Re: [dpdk-dev] [RFC PATCH v1 1/4] ethdev: add support for PMD-tuned Tx/Rx parameters

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

On Thu, Mar 15, 2018 at 01:57:13PM +0000, Ferruh Yigit wrote:
> On 3/14/2018 9:36 PM, Bruce Richardson wrote:
> > On Wed, Mar 14, 2018 at 09:02:47PM +0000, Ferruh Yigit wrote:
> >> On 3/14/2018 6:53 PM, Ananyev, Konstantin wrote:
> >>>
> >>>
> >>>> -----Original Message-----
> >>>> From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Ferruh Yigit
> >>>> Sent: Wednesday, March 14, 2018 5:52 PM
> >>>> To: Shreyansh Jain <shreyansh.jain@nxp.com>; Horton, Remy <remy.horton@intel.com>; dev@dpdk.org
> >>>> Cc: Lu, Wenzhuo <wenzhuo.lu@intel.com>; Wu, Jingjing <jingjing.wu@intel.com>; Zhang, Qi Z <qi.z.zhang@intel.com>; Xing, Beilei
> >>>> <beilei.xing@intel.com>; Thomas Monjalon <thomas@monjalon.net>
> >>>> Subject: Re: [dpdk-dev] [RFC PATCH v1 1/4] ethdev: add support for PMD-tuned Tx/Rx parameters
> >>>>
> >>>> On 3/14/2018 5:23 PM, Shreyansh Jain wrote:
> >>>>>> -----Original Message-----
> >>>>>> From: Ferruh Yigit [mailto:ferruh.yigit@intel.com]
> >>>>>> Sent: Wednesday, March 14, 2018 10:13 PM
> >>>>>> To: Remy Horton <remy.horton@intel.com>; dev@dpdk.org
> >>>>>> Cc: Wenzhuo Lu <wenzhuo.lu@intel.com>; Jingjing Wu
> >>>>>> <jingjing.wu@intel.com>; Qi Zhang <qi.z.zhang@intel.com>; Beilei Xing
> >>>>>> <beilei.xing@intel.com>; Shreyansh Jain <shreyansh.jain@nxp.com>;
> >>>>>> Thomas Monjalon <thomas@monjalon.net>
> >>>>>> Subject: Re: [dpdk-dev] [RFC PATCH v1 1/4] ethdev: add support for PMD-
> >>>>>> tuned Tx/Rx parameters
> >>>>>>
> >>>>>> On 3/14/2018 3:48 PM, Remy Horton wrote:
> >>>>>>>
> >>>>>>> On 14/03/2018 14:43, Ferruh Yigit wrote:
> >>>>>>> [..]
> >>>>>>>>>  lib/librte_ether/rte_ethdev.c | 18 ++++++++++++++++++
> >>>>>>>>>  lib/librte_ether/rte_ethdev.h | 15 +++++++++++++++
> >>>>>>>>
> >>>>>>>> Can you please remove deprecation notice in this patch.
> >>>>>>>
> >>>>>>> Done.
> >>>>>>>
> >>>>>>>>> +	/* Defaults for drivers that don't implement preferred
> >>>>>>>>> +	 * queue parameters.
> >>>>>>> [..]
> >>>>>>>> Not sure about having these defaults here. It is OK to have defaults
> >>>>>> in driver,
> >>>>>>>> in application or in config file, but I am not sure if putting them
> >>>>>> into device
> >>>>>>>> abstraction layer hides them.
> >>>>>>>>
> >>>>>>>> What about not providing any default in ethdev layer, and get zero
> >>>>>> as invalid
> >>>>>>>> when using them?
> >>>>>>>
> >>>>>>> This is something I have been thinking about, and I am going to
> >>>>>> remove
> >>>>>>> them for the V2. Original motive was to avoid breaking testpmd for
> >>>>>> PMDs
> >>>>>>> that don't give defaults (i.e. almost all of them). The alternative
> >>>>>> is
> >>>>>>> to put place-holders into all the PMDs themselves, but I am not sure
> >>>>>> if
> >>>>>>> this is appropriate.
> >>>>>>
> >>>>>> I think preferred values should be optional, PMD should have right to
> >>>>>> not
> >>>>>> provide any. Implementation in 4/4 forces preferred values, either in
> >>>>>> all PMDs
> >>>>>> or in ethdev layer.
> >>>>>>
> >>>>>> What about changing approach in application:
> >>>>>>  is preferred value provided [1] ?
> >>>>>>   yes => use it by sending value 0
> >>>>>>   no => use application provided value, same as now, so control should
> >>>>>> be in
> >>>>>> application.
> >>>>>>
> >>>>>> I am aware this breaks the comfort of just providing 0 and PMD values
> >>>>>> will be
> >>>>>> used but covers the case there is no PMD values.
> >>>>>>
> >>>>>> [1]
> >>>>>> it can be possible to check if preferred value provided by comparing 0,
> >>>>>> but if 0
> >>>>>> is a valid value that can be problem. It may not be problem with
> >>>>>> current
> >>>>>> variables but it may be when this struct extended, it may be good to
> >>>>>> think about
> >>>>>> alternative here.
> >>>>>
> >>>>> I don't think we should use the condition of "yes => use it by sending value 0". That is non-intuitive. Ideally, the application should query
> >>>> and then if query responds with value as '0' (which can be valid for some variables in future), it sends its own value to setup functions
> >>>> (whether '0' or something else, in case of 0 response, would depend on the knob).
> >>>>
> >>>> Right, at that stage application already knows what is the preferred value and
> >>>> can directly use it.
> >>>>
> >>>>
> >>>> Will it be too much to:
> >>>>
> >>>> 1) Adding a new field into "rte_eth_[rt]xconf" to say if exists prefer PMD
> >>>> values. "prefer_device_values" ?
> >>>> Application can provide values as usual, but if that field is set, abstraction
> >>>> layer overwrites the application values with PMD preferred ones. If there is no
> >>>> PMD preferred values continue using application ones.
> >>>>
> >>>>
> >>>> 2) Add a bitwise "is_set" field to new "preferred_size" struct, which may show
> >>>> status of other fields in the struct, if PMD set a valid value for them or not,
> >>>> so won't have to rely on the 0 check.
> >>>
> >>> That all seems like too much hassle for such small thing.
> >>
> >> Fair enough.
> >>
> >>> If we really want to allow PMD not to provide preferred values -
> >>> then instead of adding rte_eth_dev_pref_info into dev_info we can simply
> >>> introduce a new optional ethdev API call:
> >>> rte_eth_get_pref_params() or so.
> >>> If the PMD doesn’t want to provide preferred params to the user it simply
> >>> wouldn't implement that function. 
> >>
> >> Same can be done with updated rte_eth_dev_info.
> >> Only application needs to check and use PMD preferred values, so this will mean
> >> dropping "pass 0 to get preferred values" feature in initial set.
> >>
> >>>
> > I actually don't see the issue with having ethdev provide reasonable
> > default values. If those don't work for a driver, then let the driver
> > provide it's own values. If the defaults don't work for an app, then let
> > the app override the provided values.
> > 
> > It really is going to make the app writers job easier if we do things this
> > way. The only thing you are missing is the info as to whether it's ethdev
> > or the driver that's providing the values, but in the case that it's
> > ethdev, then the driver by definition "doesn't care", so we can treat them
> > as driver provided values. What's the downside?
> Abstraction layer having hardcoded config options doesn't look right to me. In
> long term who will ensure to make those values relevant?
> 

