Change top’s sorting back to CPU

I assume your question is basically "What's going on here?".

I will answer by explaining your output - If that helps, let me know, I'd add more detail.
(Try to edit the question so that is's more clear what you are asking, otherwise it may get closed)

So, yes, you see "huge CPU load due to high CPU usage"!

Let's look at the top output:

Cpu(s): 10.5%us, 89.2%sy, 0.0%ni, 0.0%id, 0.0%wa, 0.0%hi, 0.3%si, 0.0%st

The percentage values say where the time is spent - in user code, or in system (kernel) code. The %sy are the time in system code - and there are additional 10%us - user time. So the CPU is used by 100%! (You can see it from the 0%id - idle - also.)

But there is even more:

Tasks: 747 total, 176 running, 560 sleeping, 0 stopped, 11 zombie

There are 176 running processes. But if you have less than 176 cores, some of them are olny in the state that they could run if they had CPU time.
That means you have more load, that could get more CPUs to 100% usage.
Your CPU is not used to 89.2% - it's 100%

From this, there is no reason to look at iostat - the system does not need much IO in this state.

But the information we need it: what are these at least 176 processes or threads, there may be many more similar tasks not in running state.

And the next will be: what are they doing, and why?

So take a look at the process list in top - it may show some obvious problem.

It could help to know more about the processes in "runable" state;
The command below lists all processes and threads that are in "runable" state - the tasks that could run if they get CPU time:

ps -o comm,pid,ppid,user,time,etime,start,pcpu,state --sort=comm aH | grep '^COMMAND\|R$'

For me, that lists only one or two lines, including ps itself

top -M sorts by resident memory usage.

M sort tasks by resident memory usage.

This is the version of top on my system.

top -v top: procps version 3.2.7

If your Linux distribution supports the -M flag, you could use it as mentioned here.

However, if your top doesn't support the -M flag, you could launch your top command and get into the interactive mode by typing h to check the sort field. (I assume it is the same across various distributions)

In my system (rather the top version of my system), I could type F or O to select the sorting field and key Q of my top version lets me sort on resident memory.

If you want to save your configuration you could do something as mentioned by slm here.

Saving configuration

You can use the Shift+W to save your changes so they're the defaults:

  W         Write configuration file

The file is stored here, $HOME/.toprc, and looks like this:

$ more .toprc 
RCfile for "top with windows"       # shameless braggin'
Id:a, Mode_altscr=0, Mode_irixps=1, Delay_time=1.000, Curwin=2
Def fieldscur=AEHIoqTWKNMBcdfgjpLrsuvyzX
    winflags=129016, sortindx=19, maxtasks=0
    summclr=2, msgsclr=5, headclr=7, taskclr=7
Job fieldscur=ABcefgjlrstuvyzMKNHIWOPQDX
    winflags=63416, sortindx=13, maxtasks=0
    summclr=6, msgsclr=6, headclr=7, taskclr=6
Mem fieldscur=ANOPQRSTUVbcdefgjlmyzWHIKX
    winflags=65464, sortindx=13, maxtasks=0
    summclr=5, msgsclr=5, headclr=4, taskclr=5
Usr fieldscur=ABDECGfhijlopqrstuvyzMKNWX
    winflags=65464, sortindx=12, maxtasks=0
    summclr=3, msgsclr=3, headclr=2, taskclr=7

See section 5 of the man page for more details, "5. FILES".