netdev - Re: [PATCH] net: stmmac: Don't call _irqoff() with hardirqs enabled

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Date:   Sat, 10 Oct 2020 08:22:48 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Heiner Kallweit <hkallweit1@...il.com>
Cc:     John Keeping <john@...anate.com>, netdev@...r.kernel.org,
        Giuseppe Cavallaro <peppe.cavallaro@...com>,
        Alexandre Torgue <alexandre.torgue@...com>,
        Jose Abreu <joabreu@...opsys.com>,
        "David S. Miller" <davem@...emloft.net>,
        Maxime Coquelin <mcoquelin.stm32@...il.com>,
        linux-stm32@...md-mailman.stormreply.com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH] net: stmmac: Don't call _irqoff() with hardirqs enabled

On Sat, 10 Oct 2020 15:08:15 +0200 Heiner Kallweit wrote:
> On 09.10.2020 18:06, Heiner Kallweit wrote:
> > On 09.10.2020 17:58, Jakub Kicinski wrote:  
> >> On Fri, 9 Oct 2020 16:54:06 +0200 Heiner Kallweit wrote:  
> >>> I'm thinking about a __napi_schedule version that disables hard irq's
> >>> conditionally, based on variable force_irqthreads, exported by the irq
> >>> subsystem. This would allow to behave correctly with threadirqs set,
> >>> whilst not loosing the _irqoff benefit with threadirqs unset.
> >>> Let me come up with a proposal.  
> >>
> >> I think you'd need to make napi_schedule_irqoff() behave like that,
> >> right?  Are there any uses of napi_schedule_irqoff() that are disabling
> >> irqs and not just running from an irq handler?
> >>  
> > Right, the best approach depends on the answer to the latter question.
> > I didn't check this yet, therefore I described the least intrusive approach.
> >   
> 
> With some help from coccinelle I identified the following functions that
> call napi_schedule_irqoff() or __napi_schedule_irqoff() and do not run
> from an irq handler (at least not at the first glance).
> 
> dpaa2_caam_fqdan_cb
> qede_simd_fp_handler
> mlx4_en_rx_irq
> mlx4_en_tx_irq

Don't know the others but FWIW the mlx4 ones run from an IRQ handler,
AFAICT:

static irqreturn_t mlx4_interrupt(int irq, void *dev_ptr)
static irqreturn_t mlx4_msi_x_interrupt(int irq, void *eq_ptr)
  mlx4_eq_int()
    mlx4_cq_completion
      cq->comp()

> qeth_qdio_poll
> netvsc_channel_cb
> napi_watchdog

This one runs from a hrtimer, which I believe will be a hard irq
context on anything but RT. I could be wrong.