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

[zhoufeng]

在 2021/5/28 上午6:30, Andrew Morton 写道:
> On Thu, 27 May 2021 14:13:09 +0800 zhoufeng <zhoufeng.zf@xxxxxxxxxxxxx> 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?

Sorry, my English is poor. I mean trigger the system call.

> > 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.