From: Alison Chaiken <email@example.com> To: Jakub Kicinski <firstname.lastname@example.org> Cc: Thomas Gleixner <email@example.com>, Sebastian Andrzej Siewior <firstname.lastname@example.org>, email@example.com, Juri Lelli <firstname.lastname@example.org>, linux-rt-users <email@example.com>, Steven Rostedt <firstname.lastname@example.org>, LKML <email@example.com>, firstname.lastname@example.org, "David S. Miller" <email@example.com>, firstname.lastname@example.org Subject: Re: [PATCH net-next] net: Treat __napi_schedule_irqoff() as __napi_schedule() on PREEMPT_RT Date: Fri, 14 May 2021 12:44:15 -0700 [thread overview] Message-ID: <CAFzL-7vTcr75ho0kKs+0PxD3UFRE9=KtNQKJGTx7u-LzGK_oxA@mail.gmail.com> (raw) In-Reply-To: <20210514115649.6c84fdc3@kicinski-fedora-PC1C0HJN> On Fri, May 14, 2021 at 11:56 AM Jakub Kicinski <email@example.com> wrote: > > On Thu, 13 May 2021 00:28:02 +0200 Thomas Gleixner wrote: > > On Wed, May 12 2021 at 23:43, Sebastian Andrzej Siewior wrote: > > > __napi_schedule_irqoff() is an optimized version of __napi_schedule() > > > which can be used where it is known that interrupts are disabled, > > > e.g. in interrupt-handlers, spin_lock_irq() sections or hrtimer > > > callbacks. > > > > > > On PREEMPT_RT enabled kernels this assumptions is not true. Force- > > > threaded interrupt handlers and spinlocks are not disabling interrupts > > > and the NAPI hrtimer callback is forced into softirq context which runs > > > with interrupts enabled as well. > > > > > > Chasing all usage sites of __napi_schedule_irqoff() is a whack-a-mole > > > game so make __napi_schedule_irqoff() invoke __napi_schedule() for > > > PREEMPT_RT kernels. > > > > > > The callers of ____napi_schedule() in the networking core have been > > > audited and are correct on PREEMPT_RT kernels as well. > > > > > > Reported-by: Juri Lelli <firstname.lastname@example.org> > > > Signed-off-by: Sebastian Andrzej Siewior <email@example.com> > > > > Reviewed-by: Thomas Gleixner <firstname.lastname@example.org> > > > > > --- > > > Alternatively __napi_schedule_irqoff() could be #ifdef'ed out on RT and > > > an inline provided which invokes __napi_schedule(). > > > > > > This was not chosen as it creates #ifdeffery all over the place and with > > > the proposed solution the code reflects the documentation consistently > > > and in one obvious place. > > > > Blame me for that decision. > > > > No matter which variant we end up with, this needs to go into all stable > > RT kernels ASAP. > > Mumble mumble. I thought we concluded that drivers used on RT can be > fixed, we've already done it for a couple drivers (by which I mean two). > If all the IRQ handler is doing is scheduling NAPI (which it is for > modern NICs) - IRQF_NO_THREAD seems like the right option. > > Is there any driver you care about that we can convert to using > IRQF_NO_THREAD so we can have new drivers to "do the right thing" > while the old ones depend on this workaround for now? > > > Another thing while I have your attention - ____napi_schedule() does > __raise_softirq_irqoff() which AFAIU does not wake the ksoftirq thread. > On non-RT we get occasional NOHZ warnings when drivers schedule napi > from process context, but on RT this is even more of a problem, right? > ksoftirqd won't run until something else actually wakes it up? By "NOHZ warnings," do you mean "NOHZ: local_softirq_pending"? We see that message about once a week with 4.19. Presumably any failure of ____napi_schedule() to wake ksoftirqd could only cause problems for the NET_RX softirq, so if the pending softirq is different, the cause lies elsewhere. -- Alison Chaiken Aurora Innovation
next prev parent reply other threads:[~2021-05-14 19:44 UTC|newest] Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top 2021-05-11 6:09 [RT] Question about i40e threaded irq Juri Lelli 2021-05-11 6:24 ` Stefan Assmann 2021-05-11 7:46 ` Thomas Gleixner 2021-05-12 21:43 ` [PATCH net-next] net: Treat __napi_schedule_irqoff() as __napi_schedule() on PREEMPT_RT Sebastian Andrzej Siewior 2021-05-12 22:28 ` Thomas Gleixner 2021-05-13 0:50 ` Steven Rostedt 2021-05-13 16:43 ` Steven Rostedt 2021-05-14 12:11 ` Thomas Gleixner 2021-05-14 18:56 ` Jakub Kicinski 2021-05-14 19:44 ` Alison Chaiken [this message] 2021-05-14 21:53 ` Thomas Gleixner 2021-05-14 20:16 ` Thomas Gleixner 2021-05-14 20:38 ` Jakub Kicinski 2021-05-13 5:12 ` Juri Lelli 2021-05-13 20:20 ` patchwork-bot+netdevbpf
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