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From: "Ananyev, Konstantin" <konstantin.ananyev@intel.com>
To: Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>,
	"Gujjar, Abhinandan S" <abhinandan.gujjar@intel.com>,
	"dev@dpdk.org" <dev@dpdk.org>,
	"Doherty, Declan" <declan.doherty@intel.com>
Cc: "jerinj@marvell.com" <jerinj@marvell.com>,
	"Akhil.goyal@nxp.com" <akhil.goyal@nxp.com>,
	"Vangati, Narender" <narender.vangati@intel.com>, nd <nd@arm.com>,
	nd <nd@arm.com>
Subject: Re: [dpdk-dev] [v2 1/2] cryptodev: support enqueue callback functions
Date: Thu, 24 Sep 2020 17:06:52 +0000
Message-ID: <BYAPR11MB3301FFC21A26E5A37A9921219A390@BYAPR11MB3301.namprd11.prod.outlook.com> (raw)
In-Reply-To: <DBAPR08MB581492EEC7314057462689C298380@DBAPR08MB5814.eurprd08.prod.outlook.com>


> 
> > > > > > > > >
> > > > > > > > > +#ifdef RTE_CRYPTODEV_CALLBACKS int
> > > > > > > > > +rte_cryptodev_rcu_qsbr_add(uint8_t dev_id, struct
> > > > > > > > > +rte_rcu_qsbr
> > > > > > > > > +*qsbr) {
> > > > > > > > > +
> > > > > > > > > +	struct rte_cryptodev *dev;
> > > > > > > > > +
> > > > > > > > > +	if (!rte_cryptodev_pmd_is_valid_dev(dev_id)) {
> > > > > > > > > +		CDEV_LOG_ERR("Invalid dev_id=%" PRIu8, dev_id);
> > > > > > > > > +		return -EINVAL;
> > > > > > > > > +	}
> > > > > > > > > +
> > > > > > > > > +	dev = &rte_crypto_devices[dev_id];
> > > > > > > > > +	dev->qsbr = qsbr;
> > > > > > > > > +	return 0;
> > > > > > > > > +}
> > > > > > > >
> > > > > > > > So if I understand your patch correctly you propose a new
> > > > > > > > working model for
> > > > > > > > crypto-devs:
> > > > > > > > 1. Control-plane has to allocate/setup rcu_qsbr and do
> > > > > > > > rte_cryptodev_rcu_qsbr_add().
> > > > > > > > 2. Data-plane has somehow to obtain pointer to that rcu_qsbr
> > > > > > > > and wrap
> > > > > > > > cryptodev_enqueue()
> > > > > > > >    with rcu_qsbr_quiescent()  or
> > rcu_qsbr_online()/rcu_qsbr_offline().
> > > > > > > Yes. I think, it is not a new model. It is same as RCU
> > > > > > > integration with
> > > > LPM.
> > > > > > > Please refer: https://patches.dpdk.org/cover/73673/
> > > > > >
> > > > > > I am talking about new working model for crypto-dev
> > enqueue/dequeue.
> > > > > > As I said above now it becomes data-plane thread responsibility to:
> > > > > >  -somehow to obtain pointer to that rcu_qsbr for each cryptodev
> > > > > > it is
> > > > using.
> > > > > >  -call rcu sync functions (quiescent/online/offline) on a regular basis.
> > > > > It is not on regular basis. When data plane comes up, they report online.
> > > > > They report quiescent when they are done with critical section or
> > > > > shared
> > > > structure.
> > > >
> > > > I understand that, but it means all existing apps have to be changed that
> > way.
> > > >
> > > > > All though, there is some dataplane changes involved here, I don't
> > > > > think, it
> > > > is major.
> > > >
> > > > I still think our goal here should be to make no visible changes to
> > > > the dataplane.
> > > > I.E. all necessary data-plane changes need to be hidden inside CB
> > > > invocation part.
> > > Please note that this is being implemented using the memory
> > > reclamation framework documented at
> > > https://doc.dpdk.org/guides/prog_guide/rcu_lib.html#resource-reclamati
> > > on-framework-for-dpdk
> > >
> > > While using RCU there are couple of trade-offs that applications have to
> > consider:
> > > 1) Performance - reporting the quiescent state too often results in
> > > performance impact on data plane
> > > 2) Amount of outstanding memory to reclaim - reporting less often
> > > results in more outstanding memory to reclaim
> > >
> > > Hence, the quiescent state reporting is left to the application. The
> > > application decides how often it reports the quiescent state and has control
> > over the data plane performance and the outstanding memory to reclaim.
> > >
> > > When you say "new working model for crypto-dev enqueue/dequeue",
> > >
> > > 1) are you comparing these with existing crypto-dev enqueue/dequeue
> > APIs? If yes, these are new APIs, it is not breaking anything.
> > > 2) are you comparing these with existing call back functions in ethdev
> > > enqueue/dequeue APIs? If yes, agree that this is a new model. But, it is
> > possible to support what ethdev supports along with the RCU method used
> > in this patch.
> >
> > What I am talking about:
> >
> > Existing cryptodev enqueue/dequeue model doesn't require for the user to
> > manage any RCU QSBR state manually.
> > I believe that addition of ability to add/remove enqueue/dequeue callbacks
> > shouldn't change existing working model.
> > I think that adding/removing such callbacks has to be opaque to the user DP
> > code and shouldn't require user to change it. Same as we have now for
> > ethdev callback implementation.
> The ethdev callback implementation conveniently leaves the problem of freeing memory to the user to resolve, it does not handle the issue.
> Hence, it "looks" to be opaque to the DP code. However, if the application has to implement a safe way to free the call back memory, its DP
> is affected based on call backs are being used or not.