Let me turn that question around - in the long-term how likely are the
values to change significantly? Also, long-term all PMDs should provide
their own default values and then we can remove the values in the ethdev
layer.

> When application provides a value of 0, it won't know if it is using PMD
> preferred values or some other defaults, what if application explicitly wants
> use PMD preferred values?

If the PMD has preferred values, they will be automatically used. Is there
are case where the app would actually care about it? If the driver doesn't
provide default values, how is the app supposed to know what the correct
value for that driver is? And if the app *does* know what the best value
for a driver is - even if the driver itself doesn't, it can easily detect
when a port is using the driver and provide it's own ring setup defaults.
If you want, we can provide a flag field to indicate that fields are ethdev
defaults not driver defaults or something, but I'm struggling to come up
with a scenario where it would make a practical difference to an app.

> 
> The new fields are very similar to "default_[rt]xconf" in dev_info. Indeed
> perhaps we should use same naming convention because intention seems same.
> And we can continue to use new fields same as how "default_[rt]xconf" used.
> 
> What about having something like rte_eth_tx_queue_setup_relaxed() where
> application really don't care about values, not sure why, which can get config
> values as much as from PMDs and fill the missing ones with the values defined in
> function?
> 

Or how about having the ethdev defaults in the rx/tx setup function instead
of in the dev_info one? If user specifies a zero size, we use the dev_info
value if provided by driver, otherwise ethdev default. That allows the
majority of apps to never worry about ring sizes, but for those that do,
they can query the driver defaults directly, or if not present set their
own.

/Bruce