linux-kernel - Re: [PATCH rfc 0/2] mm: cma: make cma

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Date:   Thu, 22 Oct 2020 09:54:53 +0800
From:   "Xiaqing (A)" <saberlily.xia@...ilicon.com>
To:     Roman Gushchin <guro@...com>,
        Andrew Morton <akpm@...ux-foundation.org>
CC:     Zi Yan <ziy@...dia.com>, Joonsoo Kim <iamjoonsoo.kim@....com>,
        "Mike Kravetz" <mike.kravetz@...cle.com>,
        <linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>,
        <kernel-team@...com>
Subject: Re: [PATCH rfc 0/2] mm: cma: make cma_release() non-blocking



On 2020/10/17 6:52, Roman Gushchin wrote:

> This small patchset makes cma_release() non-blocking and simplifies
> the code in hugetlbfs, where previously we had to temporarily drop
> hugetlb_lock around the cma_release() call.
>
> It should help Zi Yan on his work on 1 GB THPs: splitting a gigantic
> THP under a memory pressure requires a cma_release() call. If it's
> a blocking function, it complicates the already complicated code.
> Because there are at least two use cases like this (hugetlbfs is
> another example), I believe it's just better to make cma_release()
> non-blocking.
>
> It also makes it more consistent with other memory releasing functions
> in the kernel: most of them are non-blocking.
>
>
> Roman Gushchin (2):
>    mm: cma: make cma_release() non-blocking
>    mm: hugetlb: don't drop hugetlb_lock around cma_release() call
>
>   mm/cma.c     | 51 +++++++++++++++++++++++++++++++++++++++++++++++++--
>   mm/hugetlb.c |  6 ------
>   2 files changed, 49 insertions(+), 8 deletions(-)
>
I don't think this patch is a good idea.It transfers part or even all of the time of
cma_release to cma_alloc, which is more concerned by performance indicators.
On Android phones, CPU resource competition is intense in many scenarios,
As a result, kernel threads and workers can be scheduled only after some ticks or more.
In this case, the performance of cma_alloc will deteriorate significantly,
which is not good news for many services on Android.