Re: [Intel-wired-lan] [PATCH iwl-net 0/4] igb: fix igb_msix_other() handling for PREEMPT_RT

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

On 2025-01-07 15:52:47 [-0300], Wander Lairson Costa wrote:
> On Tue, Jan 07, 2025 at 02:51:06PM +0100, Sebastian Andrzej Siewior wrote:
> > On 2024-12-04 08:42:23 [-0300], Wander Lairson Costa wrote:
> > > This is the second attempt at fixing the behavior of igb_msix_other()
> > > for PREEMPT_RT. The previous attempt [1] was reverted [2] following
> > > concerns raised by Sebastian [3].
> > > 
> > > The initial approach proposed converting vfs_lock to a raw_spinlock,
> > > a minor change intended to make it safe. However, it became evident
> > > that igb_rcv_msg_from_vf() invokes kcalloc with GFP_ATOMIC,
> > > which is unsafe in interrupt context on PREEMPT_RT systems.
> > > 
> > > To address this, the solution involves splitting igb_msg_task()
> > > into two parts:
> > > 
> > >     * One part invoked from the IRQ context.
> > >     * Another part called from the threaded interrupt handler.
> > > 
> > > To accommodate this, vfs_lock has been restructured into a double
> > > lock: a spinlock_t and a raw_spinlock_t. In the revised design:
> > > 
> > >     * igb_disable_sriov() locks both spinlocks.
> > >     * Each part of igb_msg_task() locks the appropriate spinlock for
> > >     its execution context.
> > 
> > - Is this limited to PREEMPT_RT or does it also occur on PREEMPT systems
> >   with threadirqs? And if this is PREEMPT_RT only, why?
> 
> PREEMPT systems configured to use threadirqs should be affected as well,
> although I never tested with this configuration. Honestly, until now I wasn't
> aware of the possibility of a non PREEMPT_RT kernel with threaded IRQs by default.

If the issue is indeed the use of threaded interrupts then the fix
should not be limited to be PREEMPT_RT only.

> > - What causes the failure? I see you reworked into two parts to behave
> >   similar to what happens without threaded interrupts. There is still no
> >   explanation for it. Is there a timing limit or was there another
> >   register operation which removed the mailbox message?
> > 
> 
> I explained the root cause of the issue in the last commit. Maybe I should
> have added the explanation to the cover letter as well.  Anyway, here is a
> partial verbatim copy of it:
> 
> "During testing of SR-IOV, Red Hat QE encountered an issue where the
> ip link up command intermittently fails for the igbvf interfaces when
> using the PREEMPT_RT variant. Investigation revealed that
> e1000_write_posted_mbx returns an error due to the lack of an ACK
> from e1000_poll_for_ack.

That ACK would have come if it would poll longer?

> The underlying issue arises from the fact that IRQs are threaded by
> default under PREEMPT_RT. While the exact hardware details are not
> available, it appears that the IRQ handled by igb_msix_other must
> be processed before e1000_poll_for_ack times out. However,
> e1000_write_posted_mbx is called with preemption disabled, leading
> to a scenario where the IRQ is serviced only after the failure of
> e1000_write_posted_mbx."

Where is this disabled preemption coming from? This should be one of the
ops.write_posted() calls, right? I've been looking around and don't see
anything obvious.
Couldn't you wait for an event instead of polling?

> The call chain from igb_msg_task():
> 
> igb_msg_task
> 	igb_rcv_msg_from_vf
> 		igb_set_vf_multicasts
> 			igb_set_rx_mode
> 				igb_write_mc_addr_list
> 					kmalloc
> 
> Cannot happen from interrupt context under PREEMPT_RT. So this part of
> the interrupt handler is deferred to a threaded IRQ handler.
>
> > > Cheers,
> > > Wander

Sebastian