Re: [dpdk-dev] [PATCH v2] telemetry: fix "in-memory" process socket conflicts

[Bruce Richardson <bruce.richardson@intel.com>]

On Wed, Sep 29, 2021 at 04:24:06PM +0100, Bruce Richardson wrote:
> On Wed, Sep 29, 2021 at 03:54:48PM +0100, Kevin Traynor wrote:
> > On 29/09/2021 14:32, Bruce Richardson wrote:
> > > On Wed, Sep 29, 2021 at 01:28:53PM +0100, Kevin Traynor wrote:
> > > > Hi Bruce,
> > > > 
> > > > On 24/09/2021 17:18, Bruce Richardson wrote:
> > > > > When DPDK is run with --in-memory mode, multiple processes can run
> > > > > simultaneously using the same runtime dir. This leads to each process
> > > > > removing another process' telemetry socket as it started up, giving
> > > > > unexpected behaviour.
> > > > > 
> > > > > This patch changes that behaviour to first check if the existing socket
> > > > > is active. If not, it's an old socket to be cleaned up and can be
> > > > > removed. If it is active, telemetry initialization fails and an error
> > > > > message is printed out giving instructions on how to remove the error;
> > > > > either by using file-prefix to have a different runtime dir (and
> > > > > therefore socket path) or by disabling telemetry if it not needed.
> > > > > 
> > > > 
> > > > telemetry is enabled by default but it may not be used by the application.
> > > > Hitting this issue will cause rte_eal_init() to fail which will probably
> > > > stop or severely limit the application.
> > > > 
> > > > So it could change a working application to a non-working one (albeit one
> > > > that doesn't interfere with other process' sockets).
> > > > 
> > > > Can it just print a warning that telemetry will not be enabled and continue
> > > > so it's not returning an rte_eal_init failure?
> > > > 
> > > 
> > > For a backported fix, yes, that would probably be better behaviour, but for
> > > the latest branch, I think returning error and having the user explicitly
> > > choose the resolution they want to occur is best. I'll see about doing a
> > > separate backport patch for 20.11.
> > > 
> > 
> > But this is a runtime message dependent on runtime environment. The user may
> > not have access or know how to change eal parameters.
> 
> True. But on the other hand, this problem only occurs with non-default EAL
> parameters anyway, so someone must have configured this with the
> --in-memory flag.
> 
> > 
> > In the case where the application doesn't care about telemetry, they have
> > gone from not having telemetry to rte_eal_init() failing, which probably has
> > severe consequence.
> > 
> 
> Yes, I agree, which I why I would suggest that for any backport of this
> fix, the error be made non-fatal as you suggest. [Having looked into it,
> having it as a non-fatal error is rather awkward, so it may be best just
> left unfixed and the current behaviour documented as known-issue].
> 
> However, for any application being updated and rebuilt against 21.11, I
> would have thought it reasonable to flag this as an error, as any such
> application would require revalidation anyway.
> 
> > I could maybe agree if telemetry was default disable and the application had
> > set the --telemetry flag indicating that they want/need it. As it is, it
> > feels like it's possibly a worse outcome for the user.
> > 
> 
> Perhaps, but I believe the only case of there being an issue would be where:
> 1) a user who cannot modify the EAL parameters
> 2) runs an application which has been updated and rebuilt against 21.11
> 3) where that application is hard-coded to use in-memory mode and
> 4) has never been verified with two or more instances of that running?
> Or am I missing something here?
> 

Let me also go back to the drawing board on the solution here a bit, and
see if I can come up with something better. If I can find a reasonable way
to make it so that we can always create a socket in in-memory mode, despite
other processes running, it would sidestep this problem completely. Not
sure if it's possible, but let me see if I can come up with some ideas.
[One idea I did try is using abstract sockets on linux, but with those we
lose out on the permissions/protection we get from having a filesystem
path, so were a no-go for me because of that]

/Bruce