Re: [PATCH 2/2 RESEND] mm: vmpressure: fix sending wrong events on underflow

[vinayak menon <vinayakm.list@gmail.com>]

On Mon, Feb 6, 2017 at 8:42 PM, Michal Hocko <mhocko@kernel.org> wrote:
> On Mon 06-02-17 20:05:21, vinayak menon wrote:
> [...]
>> By scan I meant pages scanned by shrink_node_memcg/shrink_list
>> which is passed as nr_scanned to vmpressure.  The calculation of
>> pressure for tree is done at the end of vmpressure_win and it is
>> that calculation which underflows. With this patch we want only the
>> underflow to be avoided. But if we make (reclaimed = scanned) in
>> vmpressure(), we change the vmpressure value even when there is no
>> underflow right ?
>>
>> Rewriting the above e.g again.  First call to vmpressure with
>> nr_scanned=1 and nr_reclaimed=512 (THP) Second call to vmpressure
>> with nr_scanned=511 and nr_reclaimed=0 In the second call
>> vmpr->tree_scanned becomes equal to vmpressure_win and the work
>> is scheduled and it will calculate the vmpressure as 0 because
>> tree_reclaimed = 512
>>
>> Similarly, if scanned is made equal to reclaimed in vmpressure()
>> itself as you had suggested, First call to vmpressure with
>> nr_scanned=1 and nr_reclaimed=512 (THP) And in vmpressure, we
>> make nr_scanned=1 and nr_reclaimed=1 Second call to vmpressure
>> with nr_scanned=511 and nr_reclaimed=0 In the second call
>> vmpr->tree_scanned becomes equal to vmpressure_win and the work is
>> scheduled and it will calculate the vmpressure as critical, because
>> tree_reclaimed = 1
>>
>> So it makes a difference, no?
>
> OK, I see what you meant. Thanks for the clarification. And you are
> right that normalizing nr_reclaimed to nr_scanned is a wrong thing to
> do because that just doesn't aggregate the real work done. Normalizing
> nr_scanned to nr_reclaimed should be better - or it would be even better
> to count the scanned pages properly...
>
With the slab reclaimed issue fixed separately, only the THP case exists AFAIK.
In the case of THP, as I understand from one of Minchan's reply, the scan is
actually 1. i.e. Only a single huge page is scanned to get 512 reclaimed pages.
So the cost involved was scanning a single page.
In that case, there is no need to normalize the nr_scanned, no?

> My main concern of doing this normalization late on aggregated numbers
> is just weird. We are mixing numbers from parallel reclaimers and that
> might just add more confusion. It is better to do the fixup as soon as
> possible when we still have at least an idea that this was a THP page
> scanned and reclaimed.
>
> If we get back to your example it works as you expect just due to good
> luck. Just make your nr_scanned=511 and nr_reclaimed=0 be a separate
> event and you have your critical event. You have no real control over
> when a new event is fired because parallel reclaimers are basically
> unpredictable.
> --
> Michal Hocko
> SUSE Labs

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