Re: [PATCH v1] event/dlb2: add dequeue interrupt mode support

["Mattias Rönnblom" <hofors@lysator.liu.se>]

On 2025-06-30 19:34, Pathak, Pravin wrote:
> 
> 
>> -----Original Message-----
>> From: Mattias Rönnblom <hofors@lysator.liu.se>
>> Sent: Monday, June 30, 2025 12:51 PM
>> To: Pathak, Pravin <pravin.pathak@intel.com>; Jerin Jacob
>> <jerinjacobk@gmail.com>
>> Cc: dev@dpdk.org; jerinj@marvell.com; Chen, Mike Ximing
>> <mike.ximing.chen@intel.com>; Richardson, Bruce
>> <bruce.richardson@intel.com>; thomas@monjalon.net; Marchand, David
>> <david.marchand@redhat.com>; nipun.gupta@amd.com;
>> chenbox@nvidia.com; Sarkar, Tirthendu <tirthendu.sarkar@intel.com>; Pavan
>> Nikhilesh <pbhagavatula@marvell.com>; Shijith Thotton
>> <sthotton@marvell.com>; Hemant Agrawal <hemant.agrawal@nxp.com>;
>> Sachin Saxena <sachin.saxena@oss.nxp.com>; harry.chang@intel.com;
>> Mattias Rönnblom <mattias.ronnblom@ericsson.com>
>> Subject: Re: [PATCH v1] event/dlb2: add dequeue interrupt mode support
>>
>> On 2025-06-30 18:18, Pathak, Pravin wrote:
>>>
>>>
>>>> -----Original Message-----
>>>> From: Jerin Jacob <jerinjacobk@gmail.com>
>>>> Sent: Monday, June 30, 2025 7:44 AM
>>>> To: Mattias Rönnblom <hofors@lysator.liu.se>
>>>> Cc: Pathak, Pravin <pravin.pathak@intel.com>; dev@dpdk.org;
>>>> jerinj@marvell.com; Chen, Mike Ximing <mike.ximing.chen@intel.com>;
>>>> Richardson, Bruce <bruce.richardson@intel.com>; thomas@monjalon.net;
>>>> Marchand, David <david.marchand@redhat.com>;
>> nipun.gupta@amd.com;
>>>> chenbox@nvidia.com; Sarkar, Tirthendu <tirthendu.sarkar@intel.com>;
>>>> Pavan Nikhilesh <pbhagavatula@marvell.com>; Shijith Thotton
>>>> <sthotton@marvell.com>; Hemant Agrawal <hemant.agrawal@nxp.com>;
>>>> Sachin Saxena <sachin.saxena@oss.nxp.com>; harry.chang@intel.com;
>>>> Mattias Rönnblom <mattias.ronnblom@ericsson.com>
>>>> Subject: Re: [PATCH v1] event/dlb2: add dequeue interrupt mode
>>>> support
>>>>
>>>> On Mon, Jun 30, 2025 at 4:47 PM Mattias Rönnblom
>>>> <hofors@lysator.liu.se>
>>>> wrote:
>>>>>
>>>>> On 2025-06-30 11:19, Jerin Jacob wrote:
>>>>>> On Sat, Jun 28, 2025 at 11:17 AM Pravin Pathak
>>>> <pravin.pathak@intel.com> wrote:
>>>>>>>
>>>>>>> DLB2 port interrupt is implemented using DPDK interrupt framework.
>>>>>>> This allows eventdev dequeue API to sleep when the port queue is
>>>>>>> empty and gets wakeup when event arrives at the port. Port dequeue
>>>>>>> mode is configured using devargs argument port_dequeue_wait.
>>>>>>> Supported modes are polling and interrupt. Default mode is polling.
>>>>>>> This commit also adds code to handle device error interrupts and
>>>>>>> print alarm details.
>>>>>>>
>>>>>>> Signed-off-by: Pravin Pathak <pravin.pathak@intel.com>
>>>>>>> Signed-off-by: Tirthendu Sarkar <tirthendu.sarkar@intel.com>
>>>>>>> ---
>>>>>>>     doc/guides/eventdevs/dlb2.rst              |  20 +
>>>>>>>     drivers/event/dlb2/dlb2.c                  | 236 +++++-
>>>>>>>     drivers/event/dlb2/dlb2_iface.c            |   7 +
>>>>>>>     drivers/event/dlb2/dlb2_iface.h            |   8 +
>>>>>>>     drivers/event/dlb2/dlb2_priv.h             |  18 +
>>>>>>>     drivers/event/dlb2/dlb2_user.h             | 112 +++
>>>>>>>     drivers/event/dlb2/pf/base/dlb2_hw_types.h |  70 ++
>>>>>>>     drivers/event/dlb2/pf/base/dlb2_osdep.h    |  46 ++
>>>>>>>     drivers/event/dlb2/pf/base/dlb2_regs.h     | 149 +++-
>>>>>>>     drivers/event/dlb2/pf/base/dlb2_resource.c | 825
>>>> +++++++++++++++++++++
>>>>>>>     drivers/event/dlb2/pf/base/dlb2_resource.h |   6 +
>>>>>>>     drivers/event/dlb2/pf/dlb2_pf.c            | 223 ++++++
>>>>>>>     12 files changed, 1711 insertions(+), 9 deletions(-)
>>>>>>>
>>>>>>> diff --git a/doc/guides/eventdevs/dlb2.rst
>>>>>>> b/doc/guides/eventdevs/dlb2.rst index 8ec7168f20..a4ba857351
>>>> 100644
>>>>>>> --- a/doc/guides/eventdevs/dlb2.rst
>>>>>>> +++ b/doc/guides/eventdevs/dlb2.rst
>>>>>>> @@ -477,6 +477,26 @@ Example command to use as meson option
>> for
>>>> credit handling:
>>>>>>>
>>>>>>>            meson configure -Dc_args='-DDLB_SW_CREDITS_CHECKS=0 -
>>>> DDLB_HW_CREDITS_CHECKS=1'
>>>>>>>
>>>>>>> +Interrupt Mode Support
>>>>>>> +~~~~~~~~~~~~~~~~~~~~~~
>>>>>>> +DLB dequeue supports interrupt mode for the API
>>>> rte_event_dequeue_burst().
>>>>>>> +The default port dequeue mode is polling. Dequeue wait mode can
>>>>>>> +be configured on per eventdev port basis using devargs argument
>>>>>>> +'port_dequeue_wait'. In interrupt mode, if the port queue is
>>>>>>> +empty, the application thread will block on the interrupt until a
>>>>>>> +new event arrives. It enters blocking mode only after any
>>>>>>> +specified timeout. During the timeout, it will poll the port
>>>>>>> +queue for
>>>> events as usual. Interrupt mode uses the DPDK interrupt support
>> framework.
>>>>>>> +
>>>>>>> +    .. code-block:: console
>>>>>>> +
>>>>>>> +       --allow ea:00.0,port_dequeue_wait=all:interrupt
>>>>>>
>>>>>> Adding other eventdev PMD mainatainers.
>>>>>>
>>>>>> Looks like it can be a generic feature. i.e set this option is
>>>>>> dev_configure() If there is no objection, Please send a new patch
>>>>>> around
>>>> that.
>>>>>
>>>>> I've considered implementing this in DSW, although in a different
>>>>> manner (with eventfds and poll()).
>>>>>
>>>>> The dequeue timeout will still be honored in "interrupt mode", correct?
>>>>> It wasn't obvious from the description.
>>>>
>>>> How is it in Intel PMD?
>>>
>>> It would be best if we configure it per port using RTE_EVENT_PORT_CFG_*
>> flags. Will it be, OK?
>>> The dequeue timeout will be honored, and the decision to block or return
>> will be made after the timeout.
>>
>> That doesn't sound like the timeout is honored.
>>
>> The reason an application wants the dequeue call to complete within a certain
>> time, even though there wasn't any events, is likely because it want to go do
>> something else with that thread, after the timeout.
>>
>> Thus, you can't decide to block the thread *after* the timeout. If you block,
>> you have to do some time *before* the timeout, and wake up in time to meet
>> the deadline.
>>
>> For example, if the event device is given a 1 ms dequeue timeout by the
>> application, it could go busy-poll for 10 us, then busy-poll with a short tpause
>> for 100 us, and then put the thread to sleep blocking on some fd for the
>> remaining 890 us.
>>
>> The hardware-specific nature of the timing and exact mechanism to use
>> speaks against having this kind of configuration in the Eventdev API.
> 
> The mode is not for what to do during a timeout, but after a timeout.

