Re: [dpdk-dev] [RFC 3/5] eal: lcore state FINISHED is not required

[Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>]

<snip>

> >
> > > > Subject: [RFC 3/5] eal: lcore state FINISHED is not required
> > > >
> > > > FINISHED state seems to be used to indicate that the worker's
> > > > update of the 'state' is not visible to other threads. There seems
> > > > to be no requirement to have such a state.
> > >
> > > I am not sure "FINISHED" is necessary to be removed, and I propose
> > > some of my profiles for discussion.
> > >  There are three states for lcore now:
> > > "WAIT": indicate lcore can start working
> > > "RUNNING": indicate lcore is working
> > > "FINISHED": indicate lcore has finished its working and wait to be
> > > reset
> > If you look at the definitions of "WAIT" and "FINISHED" states, they look
> similar, except for "wait to be reset" in "FINISHED" state . The code really does
> not do anything to reset the lcore. It just changes the state to "WAIT".
> >
> > >
> > > From the description above, we can find "FINISHED" is different from
> > > "WAIT", it can shows that lcore has done the work and finished it.
> > > Thus, if we remove "FINISHED", maybe we will not know whether the
> > > lcore finishes its work or just doesn't start, because this two state has the
> same tag "WAIT".
> > Looking at "eal_thread_loop", the worker thread sets the state to "RUNNING"
> before sending the ack back to main core. After that it is guaranteed that the
> worker will run the assigned function. Only case where it will not run the
> assigned function is when the 'write' syscall fails, in which case it results in a
> panic.
> 
> Quick note: it should not panic.
> We must find a way to return an error
> without crashing the whole application.
The syscalls are being used to communicate the status back to the main thread. If they fail, it is not possible to communicate the status. May be it is better to panic.
We could change the implementation using shared variables, but it would require polling the memory. May be the syscalls are being used to avoid polling. However, this polling would happen during init time (or similar) for a short duration.

> 
> 
> > > Furthermore, consider such a scenario:
> > > Core 1 need to monitor Core 2 state, if Core 2 finishes one task,
> > > Core 1 can start its working.
> > > However, if there is only  one tag "WAIT", Core 1 maybe  start its
> > > work at the wrong time, when Core 2 still does not start its task at state
> "WAIT".
> > > This is just my guess, and at present, there is no similar
> > > application scenario in dpdk.
> > To be able to do this effectively, core 1 needs to observe the state change
> from WAIT->RUNNING->FINISHED. This requires that core 1 should be calling
> rte_eal_remote_launch and rte_eal_wait_lcore functions. It is not possible to
> observe this state transition from a 3rd core (for ex: a worker might go from
> RUNNING->FINISHED->WAIT->RUNNING which a 3rd core might not be able to
> observe).
> >
> > >
> > > On the other hand, if we decide to remove "FINISHED", please
> > > consider the following files:
> > > 1. lib/librte_eal/linux/eal_thread.c: line 31
> > >     lib/librte_eal/windows/eal_thread.c: line 22
> > >     lib/librte_eal/freebsd/eal_thread.c: line 31
> > I have looked at these lines, they do not capture "why" FINISHED state is
> required.
> >
> >  2.
> > > lib/librte_eal/include/rte_launch.h: line 24, 44, 121, 123, 131 3.
> > > examples/l2fwd-
> > > keepalive/main.c: line 510
> > > rte_eal_wait_lcore(id_core) can be removed. Because the core state
> > > has been checked as "WAIT", this is a redundant operation
> 
>