linux-kernel - Re: [PATCHv4] mm: optimization on page allocation when CMA enabled

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Date:   Wed, 10 May 2023 15:08:10 -0700
From:   Roman Gushchin <roman.gushchin@...ux.dev>
To:     "zhaoyang.huang" <zhaoyang.huang@...soc.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Roman Gushchin <guro@...com>, Minchan Kim <minchan@...nel.org>,
        Joonsoo Kim <iamjoonsoo.kim@....com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Zhaoyang Huang <huangzhaoyang@...il.com>, ke.wang@...soc.com
Subject: Re: [PATCHv4] mm: optimization on page allocation when CMA enabled

On Wed, May 10, 2023 at 10:20:51AM +0800, zhaoyang.huang wrote:
> From: Zhaoyang Huang <zhaoyang.huang@...soc.com>
> 
> Let us look at the series of scenarios below with WMARK_LOW=25MB,WMARK_MIN=5MB
> (managed pages 1.9GB). We can know that current 'fixed 1/2 ratio' start to use
> CMA since C which actually has caused U&R lower than WMARK_LOW (this should be
> deemed as against current memory policy, that is, UNMOVABLE & RECLAIMABLE should
> either stay around WATERMARK_LOW when no allocation or do reclaim via entering
> slowpath)
> 
> -- Free_pages
> |
> |
> -- WMARK_LOW
> |
> -- Free_CMA
> |
> |
> --
> 
> Free_CMA/Free_pages(MB)      A(12/30)     B(12/25)     C(12/20)
> fixed 1/2 ratio                 N             N           Y
> this commit                     Y             Y           Y
> 
> Signed-off-by: Zhaoyang Huang <zhaoyang.huang@...soc.com>

I'm mostly fine with the code. The commit message is still very confusing to me,
not sure I understand what exactly this table means. And you still use "U&R".

Also I'm a bit concerned about potential performance implications. Would be
great to provide some benchmarks or some data.
Probably it's ok because of we have pcp caches on top, but I'm not 100% sure.

Thanks!