linux-kernel - Re: [External] Re: [PATCH] fs/proc/kcore.c: add mmap interface

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Message-Id: <20210527153055.aefeee8d8385da8152bdbacc@linux-foundation.org>
Date:   Thu, 27 May 2021 15:30:55 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     zhoufeng <zhoufeng.zf@...edance.com>
Cc:     adobriyan@...il.com, rppt@...nel.org, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, songmuchun@...edance.com,
        zhouchengming@...edance.com, chenying.kernel@...edance.com,
        zhengqi.arch@...edance.com
Subject: Re: [External] Re: [PATCH] fs/proc/kcore.c: add mmap interface

On Thu, 27 May 2021 14:13:09 +0800 zhoufeng <zhoufeng.zf@...edance.com> wrote:

> > I'm surprised that it makes this much difference.  Has DRGN been fully
> > optimised to minimise the amount of pread()ing which it does?  Why does
> > it do so much reading?
> DRGN is a tool similar to Crash, but much lighter. It allows users to 
> obtain kernel data structures from Python scripts. Based on this, we 
> intend to use DRGN for kernel monitoring. So we used some pressure test 
> scripts to test the loss of monitoring.
> Monitoring is all about getting current real-time data, so every time 
> DRGN tries to get kernel data, it needs to read /proc/kcore. In my 
> script, I tried to loop 1000 times to obtain the information of all the 
> processes in the machine, in order to construct a scene where kernel 
> data is frequently read. So, the frequency in the default version of 
> kcore, pread is very high. In view of this situation, our optimization 
> idea is to reduce the number of context switches as much as possible 
> under the scenario of frequent kernel data acquisition, to reduce the 
> performance loss to a minimum, and then move the monitoring system to 
> the production environment.

Why would a pread() cause a context switch?

> After running for a long time in a 
> production environment, the number of kernel data reads was added as 
> time went on, and the pread number also increased. If users use mmap, 
> it's once for all.