linux-kernel - Re: [PATCH] mm: fix pcp count beyond pcp high in pcplist allocation

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Date:   Tue, 25 Oct 2022 19:49:59 +0800
From:   Chen Wandun <chenwandun@...wei.com>
To:     Mel Gorman <mgorman@...hsingularity.net>
CC:     <akpm@...ux-foundation.org>, <vbabka@...e.cz>,
        <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
        <wangkefeng.wang@...wei.com>
Subject: Re: [PATCH] mm: fix pcp count beyond pcp high in pcplist allocation



On 2022/10/24 22:55, Mel Gorman wrote:
> On Mon, Oct 24, 2022 at 09:41:46PM +0800, Chen Wandun wrote:
>> Nowadays there are several orders in pcplist, Function __rmqueue_pcplist
>> would alloc pcp batch pages to refill pcplist, when list of target order
>> if empty meanwhile other lists is not all empty, that result in pcp count
>> beyond pcp high after allocation. This behaviour can be easily observed by
>> adding debugging information in __rmqueue_pcplist.
>>
>> Fix this by recalculate the batch pages to be allocated.
> Are any problems observed other than the PCP lists temporarily exceed
> pcp->high?
It will result frequently refill pcp page from buddy and release pcp 
page to buddy.
>   As is, the patch could result in a batch request of 0 and
  I foget this, the patch need some improve, thanks.
> fall through to allocating from the zone list anyway defeating the
> purpose of the PCP allocator and probably regressing performance in some
> csaes.
Same as I understand，how about set high/batch for each order in pcplist，
or just share pcp batch value only set high for each order?  It looks like
strange for pcp count beyond pcp high in common case.

If each order has it's own pcp high value, that behaviour is same as 
pcplist which
only contains order 0.

Thanks
Wandun.
>
> The intention was to allow high to be briefly exceeded on the refill side,
> particularly for THP pages and to always refill by at least two pages. In
> the THP case, one would be allocated and maybe one in the near future
> without acquiring the zone lock. If the limits are exceeded, it's only
> exceeded until the next free.
>