Re: [PATCH] mm: fix draining remote pageset

[Huang, Ying]

Vlastimil Babka <vbabka@xxxxxxx> writes:

> On 8/25/23 19:06, Lameter, Christopher wrote:
>> On Tue, 22 Aug 2023, Michal Hocko wrote:
>> 
>>> Yes, this doesn't really show any actual correctness problem so I do not
>>> think this is sufficient to change the code. You would need to show that
>>> the existing behavior is actively harmful.
>> 
>> Having some pages from a remote NUMA node stuck in a pcp somewhere is 
>> making that memory unusable. It is usually rate that these remote pages 
>> are needed again and so they may remain there for a long time if the 
>> situation is right.
>> 
>> And he is right that the intended behavior of freeing the remote pages 
>> has been disabled by the patch.
>> 
>> So I think there is sufficient rationale to apply these fixes.
>
> I wonder if this the optimum way to handle the NOHZ case? IIUC there we use
> quiet_vmstat() to call refresh_cpu_vm_stats(). I'd expect if there were
> pending remote pages to flush, it would be best to do it immediately, and
> not keep a worker being requeued and only do that after the pcp->expires
> goes zero.
>
> However quiet_vmstat() even calls the refresh with do_pagesets == false. Why
> do we even refresh the stats at that moment if the delayed update is pending
> anyway?

According to commit f01f17d3705b ("mm, vmstat: make quiet_vmstat
lighter") and the comments in quiet_vmstat().  The pending worker will
not be canceled to avoid long latency of idle entry.

> And could we maybe make sure that in that case the flush is done on
> the first delayed update in that case and not expiring like this?

This sounds reasonable.  How to identify whether the current CPU is in
NOHZ state? Via tick_get_tick_sched()->tick_stopped?

--
Best Regards,
Huang, Ying