Yes, I think that's big drawback in initial ethdev callback implementation -
it simply ignores DP/CP sync problem completely.
Though I think it is possible to have both here:
 keep callback "opaque" to DP code and provide some sync mechanism between DP/CP.
Hopefully one day we can fix ethdev callbacks too. 

> > I think that forcing DP code to be aware that callbacks are present or not and
> > to modify its behaviour depending on that nearly voids the purpose of having
> > callbacks at all.
> > In that case DP can just invoke callback function directly from it's codepath .
> >
> > > >
> > > > >
> > > > > > Note that now data-plane thread would have to do that always -
> > > > > > even if there are now callbacks installed for that cryptodev queue
> > right now.
> > > > > > All that changes behaviour of existing apps and I presume would
> > > > > > reduce adoption of  that fature.
> > > If I understand this correct, you are talking about a case where in
> > > the application might be registering/unregistering multiple times
> > > during its lifetime. In this case, yes, the application might be reporting the
> > quiescent state even when it has not registered the call backs. But, it has the
> > flexibility to not report it if it implements additional logic.
> > > Note that we are assuming that the application has to report quiescent
> > > state only for using callback functions. Most probably the application has
> > other requirements to use RCU.
> > > Why not support what is done for ethdev call back functions along with
> > providing RCU method?
> > >
> > > > > There is always trade off involved!
> > > > > In the previous patch, you suggested that some lazy app may not
> > > > > free up the memory allocated by add cb. For such apps, this patch
> > > > > has sync mechanism with some additional cost of CP & DP changes.
> > > >
> > > > Sigh, it is not about laziness of the app.
> > > > The problem with current ethedev cb mechanism and yours V1 (which
> > > > was just a clone of it) - CP doesn't know when it is safe after CB
> > > > removal to free related memory.
> > > >
> > > > > > I still think all this callback mechanism should be totally
> > > > > > opaque to data-plane threads - user shouldn't change his app
> > > > > > code depending on would some enqueue/dequeue callbacks be
> > installed or not.
> > > > > I am not sure, how that can be implemented with existing RCU design.
> > > >
> > > > As I said below the simplest way - with calling rcu onine/offline
> > > > inside CB invocation block.
> > > > That's why I asked you - did you try that approach and what is the
> > > > perf numbers?
> > > > I presume with no callbacks installed the perf change should be nearly
> > zero.
> > > >
> > > > > @Honnappa Nagarahalli, Do you have any suggestions?
> > > Reporting quiescent state in the call back functions has several
> > disadvantages:
> > > 1) it will have performance impacts and the impacts will increase as the
> > number of data plane threads increase.
> > > 2) It will require additional configuration parameters to control how
> > > often the quiescent state is reported to control the performance impact.
> > > 3) Does not take advantage of the fact that most probably the
> > > application is using RCU already
> > > 4) There are few difficulties as well, please see below.
> >
> > I suggested Abhinandan to use RCU library because it is already there, and I
> > thought it would be good not to re-implement the wheel.
> > Though if you feel librte_rcu doesn't match that task - fine, let's do it without
> > librte_rcu.
> > After all, what we need here - just an atomic ref count per queue that we are
> > going to increment at entering and leaving list of callbacks inside
> > enqueue/dequeue.
> Ok, looks like I missed the point that a queue is used by a single data plane thread.
> Along with ref count increment you need the memory orderings to avoid race conditions. These will be the same ones used in RCU.
> On the control plane, you need to read this counter and poll for the counter updates. All this is same cost as in RCU.

Agree.

> To control the cost, you
> will have to control the rate of quiescent state reporting and might have to expose this as a configuration parameter.
> 
> The other important information you have to consider is if the thread is making any blocking calls, which may be in some other library. The
> thread is supposed to call rcu_qsbr_thread_offline API before calling a blocking call. This allows the RCU to know that this particular thread
> will not report quiescent state. The cryptodev library might not have that information.
> 
> If you want to go ahead with this design, you can still use RCU with single thread configuration (like you have mentioned below) and hide
> the details from the application.

Yes,  same thought here -  use rcu_qsbr online/offline for DP part and hide actual sync details inside callback code.

