How to initialize the new attributes in process control block.

[Valdis.Kletnieks at vt.edu]

On Mon, 16 May 2016 10:44:56 +0200, Youcun Liu said:

>        u64 cache_miss_rate;   // the new variable I insert

> Besides, I also have trouble in finding out how to feed data into the new
> variable from user level application.

Three comments:

1) cache_miss_rate is probably a poor thing to fold into scheduling, because
it depends a large part on how much the *previous* thing to run on that core
left the cache.  If you run, and then a CPU-bound process that only touches 16K
of your 8M L2 cache runs, when you reschedule, your cache miss rate will be
very low.  If however that CPU bound job is walking an array and dirties the
entire 8M, your cache miss rate will be astronomical until you've repopulated
the cache...

2) Trying to update cache miss rate in real time from *userspace* only makes
the problem worse, because your own userspace activity ends up impacting the
cache warmth of the process you're trying to track...

2a) Oh, and doing so is inherently full of race conditions....

3) Even if you get the number in there, what exactly do you propose to do
with it?  How would you modify the scheduling algorithm?

Hint:  If you think that the scheduler should be including some new piece
of information, you should probably stop and ask yourself "Why, with close
to 2 decades of improvement by some very smart people, is it not doing so
already?"... There's a *very* good chance it's been considered and discarded
(possibly multiple times).
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