From: Jakub Kicinski <email@example.com> To: Thomas Gleixner <firstname.lastname@example.org>, Sebastian Andrzej Siewior <email@example.com> Cc: firstname.lastname@example.org, Juri Lelli <email@example.com>, linux-rt-users <firstname.lastname@example.org>, Steven Rostedt <email@example.com>, LKML <firstname.lastname@example.org>, email@example.com, "David S. Miller" <firstname.lastname@example.org>, email@example.com Subject: Re: [PATCH net-next] net: Treat __napi_schedule_irqoff() as __napi_schedule() on PREEMPT_RT Date: Fri, 14 May 2021 11:56:49 -0700 [thread overview] Message-ID: <20210514115649.6c84fdc3@kicinski-fedora-PC1C0HJN> (raw) In-Reply-To: <firstname.lastname@example.org> 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 <email@example.com> > > Signed-off-by: Sebastian Andrzej Siewior <firstname.lastname@example.org> > > Reviewed-by: Thomas Gleixner <email@example.com> > > > --- > > 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?
next prev parent reply other threads:[~2021-05-14 18:56 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 [this message] 2021-05-14 19:44 ` Alison Chaiken 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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