Re: [PATCH 9/9] mm, page_alloc: optionally disable pcplists during page isolation

[David Hildenbrand <david@redhat.com>]

On 25.09.20 13:10, Vlastimil Babka wrote:
> On 9/25/20 12:54 PM, David Hildenbrand wrote:
>>>> --- a/mm/page_isolation.c
>>>> +++ b/mm/page_isolation.c
>>>> @@ -15,6 +15,22 @@
>>>>  #define CREATE_TRACE_POINTS
>>>>  #include <trace/events/page_isolation.h>
>>>>  
>>>> +void zone_pcplist_disable(struct zone *zone)
>>>> +{
>>>> +	down_read(&pcp_batch_high_lock);
>>>> +	if (atomic_inc_return(&zone->pcplist_disabled) == 1) {
>>>> +		zone_update_pageset_high_and_batch(zone, 0, 1);
>>>> +		__drain_all_pages(zone, true);
>>>> +	}
>>> Hm, if one CPU is still inside the if-clause, the other one would
>>> continue, however pcp wpould not be disabled and zones not drained when
>>> returning.
> 
> Ah, well spotted, thanks!
> 
>>> (while we only allow a single Offline_pages() call, it will be different
>>> when we use the function in other context - especially,
>>> alloc_contig_range() for some users)
>>>
>>> Can't we use down_write() here? So it's serialized and everybody has to
>>> properly wait. (and we would not have to rely on an atomic_t)
>> Sorry, I meant down_write only temporarily in this code path. Not
>> keeping it locked in write when returning (I remember there is a way to
>> downgrade).
> 
> Hmm that temporary write lock would still block new callers until previous
> finish with the downgraded-to-read lock.
> 
> But I guess something like this would work:
> 
> retry:
>   if (atomic_read(...) == 0) {
>     // zone_update... + drain
>     atomic_inc(...);
>   else if (atomic_inc_return == 1)
>     // atomic_cmpxchg from 0 to 1; if that fails, goto retry
> 
> Tricky, but races could only read to unnecessary duplicated updates + flushing
> but nothing worse?
> 
> Or add another spinlock to cover this part instead of the temp write lock...

My gut feeling is, that that would be the cleanest approach.


-- 
Thanks,

David / dhildenb