netdev - Re: stmmac ethernet in kernel 4.9-rc6: coalescing related pauses.

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Date:   Fri, 2 Dec 2016 09:45:11 +0100
From:   Pavel Machek <pavel@....cz>
To:     Giuseppe CAVALLARO <peppe.cavallaro@...com>,
        alexandre.torgue@...com
Cc:     David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: stmmac ethernet in kernel 4.9-rc6: coalescing related pauses.

Hi!

> >>1 HZ, which is the lowest granularity of non-highres timers in the
> >>kernel, is variable as well as already too large of a delay for
> >>effective TX coalescing.
> >>
> >>I seriously think that the TX coalescing support should be ripped out
> >>or disabled entirely until it is implemented properly in this
> >>driver.
> >
> >Ok, I'd disable coalescing, but could not figure it out till. What is
> >generic way to do that?
> >
> >It seems only thing stmmac_tx_timer() does is calling
> >stmmac_tx_clean(), which reclaims tx_skbuff[] entries. It should be
> >possible to do that explicitely, without delay, but it stops working
> >completely if I attempt to do that.
> >
> >On a side note, stmmac_poll() does stmmac_enable_dma_irq() while
> >stmmac_dma_interrupt() disables interrupts. But I don't see any
> >protection between the two, so IMO it could race and we'd end up
> >without polling or interrupts...
> 
> 
> the idea behind the TX mitigation is to mix the interrupt and
> timer and this approach gave us real benefit in terms
> of performances and CPU usage (especially on SH4-200/SH4-300 platforms
> based).

Well, if you have a workload that sends and receive packets, it tends
to work ok, as you do tx_clean() in stmmac_poll(). My workload is not
like that -- it is "sending packets at 3MB/sec, receiving none". So
the stmmac_tx_timer() is rescheduled and rescheduled and rescheduled,
and then we run out of transmit descriptors, and then 40msec passes,
and then we clean them. Bad.

And that's why low-res timers do not cut it.

> In the ring, some descriptors can raise the irq (according to a
> threshold) and set the IC bit. In this path, the NAPI  poll will be
> scheduled.

Not NAPI poll but stmmac_tx_timer(), right? 

> But there is a timer that can run (and we experimented that no high
> resolution is needed) to clear the tx resources.
> Concerning the lock protection, we had reviewed long time ago and
> IIRC, no raise condition should be present. Open to review it,
> again!

Well, I certainly like the fact that we are talking :-).

And yes, I have some questions.

There's nothing that protect stmmac_poll() from running concurently
with stmmac_dma_interrupt(), right?

Best regards,
									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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