My comments were not so much concerning any DLB-specific extension, but 
rather how this kind of function should work, if it was a part of the 
standard API.

> We can enter sleep mode immediately by setting the timeout to 0 if we need to.
> This mode is not changing the current timeout behavior.  After the timeout,
> it allows HW devices supporting interrupts to block on an interrupt.

The current API specifies that control is returned to the application, 
after the timeout has expired. If you change that with a PMD parameter, 
the DLB behavior will be in violation of the API contract. Applications 
using rte_keepalive_alive() between dequeues is one example of those 
that will break. In fact, all applications that use more than one RTE 
service will break.

> It will wake up only after a new event arrives at the port. If the application needs control
> back then it can use the current default non-blocking mode. A better mode to save power during timeout
> will be to use umwait-based sleep.  Consider this as extension to current timeout behavior if device
> supports interrupts.
> 

What about applications that both want power efficiency *and* require a 
timeout?

It makes no sense to me the change the semantics of the dequeue timeout 
parameter from "the time until I want control back" to "the time I want 
the event device to use polling, after which it should put the thread to 
sleep". Those two are pretty much orthogonal.

The current API doesn't specify what happens during the timeout. If you 
by "non-blocking" mean "busy-polling", that is not an API requirement. I 
don't see why the event device couldn't put the lcore thread to sleep 
during a long timeout (and I also see why you may not want that to be 
the default behavior).

>>
>>> If not interrupt, it can be called blocking vs polling mode.  If the port config is
>> fine, I will create a new patch with it.
>>> Also, we should have this as a capability for eventdevs.
>>>
>>>>
>>>>>
>>>>> What's being configured should just be a threshold time at which the
>>>>> event device would go from busy-polling to blocking the thread.
>>>>>
>>>>> Maybe it should be called something with "blocking" or "sleeping",
>>>>> instead of "interrupt", since interrupts are never directly involved.
>>>>
>>>> Agree. or make it a power save mode or so.
>>>>
>>>>>
>>>>> Anyway, seems like a good candidate for a generic feature to me.
>>>>>
>