netdev - Re: [PATCH net-next 05/15] ice: replace custom AIM algorithm with kernel's DIM library

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Message-ID: <20210415100748.44fffef5@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com>
Date:   Thu, 15 Apr 2021 10:07:48 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     "Keller, Jacob E" <jacob.e.keller@...el.com>
Cc:     "Nguyen, Anthony L" <anthony.l.nguyen@...el.com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "sassmann@...hat.com" <sassmann@...hat.com>,
        "Brandeburg, Jesse" <jesse.brandeburg@...el.com>,
        "Brelinski, TonyX" <tonyx.brelinski@...el.com>
Subject: Re: [PATCH net-next 05/15] ice: replace custom AIM algorithm with
 kernel's DIM library

On Thu, 15 Apr 2021 17:03:23 +0000 Keller, Jacob E wrote:
> > On Wed, 14 Apr 2021 17:30:03 -0700 Tony Nguyen wrote:  
> > > +static void ice_tx_dim_work(struct work_struct *work)
> > > +{
> > > +	struct ice_ring_container *rc;
> > > +	struct ice_q_vector *q_vector;
> > > +	struct dim *dim;
> > > +	u16 itr, intrl;
> > > +
> > > +	dim = container_of(work, struct dim, work);
> > > +	rc = container_of(dim, struct ice_ring_container, dim);
> > > +	q_vector = container_of(rc, struct ice_q_vector, tx);
> > > +
> > > +	if (dim->profile_ix >= ARRAY_SIZE(tx_profile))
> > > +		dim->profile_ix = ARRAY_SIZE(tx_profile) - 1;
> > > +
> > > +	/* look up the values in our local table */
> > > +	itr = tx_profile[dim->profile_ix].itr;
> > > +	intrl = tx_profile[dim->profile_ix].intrl;
> > > +
> > > +	ice_write_itr(rc, itr);
> > > +	ice_write_intrl(q_vector, intrl);
> > > +
> > > +	dim->state = DIM_START_MEASURE;  
> > 
> > Are you only doing register writes in ice_write_itr/intrl or talk to FW?
> > Scheduler is expensive so you can save real cycles if you don't have to
> > rely on a work to do the programming (not sure how hard that is with
> > DIM, but since you're already sorta poking at the internals I thought
> > I'd ask).  
> 
> Hmm. I believe we only have to do register writes. If I recall, at
> least based on reading the other DIMLIB implementations, they seem to
> have mostly moved to a work item for apparently moving these changes
> out of the hot path.. but maybe that's not really an issue. Ofcourse
> the current dim implementation enforces use of the work queue, so I
> think it would require refactoring the library to support doing
> immediate application as opposed to using the work item..

I think it may be because of FW programming which needs to sleep. The
extra scheduler work because of this async mechanism does impact real
workloads (admittedly not more than 1%), but up to you.