On Tue 2015-06-09 16:20:03, Tejun Heo wrote:
Hello, Petr.
On Fri, Jun 05, 2015 at 05:01:06PM +0200, Petr Mladek wrote:
quoted
Many kthreads already calls set_freezable() before they enter the main
cycle. One of the reasons for creating iterant kthreads is to create
a safe point for freezing and make even more kthreads properly
freezable. Therefore it would make sense to set all iterant
kthreads freezable by default.
Actually, for most cases, making kthreads freezable is unnecessary and
often indicative of something going wrong. This is a crude mechanism
which goes along the line of "if all threads are stopped, the machine
should be safe to be put into whatever state", which isn't true at all
as there usually are a lot of stuff going on asynchronously especially
when interacting with hardware.
I think that the interaction with the hardware should be the reason to
make them properly freezable. In the current state they are stopped at
some random place, they might either miss some event from the hardware
or the hardware might get resumed into another state and the kthread
might wait forever.
Also I think that freezing kthreads on some well defined location
should help with reproducing and fixing problems.
Note that _only_ kthreads using the _new API_ should be freezable by
default. The move need to be done carefully. It is chance to review
and clean up the freezer stuff.
Of course, I might be too optimistic here.
In most cases, we want to implement proper power management callbacks
which plug new issuance of whatever work-unit the code is dealing with
and drain in-flight ones. Whether the worker threads are frozen or
not doesn't matter once that's implemented.
I see. The power management is important here.
It seems that people have been marking kthreads freezable w/o really
thinking about it - some of them are subtly broken due to missing
drainage of in-flight things while others simply don't need freezing
for correctness.
I have similar feeling.
We do want to clean up freezer usage in the kernel but definitely do
not want to make kthreads freezable by default especially given that
the freezer essentially is one giant lockdep-less system-wide lock.
I think that we both want to clean up freezing. I would like to make
it more deterministic and you suggest to make it more relaxed.
Do I understand it correctly?
Best Regards,
Petr
PS: Thanks a lot everyone for feedback. I try hard to get it
somehow sorted and more confident about the conclusions.