> 
> >
> > >
> > > > >
> > > > > >
> > > > > > >
> > > > > > > >
> > > > > > > > That seems quite a big change and I don't think it is
> > > > > > > > acceptable for most users.
> > > > > > > > From my perspective adding/installing call-backs to the dev
> > > > > > > > has to be opaque to the data-plane code.
> > > > > > > > Also note that different callbacks can be installed by
> > > > > > > > different entities (libs) and might have no idea about each other.
> > > > > > > > That's why I thought it would be better to make all this RCU
> > > > > > > > stuff internal inside cryptodev:
> > > > > > > >     hide all this rcu_qsbr allocation/setup inside cryptod
> > > > > > > > somehow to
> > > > > > obtain pointer to that rcu_qsbr ev init/queue setup
> > > > > > > >     invoke rcu_qsbr_online()/rcu_qsbr_offline() inside
> > > > > > cryptodev_enqueue().
> > > This will bring in the application related information such as the thread ID
> > into the library.
> >
> > I don't think it would.
> > Cryptodev enqueue/dequeue functions are not supposed to be thread safe
> > (same as rx/tx burst).
> > So we can always use RCU with just one thread(thread_id = 0).
> Agree, the memory that needs to be freed is accessed by a single thread on the data plane. RCU with one thread would suffice.
> 
> > But as I said above - if you feel RCU lib is an overhead here, that's fine - I
> > think it would be easy enough to do without librte_rcu.
> >
> > > If the same API calls are being made from multiple data plane threads,
> > > you need a way to configure that information to the library. So, it is
> > > better to leave those details for the application to handle.
> > >
> > > > > > > I have already tried exploring above stuffs. There are too
> > > > > > > many
> > > > constraints.
> > > > > > > The changes don't fit in, as per RCU design.
> > > > > >
> > > > > > Hmm could you be more specific here - what constraints are you
> > > > > > referring to?
> > > > > >
> > > > > > > Moreover, having rcu api under enqueue_burst() will affect the
> > > > > > performance.
> > > > > >
> > > > > > It most likely will. Though my expectation it will affect
> > > > > > performance only when some callbacks are installed. My thought
> > here:
> > > > > > callback function by itself will affect cryptdev_enqueue
> > > > > > performance anyway,
> > > > > With existing callback design, I have measured the
> > > > > performance(with
> > > > crypto perf test) on xeon.
> > > > > It was almost negligible and same was shared with Declan.
> > > >
> > > > I am asking about different thing: did you try alternate approach I
> > > > described, that wouldn't require changes in the user data-plane code.
> > > >
> > > > > That is one of the reasons, I didn't want to add to many stuffs in
> > > > > to the
> > > > callback.
> > > > > The best part of existing design is crypto lib is not much modified.
> > > > > The changes are either pushed to CP or DP.
> > > > >
> > > > > so adding extra overhead for sync is probably ok here.
> > > >
> > > > I think that extra overhead when callbacks are present is expected
> > > > and probably acceptable.
> > > > Changes in the upper-layer data-plane code - probably not.
> > > >
> > > > > > Though for situation when no callbacks are installed -
> > > > > > perfomance should be left unaffected (or impact should be as small
> > as possible).
> > > > > >
> > > > > > > The changes are more on control plane side, which is one time.
> > > > > > > The data plane changes are minimal.
> > > > > >
> > > > > > I still think upper layer data-plane code should stay unaffected
> > > > > > (zero changes).
> > > > > >
> > > > > > > >
> 
> <snip>

  reply	other threads:[~2020-09-24 17:08 UTC|newest]

Thread overview: 24+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-09-08  7:10 Abhinandan Gujjar
2020-09-09 16:21 ` Gujjar, Abhinandan S
2020-09-09 22:48   ` Honnappa Nagarahalli
2020-09-11  8:33     ` Gujjar, Abhinandan S
2020-09-16 13:39 ` Ananyev, Konstantin
2020-09-16 15:17   ` Gujjar, Abhinandan S
2020-09-17 13:24     ` Ananyev, Konstantin
2020-09-21 10:59       ` Gujjar, Abhinandan S
2020-09-21 11:45         ` Ananyev, Konstantin
2020-09-23  3:25           ` Honnappa Nagarahalli
2020-09-23 11:58             ` Ananyev, Konstantin
2020-09-23 22:41               ` Honnappa Nagarahalli
2020-09-24 17:06                 ` Ananyev, Konstantin [this message]
2020-09-29  5:14                   ` Honnappa Nagarahalli
2020-09-29  9:03                     ` Ananyev, Konstantin
2020-10-08 17:36                       ` Gujjar, Abhinandan S
2020-10-09 14:40                         ` Ananyev, Konstantin
2020-10-09 16:51                           ` Gujjar, Abhinandan S
2020-10-11 23:02                             ` Ananyev, Konstantin
2020-10-12  6:47                               ` Gujjar, Abhinandan S
2020-10-12 13:02                                 ` Ananyev, Konstantin
2020-10-19 22:04                                   ` Honnappa Nagarahalli
2020-10-20  9:25                                     ` Gujjar, Abhinandan S
2020-09-24  5:17 ` Honnappa Nagarahalli

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