically generate node migration order

[Yang Shi]

On Tue, Feb 2, 2021 at 4:43 PM Dave Hansen <dave.hansen@xxxxxxxxx> wrote:
>
> On 2/2/21 9:46 AM, Yang Shi wrote:
> > On Mon, Feb 1, 2021 at 11:13 AM Dave Hansen <dave.hansen@xxxxxxxxx> wrote:
> >> On 1/29/21 12:46 PM, Yang Shi wrote:
> >> ...
> >>>>  int next_demotion_node(int node)
> >>>>  {
> >>>> -       return node_demotion[node];
> >>>> +       /*
> >>>> +        * node_demotion[] is updated without excluding
> >>>> +        * this function from running.  READ_ONCE() avoids
> >>>> +        * reading multiple, inconsistent 'node' values
> >>>> +        * during an update.
> >>>> +        */
> >>> Don't we need a smp_rmb() here? The single write barrier might be not
> >>> enough in migration target set. Typically a write barrier should be
> >>> used in pairs with a read barrier.
> >> I don't think we need one, practically.
> >>
> >> Since there is no locking against node_demotion[] updates, although a
> >> smp_rmb() would ensure that this read is up-to-date, it could change
> >> freely after the smp_rmb().
> > Yes, but this should be able to guarantee we see "disable + after"
> > state. Isn't it more preferred?
>
> I'm debating how much of this is theoretical versus actually applicable
> to what we have in the kernel.  But, I'm generally worried about code
> like this that *looks* innocuous:
>
>         int terminal_node = start_node;
>         int next_node = next_demotion_node(start_node);
>         while (next_node != NUMA_NO_NODE) {
>                 next_node = terminal_node;
>                 terminal_node = next_demotion_node(terminal_node);
>         }
>
> That could loop forever if it doesn't go out to memory during each loop.
>
> However, if node_demotion[] *is* read on every trip through the loop, it
> will eventually terminate.  READ_ONCE() can guarantee that, as could
> compiler barriers like smp_rmb().
>
> But, after staring at it for a while, I think RCU may be the most
> clearly correct way to solve the problem.  Or, maybe just throw in the
> towel and do a spinlock like a normal human being. :)
>
> Anyway, here's what I was thinking I'd do with RCU:
>
>  1. node_demotion[] starts off in a "before" state
>  2. Writers to node_demotion[] first set the whole array such that
>     it will not induce cycles, like setting every member to
>     NUMA_NO_NODE. (the "disable" state)
>  3. Writer calls synchronize_rcu().  After it returns, no readers can
>     observe the "before" values.
>  4. Writer sets the actual values it wants.  (the "after" state)
>  5. Readers use rcu_read_lock() over any critical section where they
>     read the array.  They are guaranteed to only see one of the two
>     adjacent states (before+disabled, or disabled+after), but never
>     before+after within one RCU read-side critical section.
>  6. Readers use READ_ONCE() or some other compiler directive to ensure
>     the compiler does not reorder or combine reads from multiple,
>     adjacent RCU read-side critical sections.

Makes sense to me.

>
> Although, after writing this, plain old locks are sounding awfully tempting.