If you want to discover which functions are thread-safe the hard way, feel free
to disable the barrier syncs as shown. Please don’t push a patch like that.
c->is_mp_safe != 0 means that a human being has looked at the code and done at
least a cursory survey to make sure that vpp won’t have an accident if worker
threads are allowed to process packets while the command runs.
We are not going to push barrier sync logic into every debug CLI command.
FWIW... Dave
From: vpp-dev@lists.fd.io <vpp-dev@lists.fd.io> On Behalf Of Aleksander Djuric
Sent: Friday, November 1, 2019 3:03 AM
To: vpp-dev@lists.fd.io
Subject: Re: [vpp-dev] event-logger output format
Dave,
Thanks for your explanation!
In my opinion, it depends on what the "is_mp_safe" flag actually means. If all
the functions are actually thread-safe and these barrier synchronization calls
are required mainly for debugging, then I suggest the following:
#if CLIB_DEBUG > 0
if (!c->is_mp_safe)
vlib_worker_thread_barrier_sync (vm);
#endif
c_error = c->function (vm, si, c);
#if CLIB_DEBUG > 0
if (!c->is_mp_safe)
vlib_worker_thread_barrier_release (vm);
#endif
or....
all these barrier synchronization calls must be moved into appropriate
functions:
c_error = c->function (vm, si, c, sync);
Regards,
Aleksander
On Thu, Oct 31, 2019 at 10:27 PM, Dave Barach wrote:
Two cases: in single-core operation, debug CLI commands change the NDR / max
throughput. A single “format” call might cost as many clock cycles as
processing 50 packets. If the one and only core is busy printing something,
it’s not processing packets.
In multi-core operation, CLI commands typically don’t change the NDR / max
throughput numbers so long as they are marked thread_safe. Examples include
“show run.” Note that most debug CLI commands are not marked thread-safe.
I think that folks will complain early and often if we start printing
disclaimers every time vpp executes a non-thread-safe debug CLI command.
It wouldn’t be hard to count and [eventually] report non-thread-safe debug CLI
commands. All such barrier sync calls originate in one place, in src/vlib/cli.c:
if (!c->is_mp_safe)
vlib_worker_thread_barrier_sync (vm);
c_error = c->function (vm, si, c);
if (!c->is_mp_safe)
vlib_worker_thread_barrier_release (vm);
What do people think?
Thanks... Dave
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