linux-kernel - Re: [net-next v3 14/15] net: marvell: Convert tasklet API to new bottom half workqueue mechanism

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Message-ID: <CAOMdWSLkra7LwXO=iy+OidZ3fWt5gpXEAteDciMLwcYe0Ox+fA@mail.gmail.com>
Date: Wed, 31 Jul 2024 13:46:07 -0700
From: Allen <allen.lkml@...il.com>
To: "Russell King (Oracle)" <linux@...linux.org.uk>
Cc: Andrew Lunn <andrew@...n.ch>, kuba@...nel.org, 
	Marcin Wojtas <marcin.s.wojtas@...il.com>, "David S. Miller" <davem@...emloft.net>, 
	Eric Dumazet <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>, 
	Mirko Lindner <mlindner@...vell.com>, Stephen Hemminger <stephen@...workplumber.org>, 
	jes@...ined-monkey.org, kda@...ux-powerpc.org, cai.huoqing@...ux.dev, 
	dougmill@...ux.ibm.com, npiggin@...il.com, christophe.leroy@...roup.eu, 
	aneesh.kumar@...nel.org, naveen.n.rao@...ux.ibm.com, nnac123@...ux.ibm.com, 
	tlfalcon@...ux.ibm.com, cooldavid@...ldavid.org, nbd@....name, 
	sean.wang@...iatek.com, Mark-MC.Lee@...iatek.com, lorenzo@...nel.org, 
	matthias.bgg@...il.com, angelogioacchino.delregno@...labora.com, 
	borisp@...dia.com, bryan.whitehead@...rochip.com, 
	UNGLinuxDriver@...rochip.com, louis.peens@...igine.com, 
	richardcochran@...il.com, linux-rdma@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-acenic@...site.dk, 
	linux-net-drivers@....com, netdev@...r.kernel.org
Subject: Re: [net-next v3 14/15] net: marvell: Convert tasklet API to new
 bottom half workqueue mechanism

> On Tue, Jul 30, 2024 at 10:39:51PM +0200, Andrew Lunn wrote:
> > > - * Called only from mvpp2_txq_done(), called from mvpp2_tx()
> > > - * (migration disabled) and from the TX completion tasklet (migration
> > > - * disabled) so using smp_processor_id() is OK.
> > > + * Called only from mvpp2_txq_done().
> > > + *
> > > + * Historically, this function was invoked directly from mvpp2_tx()
> > > + * (with migration disabled) and from the bottom half workqueue.
> > > + * Verify that the use of smp_processor_id() is still appropriate
> > > + * considering the current bottom half workqueue implementation.
> >
> > What does this mean? You want somebody else to verify this? You are
> > potentially breaking this driver?
>
> I don't see how, the only thing that's changing in mvpp2 seems to be
> an outdated comment that happens to mention a tasklet, but the
> driver doesn't use tasklets.
>
> Let's look at the original comment which claims what the call sites
> are:
>
> static void mvpp2_txq_done(struct mvpp2_port *port, struct mvpp2_tx_queue *txq,
>                            struct mvpp2_txq_pcpu *txq_pcpu)
> {
> ...
>         tx_done = mvpp2_txq_sent_desc_proc(port, txq);
>
> and that is it. _This_ function is called from several places:
>
> mvpp2_tx_done()
> mvpp2_xdp_finish_tx()
> mvpp2_tx()
>
> So I suppose that the original comment was referring to the
> mvpp2_tx() -> mvpp2_txq_done() -> mvpp2_txq_sent_desc_proc() call path,
> and the others were added over time.

 You are right, I should have checked the calls before modifying the comment.

>
> mvpp2_tx_done() is called from mvpp2_hr_timer_cb(), and yes, back in
> the distant history there was a tasklet here - see:
>
> ecb9f80db23a net/mvpp2: Replace tasklet with softirq hrtimer

 I missed this bit completely,  "historically...." is completely wrong and
misleading.

>
> So, the comment referring to a tasklet was left over from that commit
> and never fixed up.
>
> Given this, I don't think the new paragraph starting "Historically"
> is correct (or even relevant) as I think it misinterprets the original
> comment - and "this function" is ambiguous in it, but either way its
> still wrong.
>
> If we assume that "this function" refers to the one below the comment,
> then this has never been called directly from mvpp2_tx() nor the
> tasklet, and talking about a bottom half workqueue makes no sense
> because "historically" it's never been called from a bottom half
> workqueue.
>
> If we assume that "this function" refers to mvpp2_txq_done(), then
> it's not historical that this was called from mvpp2_tx(), because it
> still is today. And the bit about being called from a bottom half
> workqueue is still false.
>
> Given that bottom half workqueues have absolutely nothing to do with
> this code path, the sentence beginning with "Verify" seems totally
> irrelevant (at least to me.)
>
> So, I think I've comprehensively ripped the new comment to shreds.
> It would be far better to leave the driver alone and not change the
> comment despite it incorrectly referring to a tasklet that has
> already been eliminated (and at least was historically correct),
> rather than the new comment which just seems wrong.

  I am open to leaving the comment as is or completely removing it.
I am open to suggestions, please let me know what would be
the correct thing to do.

Thanks for taking the time out to review and write back.

- Allen

>
> --
> RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
> FTTP is here! 80Mbps down 10Mbps up. Decent connectivity at last!



-- 
       - Allen