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Subject[Patch 0/9] Per-task delay accounting
This is the next iteration of the delay accounting patches
last posted at

The major changes to the patches (and names of commenters whose concerns
have been addressed) are:

- considerable revision of the synchronous block I/O delay collection (Arjan)
Now most delays seen in I/O submission are not being collected.
- removal of dependency on schedstats by using common code (Nick)
- removal of sysctl or dynamic configurability of delay accounting (Arjan)
Now it it can be set on/off at boot time only.
- providing I/O delays through /proc/<tgid>/stats (Andi)
- revisions to the generic netlink interface (Jamal)

Unaddressed comments are suggestions from Jamal on
further improving the generic netlink interface which is ongoing.



A short note on expected usage of this functionality follows which
addresses a comment from Nick in the previous iteration.
Basically, the patches measures a subset of the delays encountered
by a task waiting for a resource such as cpu, disk I/O and page
frames. Which subset is chosen depends on whether a task's delay
is something that can be controlled by playing with its priority
or rss limit etc.
e.g. cpu delays can directly be affected by its static CPU priority.
Similarly I/O delay can be affected by its I/O priority (assuming
one uses the right iosched). In page frame delay, we only count
the page faults due to swapin since that is directly affected by
the rss limits for a task (well, ok, process).

Delays can be used as feedback to decide whether something can/should
be done about a task (or group of tasks) which is not performing
as one expects.
lists a fairly complicated way of using such data in
workload management.

At a simpler level, one can think of utilities that can use this data
to control individual apps using a simple feedback loop.
At this point we do not have such a utility.

Of course, its useful to visually inspect such data. Some of it is
being exported through /proc as suggested by Andi, but the primary
interface is through a netlink socket so that userspace can get
data for exiting tasks too.

The netlink socket also allows userspace apps to efficiently
collect delay data and group it in arbitrary ways
(as envisaged by CKRM, CSA or ELSA) for reporting or
control purposes.


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