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

[Thomas Monjalon]

26/02/2021 00:33, Honnappa Nagarahalli:
> +Thomas, David, Konstantin for input
> 
> <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.

 
> > 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