Re: [PATCH v3 1/2] sched/fair: Introduce short duration task check

[Chen Yu <yu.c.chen@intel.com>]

On 2022-12-12 at 10:17:35 -0800, Josh Don wrote:
> > BTW, I've changed the threshold to (sysctl_sched_min_granularity / 8) in my next
> > version, as this is the value that fit my previous test case and also not to break
> > the case Josh mentioned.
> 
> Do you mean a hardcoded value of some number of micros, or literally
> sched_min_granularity / 8?
The latter. According to the test, the average task duration when system
is under heavy load:
6 ~ 9 us for netperf
7 ~ 70 us for hackbench
7 ~ 8 us for tbench
13 ~ 20 ms for schbench
Overall the duration of the micros are quite small(except for schbench).
The default sysctl_sched_min_granularity is 750 us in kernel if no user
has changed it. Then 750 / 8 = 93 us, which is close to what you suggested(100us).
On the other hand, if someone changes sysctl_sched_min_granularity,
then '8' can be viewed as log2(256). That is, if there are 256 CPUs online,
and the sysctl_sched_min_granularity is changed to 750 us * log2(256) by
the user, we can devide the sysctl_sched_min_granularity by 8 in case the
sysctl_sched_min_granularity is too large.

My concern of using hardcoded value is that, this value depends on how fast
the CPU runs(cpu frequency). The value I measured above is when the
CPU is running at 1.9Ghz. If a CPU runs faster, a hard code value might not
be appropriate and could not be tuned.
> I don't think the latter is necessary, and
> indeed can lead to weirdness if min_gran is too small or too large. I
> don't think the concept of what a short duration task is should
> expand/contract with min_gran.
The value of sysctl_sched_min_granularity might indicate how long the
user would like a task to run at least. If the user enlarge this value,
does it mean the user wants every task in the system to run longer?
From this point I found connection between the the definition of short task
duration and this value. I'm open to changing this value to a fixed one, may
I have more insights on how this value would be set in production environment?

thanks,
Chenyu
> 
> Best,
> Josh